{"id":1993,"date":"2026-09-30T15:26:02","date_gmt":"2026-09-30T15:26:02","guid":{"rendered":"https:\/\/stadiaorg.com\/?page_id=1993"},"modified":"2026-09-30T15:43:37","modified_gmt":"2026-09-30T15:43:37","slug":"our-planet","status":"publish","type":"page","link":"https:\/\/stadiaorg.com\/fr\/our-planet\/","title":{"rendered":"Notre plan\u00e8te"},"content":{"rendered":"\n<article id=\"sv-planet-ch1\" lang=\"en\" aria-labelledby=\"svp-page-title\">\n<style>\n#sv-planet-ch1{--svp-ink:#111530;--svp-muted:#52627b;--svp-purple:#6351d5;--svp-blue:#215ea7;--svp-line:#dcdff0;--svp-light:#f6f7fc;--svp-dark:#141634;--svp-green:#146452;--svp-amber:#8b4809;box-sizing:border-box;background:#fff;color:var(--svp-ink);font-family:Arial,Helvetica,sans-serif;font-size:17px;line-height:1.72;overflow-wrap:anywhere;width:100%;margin:0 auto;padding:24px 0 90px;isolation:isolate}\n#sv-planet-ch1 *,#sv-planet-ch1 *::before,#sv-planet-ch1 *::after{box-sizing:border-box}#sv-planet-ch1 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.svp-kicker{color:#000}#sv-planet-ch1 .svp-toc,#sv-planet-ch1 .svp-actions,#sv-planet-ch1 .svp-js-only,#sv-planet-ch1 .svp-share{display:none!important}#sv-planet-ch1 h1{font-size:28pt}#sv-planet-ch1 h2{font-size:21pt}#sv-planet-ch1 h3{font-size:15pt}#sv-planet-ch1 .svp-section{padding:22px 0}#sv-planet-ch1 .svp-split,#sv-planet-ch1 .svp-cards{display:block}#sv-planet-ch1 .svp-figure{max-width:440px;margin:15px auto;break-inside:avoid}#sv-planet-ch1 .svp-card{margin-bottom:15px;break-inside:avoid}#sv-planet-ch1 .svp-figure img{max-height:280px}#sv-planet-ch1 .svp-kind{position:static;display:block;color:#000;background:#fff}#sv-planet-ch1 .svp-image-unavailable{color:#000;background:#fff;padding:15px}#sv-planet-ch1 .svp-print-note{display:block}#sv-planet-ch1 .svp-lab{break-inside:auto}#sv-planet-ch1 .svp-detail{break-inside:avoid}#sv-planet-ch1 .svp-button{border:0;padding:2px;min-height:0;color:#000}#sv-planet-ch1 .svp-credit,#sv-planet-ch1 .svp-hero figcaption,#sv-planet-ch1 .svp-hero .svp-credit,#sv-planet-ch1 .svp-hero figcaption a{color:#222}#sv-planet-ch1 .svp-hero .svp-figure{background:#fff}#sv-planet-ch1 a{color:#000}#sv-planet-ch1 .svp-sources{font-size:9pt}}\n\n\/* Chapter 2 - layout compatibility patch 1.1.\n   Only this chapter and its ancestor content containers are widened.\n   No shared template, navigation, text, images or scripts are changed. *\/\n@media screen {\n  body:not(.wp-admin) :is(main, article, .site-main, .entry-content,\n    .wp-block-post-content, .wp-block-group):has(#sv-planet-ch1) {\n    box-sizing: border-box;\n    width: 100% !important;\n    max-width: none !important;\n    min-width: 0;\n    margin-inline: auto !important;\n  }\n  body:not(.wp-admin) #sv-planet-ch1 {\n    width: 100% !important;\n    max-width: none !important;\n    min-width: 0;\n    margin-inline: auto !important;\n  }\n}\n\n\/* Chapter 2 - print-only compatibility patch 1.2.\n   Isolates this chapter on paper. Does not edit the shared navigation,\n   the Home page, any web-screen layout, content, links or calculator scripts.\n   Print only the answers currently open; the existing buttons control them. *\/\n@media print {\n  @page { size: auto; margin: 14mm; }\n\n  html:has(#sv-planet-ch1),\n  body:has(#sv-planet-ch1) {\n    margin: 0 !important;\n    padding: 0 !important;\n    width: auto !important;\n    min-width: 0 !important;\n    height: auto !important;\n    min-height: 0 !important;\n    overflow: visible !important;\n    background: #fff !important;\n  }\n\n  \/* Keep the chapter and the ancestors needed to lay it out.\n     Site navigation, admin bars, floating language controls and unrelated\n     template content are outside the chapter and do not print. *\/\n  body:has(#sv-planet-ch1) *:not(#sv-planet-ch1):not(#sv-planet-ch1 *):not(:has(#sv-planet-ch1)):not(script):not(style):not(link) {\n    display: none !important;\n  }\n  body:has(#sv-planet-ch1) :has(#sv-planet-ch1) {\n    display: block !important;\n    position: static !important;\n    float: none !important;\n    width: 100% !important;\n    max-width: none !important;\n    min-width: 0 !important;\n    height: auto !important;\n    max-height: none !important;\n    min-height: 0 !important;\n    margin: 0 !important;\n    padding: 0 !important;\n    border: 0 !important;\n    transform: none !important;\n    overflow: visible !important;\n    contain: none !important;\n    box-shadow: none !important;\n    background: #fff !important;\n  }\n  body:has(#sv-planet-ch1) :has(#sv-planet-ch1)::before,\n  body:has(#sv-planet-ch1) :has(#sv-planet-ch1)::after {\n    content: none !important;\n  }\n  #sv-planet-ch1 {\n    width: 100% !important;\n    max-width: none !important;\n    min-width: 0 !important;\n    height: auto !important;\n    padding: 0 !important;\n    margin: 0 !important;\n    overflow: visible !important;\n    contain: none !important;\n    container-type: normal !important;\n    background: #fff !important;\n    color: #111 !important;\n    font-size: 11pt !important;\n    line-height: 1.42 !important;\n    isolation: auto !important;\n  }\n  #sv-planet-ch1 *, #sv-planet-ch1 *::before, #sv-planet-ch1 *::after {\n    text-shadow: none !important;\n    box-shadow: none !important;\n    animation: none !important;\n    transition: none !important;\n  }\n  #sv-planet-ch1 .svp-wrap, #sv-planet-ch1 .svp-intro {\n    width: 100% !important;\n    max-width: none !important;\n    margin: 0 !important;\n    padding: 0 !important;\n  }\n  #sv-planet-ch1 .svp-breadcrumb, #sv-planet-ch1 .svp-toc,\n  #sv-planet-ch1 .svp-actions, #sv-planet-ch1 .svp-share,\n  #sv-planet-ch1 .svp-js-only, #sv-planet-ch1 button,\n  #sv-planet-ch1 .svp-local-status, #sv-planet-ch1 .svp-copy-field {\n    display: none !important;\n  }\n  #sv-planet-ch1 p { margin: 0 0 8pt !important; orphans: 3; widows: 3; }\n  #sv-planet-ch1 h1, #sv-planet-ch1 h2, #sv-planet-ch1 h3, #sv-planet-ch1 h4 {\n    color: #111 !important;\n    break-after: avoid-page;\n    page-break-after: avoid;\n    line-height: 1.15 !important;\n  }\n  #sv-planet-ch1 h1 { font-size: 27pt !important; margin: 7pt 0 12pt !important; }\n  #sv-planet-ch1 h2 { font-size: 20pt !important; margin: 6pt 0 12pt !important; }\n  #sv-planet-ch1 h3 { font-size: 14pt !important; margin: 5pt 0 8pt !important; }\n  #sv-planet-ch1 h4 { font-size: 11pt !important; }\n  #sv-planet-ch1 .svp-kicker {\n    color: #333 !important;\n    font-size: 8.5pt !important;\n    margin: 0 0 7pt !important;\n    break-after: avoid-page;\n    page-break-after: avoid;\n  }\n  #sv-planet-ch1 a, #sv-planet-ch1 .svp-hero a { color: #111 !important; }\n  #sv-planet-ch1 .svp-hero {\n    display: block !important;\n    color: #111 !important;\n    background: #fff !important;\n    border: 0 !important;\n    padding: 0 !important;\n    margin: 0 0 12pt !important;\n    border-radius: 0 !important;\n    break-inside: avoid-page;\n    page-break-inside: avoid;\n  }\n  #sv-planet-ch1 .svp-hero h1 span, #sv-planet-ch1 .svp-hero p,\n  #sv-planet-ch1 .svp-hero figcaption, #sv-planet-ch1 .svp-hero .svp-credit {\n    color: #111 !important;\n  }\n  #sv-planet-ch1 .svp-lead { font-size: 13pt !important; }\n  #sv-planet-ch1 .svp-split, #sv-planet-ch1 .svp-split.reverse,\n  #sv-planet-ch1 .svp-cards, #sv-planet-ch1 .svp-card,\n  #sv-planet-ch1 .svp-quick, #sv-planet-ch1 .svp-chain,\n  #sv-planet-ch1 .svp-glossary, #sv-planet-ch1 .svp-metrics {\n    display: block !important;\n  }\n  #sv-planet-ch1 .svp-section {\n    padding: 16pt 0 !important;\n    margin: 0 !important;\n    border-bottom: 1px solid #bbb !important;\n    break-inside: auto;\n    page-break-inside: auto;\n  }\n  #sv-planet-ch1 #svp-start { break-before: page; page-break-before: always; }\n\n  \/* A figure is one printable unit. Its type label is below the image,\n     never a second flex column competing with the image. *\/\n  #sv-planet-ch1 .svp-figure,\n  #sv-planet-ch1 .svp-split > .svp-figure,\n  #sv-planet-ch1 .svp-hero .svp-figure {\n    display: block !important;\n    width: 128mm !important;\n    max-width: 100% !important;\n    margin: 12pt auto !important;\n    padding: 0 !important;\n    border: 1px solid #aaa !important;\n    border-radius: 0 !important;\n    background: #fff !important;\n    overflow: visible !important;\n    break-inside: avoid-page !important;\n    page-break-inside: avoid !important;\n  }\n  #sv-planet-ch1 .svp-image-frame {\n    display: block !important;\n    position: static !important;\n    min-height: 0 !important;\n    height: auto !important;\n    max-height: none !important;\n    padding: 0 !important;\n    margin: 0 !important;\n    background: #fff !important;\n  }\n  #sv-planet-ch1 .svp-image-frame img {\n    width: auto !important;\n    max-width: 100% !important;\n    height: auto !important;\n    max-height: 72mm !important;\n    margin: 0 auto !important;\n    object-fit: contain !important;\n  }\n  #sv-planet-ch1 .svp-kind {\n    display: block !important;\n    position: static !important;\n    width: auto !important;\n    max-width: none !important;\n    color: #222 !important;\n    background: #fff !important;\n    border-radius: 0 !important;\n    font-size: 8pt !important;\n    line-height: 1.25 !important;\n    padding: 5pt 8pt 0 !important;\n  }\n  #sv-planet-ch1 figcaption {\n    display: block !important;\n    padding: 6pt 8pt 8pt !important;\n    color: #222 !important;\n    font-size: 9pt !important;\n    line-height: 1.35 !important;\n  }\n  #sv-planet-ch1 .svp-credit {\n    display: block !important;\n    margin-top: 5pt !important;\n    color: #333 !important;\n    font-size: 8pt !important;\n    line-height: 1.35 !important;\n  }\n  #sv-planet-ch1 .svp-image-unavailable {\n    color: #111 !important;\n    padding: 12pt !important;\n    font-size: 10pt !important;\n  }\n  #sv-planet-ch1 .svp-quick { margin: 8pt 0 12pt !important; }\n  #sv-planet-ch1 .svp-quick > div, #sv-planet-ch1 .svp-chain li,\n  #sv-planet-ch1 .svp-card, #sv-planet-ch1 .svp-glossary > div,\n  #sv-planet-ch1 .svp-metrics > div {\n    height: auto !important;\n    min-height: 0 !important;\n    margin: 0 0 7pt !important;\n    padding: 7pt 9pt !important;\n    background: #fff !important;\n    border-radius: 0 !important;\n    border-color: #bbb !important;\n    break-inside: avoid-page;\n    page-break-inside: avoid;\n  }\n  #sv-planet-ch1 .svp-quick strong, #sv-planet-ch1 .svp-chain strong {\n    font-size: 10.5pt !important;\n    margin-bottom: 3pt !important;\n  }\n  #sv-planet-ch1 .svp-quick span, #sv-planet-ch1 .svp-chain li,\n  #sv-planet-ch1 .svp-card p, #sv-planet-ch1 dd {\n    font-size: 10pt !important;\n    line-height: 1.4 !important;\n    color: #333 !important;\n  }\n  #sv-planet-ch1 .svp-chain { margin: 10pt 0 !important; }\n  #sv-planet-ch1 .svp-chain li:before {\n    display: inline !important;\n    margin-right: 6pt !important;\n    color: #333 !important;\n    font-size: 9pt !important;\n  }\n  #sv-planet-ch1 .svp-chain strong { display: inline !important; margin-right: 5pt !important; }\n  #sv-planet-ch1 .svp-lab, #sv-planet-ch1 .svp-videos {\n    margin: 12pt 0 !important;\n    padding: 11pt !important;\n    border: 1px solid #aaa !important;\n    border-radius: 0 !important;\n    background: #fff !important;\n    break-inside: auto;\n    page-break-inside: auto;\n  }\n  #sv-planet-ch1 .svp-note, #sv-planet-ch1 .svp-question {\n    color: #222 !important;\n    background: #fff !important;\n    border-left: 3pt solid #666 !important;\n    border-radius: 0 !important;\n    font-size: 10.5pt !important;\n    line-height: 1.42 !important;\n    padding: 8pt 10pt !important;\n    margin: 10pt 0 !important;\n    break-inside: avoid-page;\n    page-break-inside: avoid;\n  }\n  #sv-planet-ch1 .svp-detail {\n    background: #fff !important;\n    overflow: visible !important;\n    border: 1px solid #bbb !important;\n    border-radius: 0 !important;\n    margin: 8pt 0 !important;\n    break-inside: avoid-page;\n    page-break-inside: avoid;\n  }\n  #sv-planet-ch1 .svp-detail summary {\n    background: #fff !important;\n    color: #111 !important;\n    padding: 8pt !important;\n    min-height: 0 !important;\n    font-size: 10.5pt !important;\n    break-after: avoid-page;\n    page-break-after: avoid;\n  }\n  #sv-planet-ch1 .svp-answer { padding: 8pt !important; font-size: 10.5pt !important; }\n  #sv-planet-ch1 .svp-formula {\n    margin: 10pt 0 !important;\n    padding: 9pt !important;\n    font-size: 12pt !important;\n    line-height: 1.45 !important;\n    border-radius: 0 !important;\n    color: #111 !important;\n    background: #fff !important;\n    break-inside: avoid-page;\n  }\n  \/* Print the current inputs alongside their results. Do not reveal controls\n     when JavaScript was disabled; the default worked examples remain. *\/\n  #sv-planet-ch1 .svp-lab > .svp-js-only:not([hidden]) {\n    display: block !important;\n    margin: 8pt 0 !important;\n    break-inside: avoid-page;\n  }\n  #sv-planet-ch1 .svp-lab .svp-control-grid > div {\n    display: inline-block !important;\n    width: 48% !important;\n    vertical-align: top !important;\n  }\n  #sv-planet-ch1 .svp-lab label { font-size: 9.5pt !important; color: #111 !important; }\n  #sv-planet-ch1 .svp-lab input[type=range] { display: none !important; }\n  #sv-planet-ch1 .svp-lab input[type=number] {\n    font-size: 11pt !important;\n    width: 100% !important;\n    max-width: 100% !important;\n    min-height: 0 !important;\n    height: auto !important;\n    padding: 4pt !important;\n    margin: 0 0 4pt !important;\n    border: 1px solid #aaa !important;\n    border-radius: 0 !important;\n    appearance: textfield;\n  }\n  #sv-planet-ch1 .svp-metrics { margin: 8pt 0 !important; 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Our Planet<\/span>\n <h1 id=\"svp-page-title\">Our planet has<br>a long history.<\/h1>\n <p class=\"svp-lead\">Rocks, oceans, air and life. Explore the evidence behind the world we know.<\/p>\n <nav class=\"svp-chapter-nav\" aria-label=\"Choose a chapter\">\n  <a href=\"#svp-chapter-1\"><small>CHAPTER 01<\/small><strong>When mountains were sea<\/strong><span>Read rocks, fossils and layers &#8595;<\/span><\/a>\n  <a href=\"#svp-chapter-2\"><small>NEW &#183; CHAPTER 02<\/small><strong>A planet takes shape<\/strong><span>Earth, atmosphere and oceans &#8595;<\/span><\/a>\n <\/nav>\n<\/header>\n<section id=\"svp-chapter-1\" aria-labelledby=\"svp-title\">\n<header class=\"svp-hero\">\n<div><span class=\"svp-kicker\">Stadia Science &#183; Earth history &#183; Chapter 01<\/span>\n<p class=\"svp-series\">Our planet has a long history<\/p>\n<h2 id=\"svp-title\" class=\"svp-chapter-title\">When mountains<br><span>were sea.<\/span><\/h2>\n<p class=\"svp-lead\">How can we reconstruct a past nobody witnessed?<\/p>\n<p>Begin with a fossil. Read the layers around it. Discover why the age of a rock and the age of a mountain are different questions.<\/p>\n<div class=\"svp-actions\"><a class=\"svp-button primary\" href=\"#svp-dolomites\">Begin the journey &#8595;<\/a><a class=\"svp-button ghost\" href=\"#svp-lab\">Try the layer activity<\/a><\/div><\/div>\n<figure class=\"svp-figure\"><svg class=\"svp-diagram\" viewBox=\"0 0 520 440\" role=\"img\" aria-labelledby=\"svp-mountain-title svp-mountain-desc\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\">\n<title id=\"svp-mountain-title\">Marine rocks in a mountain landscape<\/title><desc id=\"svp-mountain-desc\">Conceptual mountain cross-section. An eroded mountain surface cuts through tilted rock bands. A marine fossil symbol marks one band. This is not a measured section of the Dolomites.<\/desc>\n<defs><linearGradient id=\"svp-sky\" x2=\"0\" y2=\"1\"><stop stop-color=\"#17213c\"\/><stop offset=\"1\" stop-color=\"#385369\"\/><\/linearGradient><clipPath id=\"svp-mountain-clip\"><path d=\"M0 340L58 272L96 288L158 164L192 213L271 85L305 161L339 133L401 266L443 223L520 334V440H0Z\"\/><\/clipPath><\/defs>\n<rect width=\"520\" height=\"440\" fill=\"url(#svp-sky)\"\/>\n<circle cx=\"431\" cy=\"69\" r=\"25\" fill=\"#e5d7b6\"\/>\n<path d=\"M0 339L76 241L131 308L226 205L320 314L420 177L520 326V440H0Z\" fill=\"#617181\" opacity=\".45\"\/>\n<g clip-path=\"url(#svp-mountain-clip)\"><rect y=\"75\" width=\"520\" height=\"365\" fill=\"#ddd1b4\"\/>\n<path d=\"M-70 207L560 335V391L-70 263Z\" fill=\"#9cabac\"\/><path d=\"M-70 263L560 391V450L-70 322Z\" fill=\"#aa806c\"\/><path d=\"M-70 322L560 450V510L-70 382Z\" fill=\"#626478\"\/>\n<path d=\"M-70 153L560 281M-70 207L560 335M-70 263L560 391M-70 322L560 450\" fill=\"none\" stroke=\"#f2ead8\" stroke-width=\"3\"\/>\n<\/g>\n<g transform=\"translate(269 250) rotate(12)\" fill=\"none\" stroke=\"#263442\" stroke-width=\"2.5\"><path d=\"M0 20C-50-16-20-38 0-39C20-38 50-16 0 20Z\" fill=\"#f3e7c9\"\/><path d=\"M0 20V-38M0 20L-16-32M0 20L-29-19M0 20L16-32M0 20L29-19\"\/><\/g>\n<path d=\"M285 235L361 198H490\" fill=\"none\" stroke=\"#fff\" stroke-width=\"1.5\"\/><text x=\"369\" y=\"184\" fill=\"#fff\" font-size=\"15\">Marine fossil<\/text>\n<rect x=\"22\" y=\"24\" width=\"203\" height=\"31\" rx=\"15\" fill=\"#0d1427\"\/><text x=\"36\" y=\"44\" fill=\"#dae9ec\" font-size=\"12\" letter-spacing=\"1\">CONCEPTUAL CROSS-SECTION<\/text>\n<rect x=\"18\" y=\"371\" width=\"484\" height=\"49\" rx=\"10\" fill=\"#111a2be8\"\/><text x=\"32\" y=\"392\" fill=\"#e9eff3\" font-size=\"14\">The rock formed before the mountain.