Continental crust is made up of many different types of igneous, metamorphic, and sedimentary rocks. The average composition is granite, which is much less dense than the mafic rocks of the oceanic crust (Figure 1.3). Because it is thick and has relatively low density, continental crust rises higher on the mantle than oceanic crust, which sinks into the mantle to form basins. When filled with water, these basins form the planets oceans. Click image to the left or use the URL below. URL: A cross-section of oceanic crust.
Oceanic crust is composed of mafic magma that erupts on the seafloor to create basalt lava flows or cools deeper down to create the intrusive igneous rock gabbro (Figure 1.1). Gabbro from ocean crust. The gabbro is deformed because of intense faulting at the eruption site. Sediments, primarily mud and the shells of tiny sea creatures, coat the seafloor. Sediment is thickest near the shore, where it comes off the continents in rivers and on wind currents. The oceanic crust is relatively thin and lies above the mantle. The cross section of oceanic crust in the Figure 1.2 shows the layers that grade from sediments at the top to extrusive basalt lava, to the sheeted dikes that feed lava to the surface, to deeper intrusive gabbro, and finally to the mantle.
Earths outer surface is its crust, a cold, thin, brittle outer shell made of rock. The crust is very thin relative to the radius of the planet. There are two very different types of crust, each with its own distinctive physical and chemical properties, which are summarized in Table 1.1. Crust Oceanic Continental Thickness 5-12 km (3-8 mi) Avg. 35 km (22 mi) Density 3.0 g/cm3 2.7 g/cm3 Composition Mafic Felsic Rock types Basalt and gabbro All types
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relative to oceanic crust, continental crust is
a) thinner. b) denser. c) more mafic. --> d) made of a wider assortment of rocks.
earths crust is
a) pretty similar all the way around. --> b) very different between continental and oceanic crust types. c) made primarily of granite. d) all of the above
oceanic crust is
--> a) basalt lava on top of gabbro. b) a granitic layer with sediments on top. c) almost all formed from basalt lava. d) almost all formed from granitic rock.
the sediments that top the oceanic crust are primarily
a) sands blown from the land. b) fossils of fish and marine mammals. --> c) mud and tiny marine fossils. d) there are no sediments on the oceanic crust.
the oceanic crust is mostly made of magma that has the composition of this rock.
a) quartz b) granite --> c) basalt d) obsidian
because continental crust is thicker and less dense than oceanic crust, it sinks into the mantle to form basins.
a) true --> b) false
earth has continents and oceans because
a) continental crust sinks below oceanic crust. --> b) oceanic crust sinks below continental crust. c) oceanic crust rises above continental crust. d) none of these.
continental crust is made up of
a) igneous rock b) metamorphic rock c) sedimentary rock --> d) all of the above
ocean basins form because the upper mantle can flow and the dense oceanic crust sinks into it.
--> a) true b) false
crust is thick relative to earths radius.
a) true --> b) false
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