HISTORICAL GEOLOGY PRECAMBRIAN GEOLOGY.

1. HISTORICAL GEOLOGY PRECAMBRIAN GEOLOGY. Introduction Precambrian time = 87% of Earth history and yet much less is known about Precambrian history than Phanerozoic history (13%), because there are few fossils and the rocks are highly deformed. Harry Williams, Historical Geology

2. Formation of the Solar System and Earth – Solar Nebula Hypothesis Clues: 1. All planets (pretty much) orbit the same direction and in the same plane around the sun. Harry Williams, Historical Geology

3. 2. The inner planets are dense and rocky; the outer planets are less dense and gaseous. Harry Williams, Historical Geology

4. The Solar Nebula Hypothesis is that the whole solar system formed from a cloud of rotating dust and gas (explains why all planets rotate the same way). As the cloud contracted under gravity, it flattened into a disk (explains the orbital plane). Most material collected at the center (proto-sun), the rest collected in masses orbiting the sun (proto-planets). Compression by gravity caused heating in the proto-sun until nuclear fusion started (hydrogen combined into helium). The resulting Solar Wind (atomic radiation) drove lighter gases from the inner planets, leaving them rocky and denser (explains contrast in composition between inner and outer planets). Harry Williams, Historical Geology

5. When did all this occur? Based on dating the moon, meteorites and earth rocks, the earth formed about 4.6 billion years ago. Are any other planets like the earth? No – only earth seems to have liquid water (and lots of it). Mars probably used to have liquid water (seas) a long time ago (about 3.5 billion), but it has been stripped away by the solar wind. Harry Williams, Historical Geology

6. THE ARCHEAN (4.6-2.5 BILLION) The earth is about 4.6 billion years old. Either during or soon after its formation it was very hot and differentiated into a core and mantle. This was during the Archean Eon, which lasted about 2.1 billion years. Harry Williams, Historical Geology

7. Harry Williams, Historical Geology

8. The crust had not formed at this time – the surface was probably molten ultramafic rock, an extension of the mantle. Harry Williams, Historical Geology

9. As the surface cooled (over millions of years) the atmosphere was produced by outgassing and was probably similar to volcanic gases, modified by chemical processes e.g. H2O rained out -> oceans; H2 lost to space; CO reacted with H2O to form CO2 and H2; CO2 combined with Ca and Mg to form carbonate rocks (late in this period). The atmosphere was rich in carbon dioxide and water vapor, but had little to no oxygen. Harry Williams, Historical Geology

10. komatiite The mantle is made of ultramafic rock – dense rock made of ferromagnesium minerals such as olivine and pyroxenes (Silicate of Mg and Fe; and Silicates of Al, Ca, Mg and Fe). An intrusive example is peridotite; the extrusive form is komatiite – these solidify at higher temperatures than basalt. It is likely that patches of komatiite crust formed during the Archean, but being denser, sunk and were “recycled”. Harry Williams, Historical Geology

11. The Archean Crust Important question – “how did the modern crust originate? (basalt oceanic crust (started forming about 4.5 billion years ago; granitic continental crust – started forming about 4.4 billion years ago). Basalt formed at a mid oceanic ridge due to partial melting of peridotite. Harry Williams, Historical Geology

12. The answer involves cooler temperatures, the development of mid-oceanic ridges, subduction zones and partial melting. As the earth cooled (around the beginning of the archean) the mantle magma would have produced more basaltic magma (forms at a lower temperature and by partial melting of peridotite). Modern Plate tectonics had begun, forming basaltic oceanic crust at mid-oceanic ridges. This resulted in cooler, larger, longer-lasting crustal rock slabs. These were subducted and also underwent partial melting – felsic minerals melt first and produce a more felsic magma and more felsic rocks (e.g. tonalite – a quartz-rich intrusive igneous rock) – recycling of these rocks made them more and more felsic until granitic rocks formed (still in the early Archean). When these early volcanic rocks grew above sea level (became land) they were eroded and formed the first sedimentary rock (greywackes, conglomerates) possibly as early as 4.4 billion years ago. Harry Williams, Historical Geology

13. To recap: basaltic oceanic crust formed first at mid-oceanic ridges and was probably wide-spread (all over the world); continental crust formed later as oceanic crust was subducted and partially-melted, releasing more and more felsic magma. This magma formed small patches of continental crust near subduction zones. When these patches grew above sea-level, they were eroded and produced the first sedimentary rocks. Harry Williams, Historical Geology

14. Greenstone Belts Evidence for these processes is found in GREENSTONE BELTS – which record increasingly felsic volcanic rocks, metamorphosed and topped by sedimentary rocks (also metamorphosed) (the metamorphic mineral chlorite is green – hence greenstone belts). The sedimentary rocks in the upper part of the sequence often contain conglomerates and greywackes, suggesting steep, rugged coastlines and the absence of shallow seas. All are intruded by later granites. The entire sequence records the changing conditions – cooling temperatures, more felsic rocks and the first appearance of sedimentary rocks. Harry Williams, Historical Geology

15. Harry Williams, Historical Geology

16. Continental crusts grew larger by much the same way as they do today: • Small crustal slabs collided and became sutured together • Continental growth by accretion occurred at subduction zones • Sedimentation accumulated around the coastlines Harry Williams, Historical Geology

17. Hence, the early contiental crust consisted of greenstone belts, accretionary sediment wedges and suture zones where two crustal slabs had welded together. Harry Williams, Historical Geology

18. The Proterozoic (2.5 - 0.54 billion). These processes continued into the Proterozoic, building the North American craton. So the craton is actually separate provinces, welded together - ages of the provinces get younger further out from the center. The Proterozoic is distinguished by more modern Tectonics and the first shallow marine rocks (formed on wide continental shelves). Harry Williams, Historical Geology

19. Therefore, it is probable that cratons developed in much the same way as more recent parts of continents i.e. • 1. orogenesis forming mountains, folded and metamorphosed rocks (at suture zones). • 2. continental growth by accretion (many cratons consist of smaller continental masses, or provinces, joined together). • The Proterozoic rocks, being younger, formed around the margins, and inbetween, the older archean cratons. • In contrast to the archean, prolonged sedimentation built wide continental shelves and coastal plains allowing sandstones and carbonates to form (the first appearance of shallow marine/lowland environments). Most proterozoic rocks have been subject to varying degrees of folding, metamorphism, orogenesis and erosion. Harry Williams, Historical Geology

20. The Wopmay Belt – example of a proterozoic orogenic belt. Harry Williams, Historical Geology

21. The shields (or cratons) of all major land masses have a similar history. Harry Williams, Historical Geology

22. In the late (Neo)proterozoic, nearly all land masses were brought together by plate movements to form a single continent – RODINIA, surrounded by an ocean MIROVIA. Rodinia included LAURENTIA – the Precambrian craton of North America – it was sutured to Rodinia by the Grenville Orogeny. Harry Williams, Historical Geology