<\/text><text x=\"32\" y=\"410\" fill=\"#bbced9\" font-size=\"12\">Not to scale &#183; no single formation represented<\/text>\n<\/svg><figcaption>A teaching diagram, not a photograph or a reconstruction of a particular mountain. Colours separate imagined rock units; the shell is a symbol.<\/figcaption><\/figure>\n<\/header>\n<div class=\"svp-quick\"><div><strong>Read a landscape<\/strong><span>Distinguish a visible clue from an explanation.<\/span><\/div><div><strong>Put events in order<\/strong><span>Work through a fictional sequence of rock layers.<\/span><\/div><div><strong>Ask how we know<\/strong><span>Follow the sources and identify what remains unknown.<\/span><\/div><\/div>\n<nav class=\"svp-toc\" aria-label=\"Chapter contents\"><strong>Inside Chapter 1<\/strong><div class=\"svp-toc-links\"><a href=\"#svp-dolomites\">1 &#183; A sea in the mountains<\/a><a href=\"#svp-fossils\">2 &#183; Read the evidence<\/a><a href=\"#svp-layers\">3 &#183; Read the layers<\/a><a href=\"#svp-lab\">Mini-lab<\/a><a href=\"#svp-time\">4 &#183; Put years on events<\/a><a href=\"#svp-check\">Questions &amp; answers<\/a><a href=\"#svp-teachers\">For educators<\/a><a href=\"#svp-series\">The series<\/a><a href=\"#svp-sources\">Sources<\/a><\/div><\/nav>\n\n<section class=\"svp-section\" id=\"svp-dolomites\"><span class=\"svp-kicker\">01 &#183; Start close to home<\/span><h2>The Dolomites hold the memory of a sea.<\/h2>\n<div class=\"svp-intro\"><p>Imagine standing beneath a pale rock wall in the Dolomites. Its height belongs to today&#8217;s landscape. The rocks tell a much older story: UNESCO recognises these mountains for their exceptional preservation of ancient carbonate platforms and associated fossil records. These were environments where marine organisms and sediments helped build large bodies of carbonate material.<sup class=\"svp-cites\"><a href=\"#svp-source-1\" aria-label=\"Source 1\">[1]<\/a><\/sup><\/p>\n<p>In the Dolomiti Bellunesi, sediments that became the Dolomia Principale accumulated in a shallow sea during the Late Triassic. Other formations record later marine environments. Much later, Alpine mountain building compressed, folded and displaced the rock layers. Erosion helped reveal and shape them.<sup class=\"svp-cites\"><a href=\"#svp-source-2\" aria-label=\"Source 2\">[2]<\/a><\/sup><\/p>\n<p class=\"svp-question\">The mountain is here now. Where did its rocks begin?<\/p>\n<p>&#8220;When mountains were sea&#8221; is a shorthand: some rocks now exposed in mountains formed in marine environments. It does not mean that every mountain has this origin, or that today&#8217;s peaks already stood beneath an ancient ocean.<\/p><\/div>\n<ol class=\"svp-chain\"><li><strong>Deposition<\/strong>Material accumulates in a marine setting.<\/li><li><strong>Rock formation<\/strong>Sediment becomes part of the rock record.<\/li><li><strong>Deformation &amp; uplift<\/strong>Later tectonic processes change its position.<\/li><li><strong>Exposure<\/strong>Erosion reveals parts of that history.<\/li><\/ol>\n<p class=\"svp-mini\">A simplified sequence for this story, not a universal timetable. Processes can overlap. The park&#8217;s geological history explains the local sequence. <sup class=\"svp-cites\"><a href=\"#svp-source-2\" aria-label=\"Source 2\">[2]<\/a><\/sup><\/p>\n<a class=\"svp-button\" href=\"https:\/\/whc.unesco.org\/en\/list\/1237\/\">Explore the UNESCO Dolomites record &#8594;<\/a>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp-fossils\"><span class=\"svp-kicker\">02 &#183; Evidence needs context<\/span><h2>A fossil is a clue.<br>Its surroundings are part of the clue.<\/h2>\n<p>Fossils include preserved remains and traces of past life. A shell records part of an organism; a burrow can record its activity. The Grand Canyon offers another example of rocks containing marine fossils, including brachiopods and crinoids, alongside layers with evidence of terrestrial environments.<sup class=\"svp-cites\"><a href=\"#svp-source-3\" aria-label=\"Source 3\">[3]<\/a><\/sup><\/p>\n<div class=\"svp-table-wrap\"><table><caption class=\"svp-sr\">A fictional observation exercise: evidence, interpretation and limits<\/caption><thead><tr><th scope=\"col\">What you record<\/th><th scope=\"col\">What you investigate<\/th><th scope=\"col\">What remains open<\/th><\/tr><\/thead><tbody>\n<tr><td>A shell-like shape inside exposed rock<\/td><td>Is it a fossil? Which organism does it represent?<\/td><td>A shape alone does not identify its habitat.<\/td><\/tr>\n<tr><td>Several identified marine fossils in a mapped layer<\/td><td>A marine origin, considered with the sediment and its context<\/td><td>Water depth, transport and possible reworking need more evidence.<\/td><\/tr>\n<tr><td>The same layer traced along a slope<\/td><td>How the rock body continues and changes<\/td><td>Its present elevation does not give its original elevation.<\/td><\/tr>\n<\/tbody><\/table><\/div>\n<p class=\"svp-mini\">The table is a reasoning exercise, not a report of a new fossil discovery.<\/p>\n<details class=\"svp-detail\"><summary>Think first: would a loose shell on a summit prove an ancient sea?<\/summary><div class=\"svp-answer\"><p>No. First establish what the object is and where it came from. A loose object lacks the rock-layer context needed for this argument. A useful field record connects a specimen, its position and the surrounding geology.<\/p><\/div><\/details>\n<p>For an illustrated collection of real examples, use the <a href=\"https:\/\/www.nps.gov\/grca\/learn\/nature\/fossils.htm\">National Park Service fossil guide<\/a>. Compare its photographs and captions: which show body remains, and which show traces?<\/p>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp-layers\"><span class=\"svp-kicker\">03 &#183; Before and after<\/span><h2>The lowest layer is a starting point.<br>Check the story before using the rule.<\/h2>\n<p>In a sedimentary sequence that has not been overturned, lower layers were deposited before layers above them. This principle of superposition gives an order, not a date in years. Geological structures must also be read: deformation can change the position of layers.<sup class=\"svp-cites\"><a href=\"#svp-source-4\" aria-label=\"Source 4\">[4]<\/a><\/sup><\/p>\n<p>A missing layer is not necessarily a missing event. Deposition can stop, or erosion can remove material. A surface separating rocks across a gap in the record is an <strong>unconformity<\/strong>. Even apparently parallel layers can hide a long interval of missing time.<sup class=\"svp-cites\"><a href=\"#svp-source-5\" aria-label=\"Source 5\">[5]<\/a><\/sup><\/p>\n<div class=\"svp-note\"><strong>Ask two separate questions.<\/strong> What order do the surviving layers record? How complete is that record?<\/div>\n\n<div class=\"svp-lab\" id=\"svp-lab\"><span class=\"svp-kicker\">Mini-lab &#183; Fictional outcrop &#183; About 5 minutes<\/span><h3>Reconstruct the surviving pages.<\/h3>\n<p>Assume these layers formed where they are shown and were never overturned or faulted. An erosion surface separates B from C. Colours and thicknesses are illustrative.<\/p>\n<div class=\"svp-stack\" role=\"img\" aria-label=\"From top to bottom: layer C, an erosion surface, layer B, layer A.\"><div style=\"padding:10px 20px;background:#fff;font-size:12px\">TOP &#183; Present surface<\/div><ul style=\"list-style:none;margin:0;padding:0\"><li><strong>C &#183; Sandstone<\/strong>Deposited above the erosion surface.<\/li><li class=\"svp-gap\" style=\"background:#fff7ef;border-top:3px dashed #7a5140\">Erosion surface &#183; Some of the record is missing.<\/li><li style=\"background:#e3daf1\"><strong>B &#183; Mudstone<\/strong>Its upper part was partly removed.<\/li><li style=\"background:#edd9b7\"><strong>A &#183; Limestone<\/strong>The lowest surviving layer in this exercise.<\/li><\/ul><\/div>\n<p><strong>Your task:<\/strong> order the deposition of A, B and C from oldest to youngest. Then place the erosion event in the sequence. Can you calculate the duration of the gap?<\/p>\n<form class=\"svp-interactive\" id=\"svp-order-form\" hidden>\n<div class=\"svp-rank-grid\"><div><label for=\"svp-rank-1\">Oldest layer<\/label><select id=\"svp-rank-1\" name=\"rank1\"><option value=\"\">Choose a layer<\/option><option value=\"A\">A &#183; Limestone<\/option><option value=\"B\">B &#183; Mudstone<\/option><option value=\"C\">C &#183; Sandstone<\/option><\/select><\/div><div><label for=\"svp-rank-2\">Middle layer<\/label><select id=\"svp-rank-2\" name=\"rank2\"><option value=\"\">Choose a layer<\/option><option value=\"A\">A &#183; Limestone<\/option><option value=\"B\">B &#183; Mudstone<\/option><option value=\"C\">C &#183; Sandstone<\/option><\/select><\/div><div><label for=\"svp-rank-3\">Youngest layer<\/label><select id=\"svp-rank-3\" name=\"rank3\"><option value=\"\">Choose a layer<\/option><option value=\"A\">A &#183; Limestone<\/option><option value=\"B\">B &#183; Mudstone<\/option><option value=\"C\">C &#183; Sandstone<\/option><\/select><\/div><\/div><button class=\"svp-button primary\" type=\"submit\">Check my order<\/button> <button class=\"svp-button\" type=\"reset\">Start again<\/button><div id=\"svp-order-status\" class=\"svp-lab-result\" role=\"status\" aria-live=\"polite\">Choose all three layers, then check your reasoning.