{"id":3603,"date":"2014-05-06T19:39:46","date_gmt":"2014-05-06T19:39:46","guid":{"rendered":"http:\/\/all-geo.org\/metageologist\/?p=3603"},"modified":"2014-05-06T19:39:46","modified_gmt":"2014-05-06T19:39:46","slug":"a-world-without-subduction","status":"publish","type":"post","link":"https:\/\/all-geo.org\/metageologist\/2014\/05\/a-world-without-subduction\/","title":{"rendered":"A world without subduction"},"content":{"rendered":"

The greatest achievement of the generation of Earth Scientists now retiring is the concept of plate tectonics. The insight that the earth’s surface is made up of rigid plates that move has shed light on all aspects of Earth Science, from palaeontology to geophysics to the study of ancient climates. What’s less well known is that the way\u00a0the plates interact has changed over time. Key plate tectonic features such as subduction, didn’t happen for large periods\u00a0of earth history<\/em>.<\/p>\n

\"Cut-away<\/a>

Cut-away diagram showing modern convection from computer modelling. White is hot rising plumes, black cold sinking plates .\u00a0 Image used with permission of Fabio Crameri<\/a>.<\/p><\/div>\n

Earth scientists have\u00a0a pretty good idea of the details of how modern plate tectonics works. This has required the integration of\u00a0indirect observation of modern subduction zones (using geophysical techniques) with direct study of rocks that have been inside subduction zones (such as eclogites) plus the creation of subduction zones ‘in silico’ (with computer modelling).<\/p>\n

Of these 3 methods of study, only the first (direct observation) cannot be used on the ancient earth. So what do the rocks and computer models tell us?<\/p>\n

Old rocks are odd<\/h2>\n

We’ve known for a while that ancient rocks (Eoarchean\u2013Mesoarchean, older than 2.5 Ga1<\/a>)\u00a0<\/span>are very different from modern ones.\u00a0Often\u00a0they consist of greenstone belts<\/em> –\u00a0containing\u00a0an unusual lava called komatiite – surrounded by large areas of granitic gneiss. The pattern of metamorphism in these rocks shows high temperatures, even at shallow depths.<\/p>\n

The chemistry of the igneous rocks tells a similar tale. Komatiites only melt at temperatures of around 1600\u00b0C – 400 degrees hotter than modern basalt lava. Granitic rocks have tonalite\u2013trondhjemite\u2013granodiorite compositions and are thought to have formed from direct melting of basaltic rock – unlike granites formed above subduction zones today.<\/span><\/span><\/p>\n

Rocks characteristic of modern subduction\u00a0– blueschists and eclogites \u00a0– are not found in rocks this age. There is a pretty good consensus, based on field evidence and model modelling, that subduction\u00a0did not happen in the early earth<\/em>. The earth’s mantle was much hotter and more heat was flowing up through the crust. Hot rocks are weak rocks –\u00a0forcing a slab of rock into the deep mantle requires it to be cold and hard. Hotter rocks act not as rigid slabs but as soft blobs.<\/p>\n

Computer modelling confirms the importance of temperature, both of the crust and the underlying mantle. Models are our best hope of understanding what a hot planet without subduction looked like. More like a bubbling pan of porridge perhaps, with tectonics dominated by hot upwelling plumes and lithospheric delamination, with\u00a0blobs dripping-off down again. Some\u00a0studies of mantle mixing suggest a ‘stagnant-lid’ model where the earth’s surface layer doesn’t move at all.<\/p>\n

Subduction starts<\/h2>\n

At some point in time between\u00a03.2\u20132.5 Ga, subduction started. The planet\u00a0had cooled enough that a lithospheric plate stayed rigid enough to sink down into the mantle. Evidence for this is found in ‘paired metamorphic belts’. Rocks within the subduction zone remain cool at depth (as they are pushed down before they can get as hot as the surrounding rocks) and form eclogites or high-pressure granulite rocks. Rocks nearby in the overriding plate are much hotter and enjoyed granulite\u2013ultrahigh temperature metamorphism<\/span>.<\/p>\n

Mathematical modelling of the earth\u00a0suggests subduction started because the earth cooled below a particular threshold. As an explanation, this is a little dull. Much more excitingly,\u00a0coverage of a\u00a0recent paper<\/a> suggests massive meteorite impacts<\/strong> about 3.2 Ga could have broken up the surface and somehow kickstarted plate tectonics. Scientists\u00a0who study impacts are always really keen to use them to explain events or features\u00a0on earth, whereas other scientists are sceptical, preferring to explain them via things that they<\/em>\u00a0study. We’ll need to wait to see who is right about this one (but my money is on the dull explanation).<\/p>\n

