Above Mohorovicic discontinuity is surface layer {crust}|.
Layer {Mohorovicic discontinuity} above mantle is thin.
Upper-mantle serpentine layer and lower-crust sial layer make layer {lithosphere}.
Lower crust is a three-mile thick heavy iron-and-magnesium-silicate basalt layer {sial layer}. Basalt forms from melted upper-mantle serpentine under lower pressure. Basalt crust density is 2.3 g/cm^3.
Upper crust has landmasses {continent, land}|. Continents now cover 25% of Earth surface. Continents average 20 miles thick and can be 40 miles thick. True continent edge is below ocean at continental-shelf edge, up to 400 miles from shore.
rocks
Continental rock is permanent, with no recycling back into crust or mantle. Continents are mostly granite, with density 2.1 g/cm^3 {sima layer}, so they rise above seas.
formation
First continent rocks appeared 4,000,000,000 years ago, as continents grew from upper mantle. After first continent-formation period ended 3,500,000,000 to 3,800,000,000 years ago, continents were 5% to 10% of crust. First-formation-period rocks are in Isua in southwest Greenland. These rocks have greenstone belts, granite-gneiss terrains, or igneous rocks cutting through them from upper mantle. Greenstone belts contain ultramafic rock and mafic rock, as xenolith.
Second continent-formation period, from 2,600,000,000 to 2,900,000,000 years ago, formed 50% to 60% of continental Archean rock.
Third continent-formation period was 1,700,000,000 to 1,900,000,000 years ago.
Fourth continent-formation period was 900,000,000 to 1,100,000,000 years ago.
Fifth continent-formation period was 600,000,000 years ago.
Igneous rock {mafic rock} can be mostly iron and magnesium. Mantle basalt, ocean-floor bedrock, and lava are mafic.
Greenstone belts contain volcanic rock {ultramafic rock} and partially melted mafic rock, which have no water.
Greenstone belts contain ultramafic and mafic rock, which have no water {xenolith}.
The second continent formation period, from 2,600,000,000 to 2,900,000,000 years ago, formed 50% to 60% of continents {Archean rock}.
Continent has independent masses {subcontinent} that have come together by plate movement.
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Date Modified: 2022.0225