<\/div><\/form><details class=\"svp-detail\"><summary>Worked answer &#183; Open after making your prediction<\/summary><div class=\"svp-answer\"><p><strong>A &#8594; B &#8594; erosion &#8594; C.<\/strong> A was deposited first, then B. Erosion removed part of B before C accumulated. The diagram supplies no numerical ages or sedimentation rates, so the duration of the gap cannot be calculated. The thickest layer need not represent the longest interval.<\/p><\/div><\/details><noscript><p class=\"svp-mini\">Write your order on paper, then open the worked answer. The activity remains usable without JavaScript.<\/p><\/noscript><\/div><\/section>\n<section class=\"svp-section\" id=\"svp-time\"><span class=\"svp-kicker\">04 &#183; Order is not duration<\/span><h2>How do we put years on the story?<\/h2>\n<p>Relative dating establishes a sequence. Numerical dating estimates when an event occurred. Radiometric methods use radioactive decay in suitable minerals, but a date must be connected to the geological event it records. Grains in a sedimentary rock can be older than the sediment&#8217;s deposition.<sup class=\"svp-cites\"><a href=\"#svp-source-6\" aria-label=\"Source 6\">[6]<\/a><\/sup><\/p>\n<p>Suitable primary volcanic ash layers can help constrain the age of sediments above and below them. Reworked ash needs a different interpretation. Geological context and measurement uncertainty remain essential.<sup class=\"svp-cites\"><a href=\"#svp-source-6\" aria-label=\"Source 6\">[6]<\/a><\/sup><\/p>\n<p>Carbon-14 is useful for relatively recent organic material, not for dating the formation of a Triassic rock. Other isotope systems reach much further back in time. Earth&#8217;s estimated age is about <strong>4.54 billion years<\/strong>; a single mountain outcrop records only part of that vast history.<sup class=\"svp-cites\"><a href=\"#svp-source-7\" aria-label=\"Source 7\">[7]<\/a><\/sup><\/p>\n<div class=\"svp-lab\"><span class=\"svp-kicker\">Paper challenge &#183; Invented numbers<\/span><h3>Between two dated ash beds<\/h3><p>A fossil-bearing layer lies above a primary ash bed dated to <strong>220 million years<\/strong> ago and below another dated to <strong>210 million years<\/strong> ago. The sequence is undisturbed. For this first exercise, ignore dating uncertainty and assume the ash crystals date the eruptions and immediate deposition.<\/p>\n<p>What age interval can you assign to the deposition of the fossil-bearing layer? Do you know that it is exactly 215 million years old?<\/p>\n<details class=\"svp-detail\"><summary>Worked answer &#183; An interval, not an invented birthday<\/summary><div class=\"svp-answer\"><p>Its deposition was after the 220-million-year-old ash and before the 210-million-year-old ash: between 210 and 220 million years ago. The midpoint, 215, is not a measured age. In real work, the uncertainties and the origin of both ash beds must also be assessed.<\/p><\/div><\/details>\n<p class=\"svp-mini\">These classroom numbers do not date any particular Dolomite formation.<\/p><\/div>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp-check\"><span class=\"svp-kicker\">Pause &amp; explain<\/span><h2>What does the evidence let you say?<\/h2><p>Try answering aloud or on paper before opening each explanation.<\/p>\n<details class=\"svp-detail\"><summary>1 &#183; Does an ancient marine rock date the rise of the mountain?<\/summary><div class=\"svp-answer\"><p>No. Deposition and later mountain building are different events. A date for one does not automatically date the other.<\/p><\/div><\/details>\n<details class=\"svp-detail\"><summary>2 &#183; Does a layer above another always have to be younger?<\/summary><div class=\"svp-answer\"><p>Use that rule for an intact sequence that has not been overturned. Folding, faulting and thrusting require the structure to be reconstructed first.<\/p><\/div><\/details>\n<details class=\"svp-detail\"><summary>3 &#183; Can two touching layers be separated by missing time?<\/summary><div class=\"svp-answer\"><p>Yes. An unconformity can mark erosion or an interval without deposition. Physical contact does not guarantee a continuous record.<\/p><\/div><\/details>\n<details class=\"svp-detail\"><summary>4 &#183; What extra evidence would strengthen the ancient-sea interpretation?<\/summary><div class=\"svp-answer\"><p>Identify fossils in context, examine the sediment, map the layer and compare published studies. Say which observation supports each step of the interpretation.<\/p><\/div><\/details>\n<div class=\"svp-note green\"><strong>Take one idea away:<\/strong> a landscape is the outcome of several histories. Learn to separate the formation of its materials from the processes that moved and exposed them.<\/div>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp-teachers\"><span class=\"svp-kicker\">For readers, students &amp; educators<\/span><h2>One chapter. A shared investigation.<\/h2>\n<p><strong>Suggested session: 35&#8211;45 minutes.<\/strong> Start with the mountain diagram and collect explanations. Read the Dolomites example, complete the layer activity in pairs, then work through the ash-bed challenge. Finish by distinguishing observation, interpretation and unresolved questions.<\/p>\n<p><strong>Exit ticket:<\/strong> write one sentence describing an observation, one explaining what it supports, and one naming a limitation. Assess the reasoning as well as the answer.<\/p>\n<p><strong>Materials:<\/strong> this page and paper. No sample collection is required. Use the linked institutional image records for a comparison with real examples.<\/p>\n<div class=\"svp-actions svp-js-only\" hidden><button class=\"svp-button\" type=\"button\" id=\"svp-open-answers\">Open all page answers<\/button><button class=\"svp-button\" type=\"button\" id=\"svp-close-answers\">Close all page answers<\/button><button class=\"svp-button\" type=\"button\" id=\"svp-print\">Print this page<\/button><button class=\"svp-button primary\" type=\"button\" id=\"svp-print-answers\">Print page with answers<\/button><\/div>\n<p class=\"svp-mini\">Printing uses a simplified layout. The reading copy includes answers currently open; &#8220;Print page with answers&#8221; temporarily opens every explanation in both chapters. Browser printing also works without JavaScript.<\/p>\n<\/section>\n<section class=\"svp-section\" id=\"svp-glossary\"><span class=\"svp-kicker\">Keep these words<\/span><h2>A small geological vocabulary.<\/h2><dl class=\"svp-glossary\">\n<div><dt>Stratum<\/dt><dd>A layer of rock; the plural is strata.<\/dd><\/div><div><dt>Fossil<\/dt><dd>A preserved remain or trace of past life.<\/dd><\/div><div><dt>Superposition<\/dt><dd>The relative ordering of deposits from older below to younger above in a sequence that has not been overturned.<\/dd><\/div><div><dt>Unconformity<\/dt><dd>A surface representing a gap in the geological record.<\/dd><\/div><div><dt>Uplift<\/dt><dd>Upward movement of rock or land relative to a reference level.<\/dd><\/div><div><dt>Numerical age<\/dt><dd>An age in years, associated with an event and an uncertainty.<\/dd><\/div><\/dl><\/section>\n\n<section class=\"svp-section\" id=\"svp-sources\"><span class=\"svp-kicker\">Follow the evidence<\/span><h2>Sources &amp; visual notes.<\/h2><p>The explanations use the institutional resources below. The outcrop and ash-bed exercises are invented teaching examples. The diagram is original and schematic; it is not geological survey data.<\/p><ol class=\"svp-sources\"><li id=\"svp-source-1\"><strong>UNESCO World Heritage Centre.<\/strong> <a href=\"https:\/\/whc.unesco.org\/en\/list\/1237\/\">The Dolomites<\/a><span class=\"svp-source-note\">Carbonate platforms, fossil records and the geological significance of the site.<\/span><span class=\"svp-source-url\">https:\/\/whc.unesco.org\/en\/list\/1237\/<\/span><\/li><li id=\"svp-source-2\"><strong>Parco Nazionale Dolomiti Bellunesi.<\/strong> <a href=\"https:\/\/www.dolomitipark.it\/en\/nature-and-history\/geology\/geological-history\/\">Geological history<\/a><span class=\"svp-source-note\">Marine sedimentary environments and later Alpine deformation. This chapter uses the qualitative sequence, not the older numerical period boundaries in the source.