\"Cut-away<\/a>

Cut-away diagram showing modern convection from computer modelling. Red is \u00a0hot rising plumes, blue cold sinking plates. Image used with permission of Fabio Crameri<\/a>.<\/p><\/div>\n

Subduction as a cure for boredom<\/h2>\n

When subduction first started, mantle temperatures were still\u00a0175\u2013250 \u00b0C hotter than today. Hotter, softer slabs are more likely to\u00a0break off, perhaps making subduction something that stopped and started.<\/p>\n

Blueschists and low-temperature eclogites, high-pressure & low-temperature rocks that are found in modern subduction zones are not found until the the Neoproterozoic at 600\u2013800 Ma. Mantle temperatures by then were less than\u00a0100\u00a0\u00b0C greater than today – this marks the w<\/span>ide spread development of modern-style (cold) subduction\u00a0on Earth.\u00a0Cold\u00a0slabs of oceanic lithosphere break-off deep, allowing large volumes of dense oceanic crust to pull continental lithosphere down, creating the first ultra-high pressure metamorphic complexes.<\/p>\n

The Neoproterozoic is the end of what is known as the ‘boring billion’ – a time of tedious environmental and evolutionary stability. A recent open acess paper in Geology<\/a>\u00a0suggests a link between the exciting changes that followed (glaciations! Cambrian explosion!) and the onset of subduction. The boring billion was stable in part because most continental crust was part of a supercontinent called Rodinia. The paper argues that the disruptive effects of the onset of cold subduction broke Rodinia apart, setting off a chain of events that transformed the world.<\/p>\n

—–<\/p>\n

The early earth was a very different planet. Understanding it better informs the general subject of planetology. As we get more and more data about other planets (both within and beyond our solar system) it’s natural to speculate on their tectonic activity. Why does Venus not have subduction? Does subduction here exist because of life and its role in moderating climate and creating the earth’s oceans? Ancient rocks and computer models may help us answer these questions as much as probes and telescopes.<\/p>\n

REFERENCES<\/h2>\n

Brown M. (2014). The contribution of metamorphic petrology to understanding lithosphere evolution and geodynamics, Geoscience Frontiers, <\/span> DOI: 10.1016\/j.gsf.2014.02.005<\/a><\/span>
\nAvailable here<\/a><\/p>\n

Cawood P.A. & Hawkesworth C.J. Earth’s middle age, Geology, <\/span> DOI: 10.1130\/G35402.1<\/a><\/span>
\nAvailable here<\/a><\/p>\n

Gerya T. (2014). Precambrian geodynamics: Concepts and models, Gondwana Research, 25<\/span> (2) 442-463. DOI: 10.1016\/j.gr.2012.11.008<\/a><\/span>
\nAvailable here<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

The greatest achievement of the generation of Earth Scientists now retiring is the concept of plate tectonics. The insight that the earth’s surface is made up of rigid plates that move has shed light on all aspects of Earth Science, … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[27,18,6,28,21],"tags":[],"class_list":["post-3603","post","type-post","status-publish","format-standard","hentry","category-eclogites","category-impacts","category-metamorphism","category-subduction","category-tectonics"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/all-geo.org\/metageologist\/wp-json\/wp\/v2\/posts\/3603","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/all-geo.org\/metageologist\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/all-geo.org\/metageologist\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/all-geo.org\/metageologist\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/all-geo.org\/metageologist\/wp-json\/wp\/v2\/comments?post=3603"}],"version-history":[{"count":12,"href":"https:\/\/all-geo.org\/metageologist\/wp-json\/wp\/v2\/posts\/3603\/revisions"}],"predecessor-version":[{"id":3665,"href":"https:\/\/all-geo.org\/metageologist\/wp-json\/wp\/v2\/posts\/3603\/revisions\/3665"}],"wp:attachment":[{"href":"https:\/\/all-geo.org\/metageologist\/wp-json\/wp\/v2\/media?parent=3603"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/all-geo.org\/metageologist\/wp-json\/wp\/v2\/categories?post=3603"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/all-geo.org\/metageologist\/wp-json\/wp\/v2\/tags?post=3603"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}