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ssss1 Characteristics of oceanic and continental crust: a comparison.

Properties Oceanic crust Continental crust Composition Dark colored, mafic rocks enriched in MgO, FeO, and CaO Complex; many lighter colored felsic rocks 222 Averages ~50% SiO2 Averages ~60% SiO2 Density Higher; less buoyant Lower; more buoyant Average 2.9–3.1 g/cm³ Average 2.6–2.9 g/cm³ Thickness Thinner; average 5–7 km thickness Thicker; average 30 km thickness Up to 15 km under islands Up to 80 km under mountains Elevation Low surface elevation; mostly submerged below sea level Higher surface elevations; mostly emergent above sea level Age Up to 190 Ma for in‐place crust Up to more than 4000 Ma ~3.5% of Earth history 85–90% of Earth history

Continental crust222

Whereas modern oceans, with the exception of a small area in the Mediterranean Sea, are underlain by oceanic crust younger than 190 Ma, the oldest well‐documented continental crust includes 4.03 Ga rocks from the Northwest Territories of Canada (Stern and Bleeker 1998). Approximately 4 Ga rocks also occur in Greenland and Australia. Greenstone belts (ssss1) may date back as far as 4.28 Ga (O'Neill et al. 2008) which suggests that continental crust began forming within 300 million years of Earth's birth. Individual detrital zircon grains, derived from the erosion of older continental crust, occur in metamorphosed sedimentary rocks in Australia. These zircons have been dated at 4.4 Ga (Wilde et al. 2001) an age recently confirmed by Valley et al. (2014). These data suggest that continental crust may have existed no more than 150 Ma after Earth formed. The great age of some continental crust results from its relative buoyancy. In contrast to ocean crust, continental crust is largely preserved as its density is generally too low for it to be subducted on as large a scale. ssss1 summarizes the major differences between oceanic and continental crust.