<\/span><span class=\"svp-source-url\">https:\/\/www.dolomitipark.it\/en\/nature-and-history\/geology\/geological-history\/<\/span><\/li><li id=\"svp-source-3\"><strong>National Park Service.<\/strong> <a href=\"https:\/\/www.nps.gov\/grca\/learn\/nature\/fossils.htm\">Fossils &#183; Grand Canyon<\/a><span class=\"svp-source-note\">Photographs of marine fossils and examples of body and trace fossils.<\/span><span class=\"svp-source-url\">https:\/\/www.nps.gov\/grca\/learn\/nature\/fossils.htm<\/span><\/li><li id=\"svp-source-4\"><strong>U.S. Geological Survey.<\/strong> <a href=\"https:\/\/www.usgs.gov\/geology-and-ecology-of-national-parks\/geology-grand-canyon-national-park\">Geology of Grand Canyon National Park<\/a><span class=\"svp-source-note\">Superposition, stratigraphy and the interpretation of a layered landscape.<\/span><span class=\"svp-source-url\">https:\/\/www.usgs.gov\/geology-and-ecology-of-national-parks\/geology-grand-canyon-national-park<\/span><\/li><li id=\"svp-source-5\"><strong>National Park Service.<\/strong> <a href=\"https:\/\/www.nps.gov\/articles\/000\/grcatime-missing-time.htm\">Missing Time at Grand Canyon<\/a><span class=\"svp-source-note\">Unconformities and the incomplete geological record.<\/span><span class=\"svp-source-url\">https:\/\/www.nps.gov\/articles\/000\/grcatime-missing-time.htm<\/span><\/li><li id=\"svp-source-6\"><strong>National Park Service.<\/strong> <a href=\"https:\/\/www.nps.gov\/articles\/000\/grcatime-timescale.htm\">Geologic Timescale, Geologic Dating Techniques, and Numeric Ages<\/a><span class=\"svp-source-note\">Relative ages, mineral dates, volcanic ash and uncertainty.<\/span><span class=\"svp-source-url\">https:\/\/www.nps.gov\/articles\/000\/grcatime-timescale.htm<\/span><\/li><li id=\"svp-source-7\"><strong>National Institute of Standards and Technology.<\/strong> <a href=\"https:\/\/www.nist.gov\/how-do-you-measure-it\/how-do-you-know-age-fossils-and-other-old-things\">How Do You Know the Age of Fossils and Other Old Things?<\/a><span class=\"svp-source-note\">Radiometric methods, the limits of carbon dating and the estimated age of Earth.<\/span><span class=\"svp-source-url\">https:\/\/www.nist.gov\/how-do-you-measure-it\/how-do-you-know-age-fossils-and-other-old-things<\/span><\/li><\/ol><p class=\"svp-mini\">Institutional photographs are available through their original pages; none are reproduced here. Stadia is an independent educational initiative.<\/p><\/section>\n\n<div class=\"svp-actions svp-chapter-next\"><a class=\"svp-button primary\" href=\"#svp-chapter-2\">Continue to Chapter 2: A planet takes shape &#8594;<\/a><\/div>\n<\/section>\n\n<section id=\"svp-chapter-2\" class=\"svp-chapter-two\" aria-labelledby=\"svp2-title\">\n<header class=\"svp-hero svp-hero-two\">\n <div><span class=\"svp-kicker\">Stadia Science &#183; Earth history &#183; Chapter 02<\/span>\n <p class=\"svp-series\">Our planet has a long history<\/p>\n <h2 id=\"svp2-title\" class=\"svp-chapter-title\">A planet<br><span>takes shape.<\/span><\/h2>\n <p class=\"svp-lead\">How did a growing rocky world acquire oceans and an atmosphere?<\/p>\n <p>Travel further back than the mountains. Follow the clues in meteorites, lunar samples and tiny surviving crystals.<\/p>\n <div class=\"svp-actions\"><a class=\"svp-button primary\" href=\"#svp2-building\">Begin Chapter 2 &#8595;<\/a><a class=\"svp-button ghost\" href=\"#svp2-time-lab\">Explore deep time<\/a><\/div><\/div>\n <figure class=\"svp-figure\"><svg class=\"svp-diagram\" viewBox=\"0 0 520 450\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" role=\"img\" aria-labelledby=\"svp2-diagram-title svp2-diagram-desc\">\n<title id=\"svp2-diagram-title\">Differentiation: separating a rocky body&#8217;s materials<\/title><desc id=\"svp2-diagram-desc\">Two schematic cross-sections. At the top, metal is dispersed through silicate material. Below, metal is concentrated in a central core surrounded by a silicate-rich region. This is a conceptual comparison, not a simulation or an image of early Earth.<\/desc>\n<rect width=\"520\" height=\"450\" fill=\"#18233a\"\/>\n<text x=\"28\" y=\"34\" fill=\"#c3dfdc\" font-family=\"Arial,sans-serif\" font-size=\"12\" letter-spacing=\"1\">A CHANGING INTERIOR<\/text>\n<circle cx=\"143\" cy=\"133\" r=\"73\" fill=\"#bb7551\" stroke=\"#f0cda0\" stroke-width=\"3\"\/>\n<g fill=\"#f7d191\"><circle cx=\"107\" cy=\"111\" r=\"9\"\/><circle cx=\"151\" cy=\"95\" r=\"12\"\/><circle cx=\"177\" cy=\"128\" r=\"8\"\/><circle cx=\"119\" cy=\"154\" r=\"12\"\/><circle cx=\"162\" cy=\"174\" r=\"9\"\/><circle cx=\"151\" cy=\"137\" r=\"6\"\/><circle cx=\"95\" cy=\"133\" r=\"5\"\/><\/g>\n<g fill=\"#fff\" font-family=\"Arial,sans-serif\" font-size=\"17\"><text x=\"250\" y=\"112\">Initially mixed<\/text><text x=\"250\" y=\"136\">Metal dispersed in<\/text><text x=\"250\" y=\"158\">silicate material<\/text><\/g>\n<path d=\"M143 218V244M135 236L143 245L151 236\" fill=\"none\" stroke=\"#b5e7da\" stroke-width=\"3\"\/>\n<circle cx=\"143\" cy=\"329\" r=\"73\" fill=\"#bb7551\" stroke=\"#f0cda0\" stroke-width=\"3\"\/><circle cx=\"143\" cy=\"329\" r=\"34\" fill=\"#f7d191\" stroke=\"#fff0c6\" stroke-width=\"2\"\/>\n<path d=\"M179 329H237M194 284H237\" fill=\"none\" stroke=\"#d5e3ea\" stroke-width=\"1.5\"\/>\n<g fill=\"#fff\" font-family=\"Arial,sans-serif\" font-size=\"17\"><text x=\"250\" y=\"290\">Silicate-rich region<\/text><text x=\"250\" y=\"335\">Metal-rich core<\/text><\/g>\n<text x=\"28\" y=\"432\" fill=\"#c6d3e2\" font-family=\"Arial,sans-serif\" font-size=\"12\">Conceptual cross-sections &#183; Not to scale<\/text><\/svg><figcaption>Material separation in a partly molten rocky body. A simplified teaching diagram; it does not show the present-day solid inner core or reproduce a measured Earth cross-section. See source <sup class=\"svp-cites\"><a href=\"#svp2-source-3\" aria-label=\"Chapter 2, source 3\">[3]<\/a><\/sup>.<\/figcaption><\/figure>\n<\/header>\n<div class=\"svp-quick\"><div><strong>Build a planet<\/strong><span>Separate growth, internal layering and surface change.<\/span><\/div><div><strong>Trace water and air<\/strong><span>Distinguish origins, reservoirs and later transformations.<\/span><\/div><div><strong>Read the clock<\/strong><span>Convert ages into elapsed time using an explicit model.<\/span><\/div><\/div>\n<nav class=\"svp-toc\" aria-label=\"Chapter 2 contents\"><strong>Inside Chapter 2<\/strong><div class=\"svp-toc-links\"><a href=\"#svp2-building\">1 &#183; Building Earth<\/a><a href=\"#svp2-moon\">2 &#183; The Moon<\/a><a href=\"#svp2-atmosphere\">3 &#183; Early air<\/a><a href=\"#svp2-water\">4 &#183; Water &amp; oceans<\/a><a href=\"#svp2-oxygen\">5 &#183; Oxygen arrives later<\/a><a href=\"#svp2-time-lab\">Deep-time lab<\/a><a href=\"#svp2-evidence\">Evidence workshop<\/a><a href=\"#svp2-check\">Questions &amp; answers<\/a><a href=\"#svp2-teachers\">For educators<\/a><a href=\"#svp2-sources\">Sources<\/a><\/div><\/nav>\n\n<section class=\"svp-section\" id=\"svp2-building\"><span class=\"svp-kicker\">01 &#183; Growth and separation<\/span><h3 class=\"svp-subtitle\">Earth did not arrive fully assembled.<\/h3>\n<p>Earth grew from material in the young Solar System. Small bodies combined into larger ones in a process called <strong>accretion<\/strong>. Gravity shaped that growth. The result was a rocky planet with a core, mantle and crust, rather than a uniform ball of material.<sup class=\"svp-cites\"><a href=\"#svp2-source-1\" aria-label=\"Chapter 2, source 1\">[1]<\/a><\/sup><\/p>\n<p>Its estimated age is about <strong>4.54 billion years<\/strong>. This figure comes from isotopic evidence, including meteorites; it is not the age of the oldest surviving mountain or a date obtained by simply counting sedimentary layers. Planet formation was a process, so this rounded age is a reference point rather than a precisely timed birthday.<sup class=\"svp-cites\"><a href=\"#svp2-source-2\" aria-label=\"Chapter 2, source 2\">[2]<\/a><\/sup><\/p>\n<p>When enough material melted, dense metal could move inward, leaving a more silicate-rich outer region. This separation is called <strong>differentiation<\/strong>. Meteorites from differentiated parent bodies preserve evidence of related processes. They are comparison samples from other bodies, not pieces of Earth&#8217;s core.<sup class=\"svp-cites\"><a href=\"#svp2-source-3\" aria-label=\"Chapter 2, source 3\">[3]<\/a><\/sup><\/p>\n<details class=\"svp-detail\"><summary>Think first: are accretion and differentiation the same process?<\/summary><div class=\"svp-answer\"><p>No. Accretion describes growth by adding material. Differentiation describes the separation of material within a body. They can overlap in time, but they answer different questions: how did it grow, and how did its interior become organised?<\/p><\/div><\/details>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp2-moon\"><span class=\"svp-kicker\">02 &#183; A companion with clues<\/span><h3 class=\"svp-subtitle\">The Moon preserves part of the story.<\/h3>\n<p>The leading explanation for the Moon&#8217;s origin involves a giant collision between the young Earth and another large body. Material displaced by the impact contributed to the Moon. Researchers compare lunar samples, physical models and the Earth&#8211;Moon system to test this idea.<sup class=\"svp-cites\"><a href=\"#svp2-source-4\" aria-label=\"Chapter 2, source 4\">[4]<\/a><\/sup><\/p>\n<p>The broad impact explanation is stronger than any one illustration of the event. The collision&#8217;s details and the path from debris to a Moon remain research questions. An animation is a model of what could have happened, not a recording of the past.<sup class=\"svp-cites\"><a href=\"#svp2-source-4\" aria-label=\"Chapter 2, source 4\">[4]<\/a><\/sup><\/p>\n<div class=\"svp-note\"><strong>A useful question for every reconstruction:<\/strong> which features are constrained by evidence, and which depend on the model&#8217;s starting assumptions?<\/div>\n<a class=\"svp-button\" href=\"https:\/\/science.nasa.gov\/moon\/formation\/\">Explore NASA&#8217;s Moon-formation explanation &#8594;<\/a>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp2-atmosphere\"><span class=\"svp-kicker\">03 &#183; An unfamiliar sky<\/span><h3 class=\"svp-subtitle\">An atmosphere is not automatically breathable.<\/h3>\n<p>The early Earth experienced intense impacts and hot conditions. Gases released from the interior helped build an atmosphere very different from the one around us today. Cooling and exchanges between the surface and the air changed conditions over time.<sup class=\"svp-cites\"><a href=\"#svp2-source-5\" aria-label=\"Chapter 2, source 5\">[5]<\/a><\/sup><\/p>\n<p>Water vapour in an atmosphere and liquid water at the surface are different states of the same substance. Cooling can allow condensation, but temperature, pressure and the surrounding gases matter. There is no single universal temperature that describes ocean formation under every possible early atmosphere.<sup class=\"svp-cites\"><a href=\"#svp2-source-6\" aria-label=\"Chapter 2, source 6\">[6]<\/a><\/sup><\/p>\n<p class=\"svp-question\">A world can have air and liquid water long before it has air that humans could breathe.<\/p>\n<p>The crucial distinction is between oxygen atoms bound in water or minerals and free oxygen gas, O<sub>2<\/sub>, in the atmosphere. These are not interchangeable measures of how much breathable oxygen a planet has.<\/p>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp2-water\"><span class=\"svp-kicker\">04 &#183; Two different water questions<\/span><h3 class=\"svp-subtitle\">Where did the water come from?<br>When could it remain liquid?<\/h3>\n<p>These questions are related, but they are not identical. Researchers consider water associated with Earth&#8217;s building materials, delivery by other bodies and exchange with the interior. Models can combine several contributions; a picture of raining comets is not a complete explanation.<sup class=\"svp-cites\"><a href=\"#svp2-source-7\" aria-label=\"Chapter 2, source 7\">[7]<\/a><\/sup><\/p>\n<p>One clue is the ratio of deuterium, a heavier hydrogen isotope, to ordinary hydrogen. Comparing this ratio in different reservoirs can test possible connections. Measurements of comet water also depend on what was sampled and how it was interpreted, so one matching ratio does not establish a unique source.<sup class=\"svp-cites\"><a href=\"#svp2-source-8\" aria-label=\"Chapter 2, source 8\">[8]<\/a><\/sup><\/p>\n<div class=\"svp-note\"><strong>Timing matters.<\/strong> A NASA-reported study of lunar soils placed limits on water delivered by meteorites since about four billion years ago. Its conclusion concerns late delivery; it does not rule out earlier contributions during Earth&#8217;s assembly.<sup class=\"svp-cites\"><a href=\"#svp2-source-9\" aria-label=\"Chapter 2, source 9\">[9]<\/a><\/sup><\/div>\n<h4 class=\"svp-small-title\">A tiny crystal, a very old clue<\/h4>\n<p>Some surviving zircons are about 4.4 billion years old. Their ages and oxygen-isotope signatures have been used to argue for early crust and interaction with liquid water. This is an inference from a mineral record. It does not give us a photograph of the first ocean, its shoreline or the exact date when oceans began.<sup class=\"svp-cites\"><a href=\"#svp2-source-10\" aria-label=\"Chapter 2, source 10\">[10]<\/a><\/sup><\/p>\n<details class=\"svp-detail\"><summary>Think first: does the oldest water-related evidence date the very first water?<\/summary><div class=\"svp-answer\"><p>No. It shows that relevant conditions existed by the time recorded by the evidence, under that interpretation. Earlier records may be missing. The oldest surviving clue is not necessarily the beginning of the process.<\/p><\/div><\/details>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp2-oxygen\"><span class=\"svp-kicker\">05 &#183; A later transformation<\/span><h3 class=\"svp-subtitle\">Life changed the air.<\/h3>\n<p>Photosynthetic microbes, including cyanobacteria, released oxygen. Oxygen also reacted with other materials, including dissolved iron, so production did not mean immediate accumulation everywhere. Rocks preserve traces of these chemical changes.<sup class=\"svp-cites\"><a href=\"#svp2-source-11\" aria-label=\"Chapter 2, source 11\">[11]<\/a><\/sup><\/p>\n<p>A major rise in atmospheric oxygen, the <strong>Great Oxidation Event<\/strong>, occurred around 2.4 billion years ago. This was much later than Earth&#8217;s formation. It was a transition, not an overnight switch to the modern atmosphere.<sup class=\"svp-cites\"><a href=\"#svp2-source-12\" aria-label=\"Chapter 2, source 12\">[12]<\/a><\/sup><\/p>\n<div class=\"svp-note green\"><strong>Keep the milestones separate:<\/strong> a planet, an ocean, life and an oxygen-rich atmosphere do not all appear at the same moment.<\/div>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp2-time-lab\"><span class=\"svp-kicker\">Mini-lab &#183; A scale model of time<\/span><h3 class=\"svp-subtitle\">&#8220;Long ago&#8221; needs a ruler.<\/h3>\n<p>For this exercise, use <strong>4,540 million years<\/strong> as the rounded age of Earth. Choose an age before the present and calculate how much time had elapsed since that reference point. The ruler spans the whole model, from formation on the left to today on the right.<\/p>\n<div class=\"svp-lab\">\n<div class=\"svp2-controls svp-interactive\" id=\"svp2-clock-controls\" hidden>\n <label for=\"svp2-age\">Age before present, in millions of years (Ma)<\/label>\n <div class=\"svp2-clock-inputs\"><input id=\"svp2-age\" type=\"number\" min=\"0\" max=\"4540\" step=\"1\" value=\"4400\" inputmode=\"numeric\" aria-describedby=\"svp2-clock-help svp2-clock-error\"><label class=\"svp-sr\" for=\"svp2-age-slider\">Choose age before present on the slider<\/label><input id=\"svp2-age-slider\" type=\"range\" min=\"0\" max=\"4540\" step=\"1\" value=\"4400\"><\/div>\n <p class=\"svp-mini\" id=\"svp2-clock-help\">Enter a whole number from 0 to 4,540. The examples are rounded reference points, not exact event dates.<\/p>\n <div class=\"svp-actions svp2-presets\"><button class=\"svp-button\" type=\"button\" data-svp2-age=\"4540\">Earth&#8217;s age &#183; 4,540 Ma<\/button><button class=\"svp-button\" type=\"button\" data-svp2-age=\"4400\">Ancient zircon &#183; 4,400 Ma<\/button><button class=\"svp-button\" type=\"button\" data-svp2-age=\"2400\">Oxygen transition &#183; 2,400 Ma<\/button><button class=\"svp-button\" type=\"button\" data-svp2-age=\"0\">Today &#183; 0 Ma<\/button><\/div>\n <p id=\"svp2-clock-error\" class=\"svp-local-status\" role=\"status\"><\/p>\n<\/div>\n<div class=\"svp-metrics\" id=\"svp2-clock-results\" aria-live=\"polite\" aria-atomic=\"true\"><div><small>Before the present<\/small><strong id=\"svp2-before\">4,400 Ma<\/strong><\/div><div><small>Elapsed since model formation<\/small><strong id=\"svp2-elapsed\">140 Ma<\/strong><\/div><div><small>Share of the model elapsed<\/small><strong id=\"svp2-percent\">3.1%<\/strong><\/div><\/div>\n<div class=\"svp2-ruler\" role=\"img\" id=\"svp2-ruler\" aria-label=\"Linear time ruler: the selected age is 3.1 percent of the way from Earth's formation to the present.\"><span class=\"svp2-ruler-fill\" id=\"svp2-ruler-fill\" style=\"width:3.0837%\"><\/span><span class=\"svp2-ruler-marker\" id=\"svp2-ruler-marker\" style=\"left:3.0837%\"><\/span><\/div>\n<div class=\"svp2-ruler-labels\"><span>Formation &#183; 4,540 Ma ago<\/span><span>Present &#183; 0 Ma ago<\/span><\/div>\n<p class=\"svp-formula\">Elapsed time = 4,540 &#8722; age before present<\/p>\n<p><strong>Predict, then explore:<\/strong> would a 4,400-million-year-old crystal sit near the beginning or the middle of this ruler? What changes when you select 2,400 million years ago?<\/p>\n<details class=\"svp-detail\"><summary>Worked example &#183; Keep the two clocks separate<\/summary><div class=\"svp-answer\"><p>For 4,400 Ma ago: 4,540 &#8722; 4,400 = <strong>140 million years<\/strong> after the model starts, or about <strong>3.1%<\/strong> of its total length. For 2,400 Ma ago: 4,540 &#8722; 2,400 = <strong>2,140 million years<\/strong>, about <strong>47.1%<\/strong>. A smaller &#8220;years ago&#8221; value means a later point on the ruler. These are calculations from rounded inputs, not new dating measurements.<\/p><\/div><\/details>\n<noscript><p class=\"svp-mini\">The default example and worked answers remain readable. Use the formula on paper to explore the other ages.<\/p><\/noscript>\n<p class=\"svp-mini\">The ruler converts time units only. It does not simulate cooling, impacts, ocean formation or oxygen concentrations. Geological uncertainties are not plotted. Age references: <sup class=\"svp-cites\"><a href=\"#svp2-source-2\" aria-label=\"Chapter 2, source 2\">[2]<\/a><\/sup> <sup class=\"svp-cites\"><a href=\"#svp2-source-10\" aria-label=\"Chapter 2, source 10\">[10]<\/a><\/sup> <sup class=\"svp-cites\"><a href=\"#svp2-source-12\" aria-label=\"Chapter 2, source 12\">[12]<\/a><\/sup><\/p>\n<\/div><\/section>\n\n<section class=\"svp-section\" id=\"svp2-evidence\"><span class=\"svp-kicker\">Evidence workshop &#183; On paper or in conversation<\/span><h3 class=\"svp-subtitle\">What was measured?<br>What was inferred?<\/h3>\n<p>Choose one row. Explain the link between its measurement and interpretation, then say what additional evidence you would want.<\/p>\n<div class=\"svp-table-wrap\"><table><caption class=\"svp-sr\">Examples of evidence, interpretation and limits in early Earth research<\/caption><thead><tr><th scope=\"col\">Evidence<\/th><th scope=\"col\">Interpretation to test<\/th><th scope=\"col\">A limit to remember<\/th><\/tr><\/thead><tbody>\n<tr><td>An old zircon&#8217;s age and isotope composition <sup class=\"svp-cites\"><a href=\"#svp2-source-10\" aria-label=\"Chapter 2, source 10\">[10]<\/a><\/sup><\/td><td>Early crust with a history involving surface water<\/td><td>One mineral does not map a whole ocean.<\/td><\/tr>\n<tr><td>A hydrogen-isotope ratio in comet water <sup class=\"svp-cites\"><a href=\"#svp2-source-8\" aria-label=\"Chapter 2, source 8\">[8]<\/a><\/sup><\/td><td>A possible connection to a water reservoir on Earth<\/td><td>A matching ratio does not give a unique delivery history.<\/td><\/tr>\n<tr><td>Changing oxidation signatures in ancient rocks <sup class=\"svp-cites\"><a href=\"#svp2-source-11\" aria-label=\"Chapter 2, source 11\">[11]<\/a><\/sup><\/td><td>A change in environmental oxygen conditions<\/td><td>A local record is not a complete global atmosphere measurement.<\/td><\/tr>\n<\/tbody><\/table><\/div>\n<details class=\"svp-detail\"><summary>Discussion guide &#183; How to strengthen an explanation<\/summary><div class=\"svp-answer\"><p>Ask for independent samples, reliable ages and agreement between different kinds of evidence. Separate what the instrument measured from the history proposed to explain it. A stronger explanation accounts for more observations without hiding conflicting results.<\/p><\/div><\/details>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp2-check\"><span class=\"svp-kicker\">Pause &amp; explain<\/span><h3 class=\"svp-subtitle\">Five questions to take with you.<\/h3>\n<details class=\"svp-detail\"><summary>1 &#183; Why is a meteorite relevant to the story of Earth?<\/summary><div class=\"svp-answer\"><p>It can preserve information about early Solar System materials or processes in other rocky bodies. Its relevance comes from the comparison and its context; it is not automatically a fragment of Earth.<\/p><\/div><\/details>\n<details class=\"svp-detail\"><summary>2 &#183; Does a giant-impact illustration show exactly what happened?<\/summary><div class=\"svp-answer\"><p>No. It visualises a reconstruction. Samples and physical constraints help evaluate the model, while important details remain open to investigation.<\/p><\/div><\/details>\n<details class=\"svp-detail\"><summary>3 &#183; Does condensation explain the original source of all water?<\/summary><div class=\"svp-answer\"><p>No. Condensation describes a change from vapour to liquid. The original source and delivery history of that water are additional questions.<\/p><\/div><\/details>\n<details class=\"svp-detail\"><summary>4 &#183; Why distinguish early water from later oxygen accumulation?<\/summary><div class=\"svp-answer\"><p>They mark different aspects of planetary history. An environment can contain liquid water while having very little free oxygen gas in the air.<\/p><\/div><\/details>\n<details class=\"svp-detail\"><summary>5 &#183; Does 140 million years after formation mean 140 million years ago?<\/summary><div class=\"svp-answer\"><p>No. With a model age of 4,540 Ma, 140 Ma after formation corresponds to 4,400 Ma before the present. Always state the reference point.<\/p><\/div><\/details>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp2-teachers\"><span class=\"svp-kicker\">For students &amp; educators<\/span><h3 class=\"svp-subtitle\">Build an explanation, not just a timeline.<\/h3>\n<p><strong>Suggested session: 40&#8211;50 minutes.<\/strong> Ask learners to draw their initial idea of a young Earth. Read the five sections, use the deep-time ruler, then annotate the drawing with three labels: evidence, interpretation and open question.<\/p>\n<p><strong>Pair task:<\/strong> one learner makes a claim from the evidence table; the other asks what observation would challenge it. Swap roles.<\/p>\n<p><strong>Exit ticket:<\/strong> explain why &#8220;Earth formed&#8221;, &#8220;liquid water existed&#8221; and &#8220;oxygen accumulated&#8221; need separate entries in a history of the planet.<\/p>\n<p class=\"svp-mini\">All activities are desk-based. The page works without submitting answers or creating an account. Scientific sources and worked examples remain available without JavaScript.<\/p>\n<div class=\"svp-actions svp-js-only\" hidden><button class=\"svp-button\" type=\"button\" data-svp-action=\"open\">Open all page answers<\/button><button class=\"svp-button\" type=\"button\" data-svp-action=\"close\">Close all page answers<\/button><button class=\"svp-button\" type=\"button\" data-svp-action=\"print\">Print this page<\/button><button class=\"svp-button primary\" type=\"button\" data-svp-action=\"print-answers\">Print page with answers<\/button><\/div>\n<p class=\"svp-mini\">Print controls apply to both chapters. Answers are included when open; &#8220;Print page with answers&#8221; opens them temporarily.<\/p>\n<\/section>\n\n<section class=\"svp-section\" id=\"svp2-glossary\"><span class=\"svp-kicker\">Keep these words<\/span><h3 class=\"svp-subtitle\">A vocabulary for a young planet.<\/h3><dl class=\"svp-glossary\">\n<div><dt>Accretion<\/dt><dd>Growth as material is added to a body.<\/dd><\/div><div><dt>Differentiation<\/dt><dd>Separation of a body&#8217;s materials into regions with different compositions.<\/dd><\/div><div><dt>Outgassing<\/dt><dd>Release of gases from a planet&#8217;s interior.<\/dd><\/div><div><dt>Condensation<\/dt><dd>A change from vapour to liquid.<\/dd><\/div><div><dt>Isotope<\/dt><dd>A form of an element with a particular number of neutrons.<\/dd><\/div><div><dt>Reservoir<\/dt><dd>A store of material, such as water in an ocean or within a planet.<\/dd><\/div><div><dt>Ma<\/dt><dd>Millions of years; in an age label, millions of years before the present.<\/dd><\/div><div><dt>Ga<\/dt><dd>Billions of years. One Ga equals 1,000 Ma.<\/dd><\/div><\/dl><\/section>\n\n<section class=\"svp-section\" id=\"svp2-sources\"><span class=\"svp-kicker\">Follow the evidence &#183; Chapter 02<\/span><h3 class=\"svp-subtitle\">Sources &amp; reading notes.<\/h3><p>These sources support different parts of the explanation. A research result is identified as such; it is not presented as a complete solution to every question about early Earth.<\/p><ol class=\"svp-sources\"><li id=\"svp2-source-1\"><strong>NASA Science.<\/strong> <a href=\"https:\/\/science.nasa.gov\/earth\/facts\/\">Facts About Earth<\/a><span class=\"svp-source-note\">Planetary formation and large-scale structure.<\/span><span class=\"svp-source-url\">https:\/\/science.nasa.gov\/earth\/facts\/<\/span><\/li>\n<li id=\"svp2-source-2\"><strong>NIST.<\/strong> <a href=\"https:\/\/www.nist.gov\/how-do-you-measure-it\/how-do-you-know-age-fossils-and-other-old-things\">How Do You Know the Age of Fossils and Other Old Things?<\/a><span class=\"svp-source-note\">Isotopic dating and the approximate 4.54-billion-year age of Earth.<\/span><span class=\"svp-source-url\">https:\/\/www.nist.gov\/how-do-you-measure-it\/how-do-you-know-age-fossils-and-other-old-things<\/span><\/li>\n<li id=\"svp2-source-3\"><strong>Natural History Museum, London.<\/strong> <a href=\"https:\/\/www.nhm.ac.uk\/discover\/types-of-meteorites.html\">Types of meteorites<\/a><span class=\"svp-source-note\">Differentiated parent bodies and the evidence preserved in meteorites.<\/span><span class=\"svp-source-url\">https:\/\/www.nhm.ac.uk\/discover\/types-of-meteorites.html<\/span><\/li>\n<li id=\"svp2-source-4\"><strong>NASA Science.<\/strong> <a href=\"https:\/\/science.nasa.gov\/moon\/formation\/\">Moon Formation<\/a><span class=\"svp-source-note\">Giant-impact models, lunar evidence and continuing questions.<\/span><span class=\"svp-source-url\">https:\/\/science.nasa.gov\/moon\/formation\/<\/span><\/li>\n<li id=\"svp2-source-5\"><strong>NASA Astrobiology.<\/strong> <a href=\"https:\/\/science.nasa.gov\/astrobiology\/learning-resources\/alp\/earth-right-after-it-formed\/\">What was the Earth like right after it formed?<\/a><span class=\"svp-source-note\">Introductory explanation of impacts, hot conditions and early atmospheric evolution.<\/span><span class=\"svp-source-url\">https:\/\/science.nasa.gov\/astrobiology\/learning-resources\/alp\/earth-right-after-it-formed\/<\/span><\/li>\n<li id=\"svp2-source-6\"><strong>NOAA.<\/strong> <a href=\"https:\/\/www.noaa.gov\/education\/resource-collections\/freshwater\/water-cycle\">The water cycle<\/a><span class=\"svp-source-note\">Water phases, condensation and exchanges between reservoirs. This is process background, not a date for the first ocean.<\/span><span class=\"svp-source-url\">https:\/\/www.noaa.gov\/education\/resource-collections\/freshwater\/water-cycle<\/span><\/li>\n<li id=\"svp2-source-7\"><strong>NASA Astrobiology.<\/strong> <a href=\"https:\/\/astrobiology.nasa.gov\/news\/the-origin-of-earths-water\/\">The Origin of Earth&#8217;s Water<\/a><span class=\"svp-source-note\">A model combining possible water contributions; an example of the research question, not a settled inventory.<\/span><span class=\"svp-source-url\">https:\/\/astrobiology.nasa.gov\/news\/the-origin-of-earths-water\/<\/span><\/li>\n<li id=\"svp2-source-8\"><strong>NASA Science.<\/strong> <a href=\"https:\/\/science.nasa.gov\/solar-system\/comets\/nasa-led-team-links-comet-water-to-earths-oceans\/\">NASA-Led Team Links Comet Water to Earth&#8217;s Oceans<\/a><span class=\"svp-source-note\">Deuterium-to-hydrogen ratios, comet measurements and interpretation.<\/span><span class=\"svp-source-url\">https:\/\/science.nasa.gov\/solar-system\/comets\/nasa-led-team-links-comet-water-to-earths-oceans\/<\/span><\/li>\n<li id=\"svp2-source-9\"><strong>NASA Science.<\/strong> <a href=\"https:\/\/science.nasa.gov\/science-research\/astromaterials\/nasa-finds-lunar-regolith-limits-meteorites-as-source-of-earths-water\/\">NASA Finds Lunar Regolith Limits Meteorites as Source of Earth&#8217;s Water<\/a><span class=\"svp-source-note\">Research reported in January 2026 on limits to late water delivery, since about four billion years ago.<\/span><span class=\"svp-source-url\">https:\/\/science.nasa.gov\/science-research\/astromaterials\/nasa-finds-lunar-regolith-limits-meteorites-as-source-of-earths-water\/<\/span><\/li>\n<li id=\"svp2-source-10\"><strong>University of Wisconsin&#8211;Madison.<\/strong> <a href=\"https:\/\/news.wisc.edu\/professors-study-of-ancient-crystals-sheds-light-on-earths-early-years\/\">Professor&#8217;s study of ancient crystals sheds light on earth&#8217;s early years<\/a><span class=\"svp-source-note\">Old zircons and interpretations of early crust and water. The oldest preserved evidence need not mark the start of the process.<\/span><span class=\"svp-source-url\">https:\/\/news.wisc.edu\/professors-study-of-ancient-crystals-sheds-light-on-earths-early-years\/<\/span><\/li>\n<li id=\"svp2-source-11\"><strong>Smithsonian National Museum of Natural History.<\/strong> <a href=\"https:\/\/naturalhistory.si.edu\/education\/teaching-resources\/life-science\/early-life-earth-animal-origins\">Early Life on Earth &#8211; Animal Origins<\/a><span class=\"svp-source-note\">Microbial photosynthesis, oxygen and iron-rich rock records.<\/span><span class=\"svp-source-url\">https:\/\/naturalhistory.si.edu\/education\/teaching-resources\/life-science\/early-life-earth-animal-origins<\/span><\/li>\n<li id=\"svp2-source-12\"><strong>Goldblatt, Lenton &amp; Watson &#183; Nature (2006).<\/strong> <a href=\"https:\/\/doi.org\/10.1038\/nature05169\">Bistability of atmospheric oxygen and the Great Oxidation<\/a><span class=\"svp-source-note\">Research context for the oxygen transition around 2.4 billion years ago. Full-text access may depend on the provider.<\/span><span class=\"svp-source-url\">https:\/\/doi.org\/10.1038\/nature05169<\/span><\/li>\n<\/ol><p class=\"svp-mini\">Visuals: original teaching diagrams and a numerical time ruler. No diagram is a photograph of early Earth. Sources consulted 30 September 2026.<\/p><\/section>\n<div class=\"svp-actions svp-chapter-next\"><a class=\"svp-button\" href=\"#svp-chapter-1\">Return to Chapter 1 &#8593;<\/a><a class=\"svp-button primary\" href=\"#svp-series\">See the series &#8594;<\/a><\/div>\n<\/section>\n<section class=\"svp-section\" id=\"svp-series\"><span class=\"svp-kicker\">Our planet has a long history<\/span><h2>The journey ahead.<\/h2><p>Two chapters are ready to explore. The remaining chapters are planned.<\/p><ol class=\"svp-roadmap\">\n<li><strong><a href=\"#svp-chapter-1\">How do we read Earth&#8217;s history?<\/a><\/strong><small>Read Chapter 1 &#183; When mountains were sea<\/small><\/li><li><strong><a href=\"#svp-chapter-2\">A planet takes shape<\/a><\/strong><small>Read Chapter 2 &#183; Earth, atmosphere and oceans<\/small><\/li><li><strong>Continents on the move<\/strong><small>Planned &#183; Changing oceans and landscapes<\/small><\/li><li><strong>Life leaves traces<\/strong><small>Planned &#183; Evolution, fossils and extinctions<\/small><\/li><li><strong>The Mediterranean tells a story<\/strong><small>Planned &#183; Reading places close to home<\/small><\/li><li><strong>We are recent arrivals<\/strong><small>Planned &#183; Humanity in deep time<\/small><\/li><\/ol><\/section><aside class=\"svp-share\"><span class=\"svp-kicker\">From space to Earth<\/span><h2>Different traces. The same question.<\/h2><p>In astronomy we ask what an instrument measured. In geology we ask what a rock preserves. Both journeys begin by connecting a claim to evidence.<\/p><div class=\"svp-actions\"><a class=\"svp-button primary\" href=\"https:\/\/stadiaorg.com\/astronomy-science\/\">Explore Astronomy &amp; Science &#8594;<\/a><a class=\"svp-button\" href=\"#svp-top\">Back to the chapter &#8593;<\/a><\/div><\/aside>\n<footer class=\"svp-footer\"><p><strong>Stadia Science &#183; Earth history &#183; Chapters 01&#8211;02 &#183; v2.0.<\/strong> Sources consulted 30 September 2026. Activities run in your browser and do not send or store your answers.<\/p><p class=\"svp-print-note\">Our planet has a long history &#183; Chapters 1 and 2. Institutional source addresses are listed above.<\/p><\/footer>\n<\/div>\n<script>\n(function(){\n 'use strict';\n function init(){\n  var root=document.getElementById('sv-planet-ch1');\n  if(!root||root.dataset.initialized){return;}root.dataset.initialized='true';\n  root.querySelectorAll('.svp-js-only').forEach(function(el){el.hidden=false;});\n  var form=root.querySelector('#svp-order-form'),status=root.querySelector('#svp-order-status');\n  if(form&&status){\n   form.hidden=false;\n   form.addEventListener('submit',function(event){\n    event.preventDefault();var values=[1,2,3].map(function(n){return root.querySelector('#svp-rank-'+n).value;});\n    if(values.some(function(v){return !v;})){status.textContent='Choose a layer for every position.';return;}\n    if(new Set(values).size!==3){status.textContent='Use each layer once: A, B and C.';return;}\n    status.textContent=values.join('')==='ABC'?'Correct: A, then B, then C. Now add the erosion event between B and C. 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