The Earths crust is the outermost layer of our planet and is less than 1% of the Earth's volume. The crust and the mantle contain different kinds of rocks making them chemically different. The upper mantle rocks are primarily peridotite. The crust contains of a variety of rocks. Igneous rocks, sedimentary rocks, and metamorphic rocks are the main categories of rocks found on the Earth's surface.
Continental and oceanic crust, USGS
What is the Moho boundary
The speed of earthquake waves led to the discovery of the boundary between the Earths crust and the mantle. The Mohorovicic Discontinuity (MOHO) is the boundary between these two layers.
Depth of Moho boundary
The Moho boundary was named for the Croatian seismologist who discovered it. It is called a discontinuity because the Earths crust varies in depth. The boundary is very shallow beneath oceanic crust and much deeper beneath great mountain ranges like the Himalayas
The oceanic crust is between five and ten kilometers thick. Separating plates form a low pressure area where molten rock collects in magma chambers. As two oceanic crusts separate basalt flows out onto the ocean floor creating new oceanic crust. Basalt, diabase and gabbro are the primary rocks found in the oceanic crust. These rocks are iron rich, dense and heavier than continental crust rocks.
When a continental crust and an oceanic crust meet they form a subduction zone. The heavier oceanic crust is forced beneath the lighter continental crust where it subducts into the lithosphere and melts. The oceanic crust is much younger than continental crust that is not destroyed when it collides with another tectonic plate.
Continental crust rocks
The continental crust contains a wide variety of rocks. Two thousand different minerals have been identified in the crust. Only twenty of these minerals are common. Just ten of the two thousand minerals in the crust combine together in rocks to make up 90% of the crust.
Ten silicate minerals
Ten minerals that make up 90% of the crust are all silicates because all contain silicon and oxygen. Quartz is pure silicon and oxygen. The rest of the silicate minerals include additional elements in the minerals structure.
The silicate minerals that make up the continents are lighter than the iron-rich minerals that form in oceanic crust which are derived from the upper mantle.
Both the oceanic crust and the continental crust "float" on the asthenosphere that lies beneath the lithosphere. Continental crusts "floats" on the asthenosphere similar to an ice cube floating in water. When they collide with another continental crust they smash together forming great mountain chains instead of being recycled. The continents rise above and sink below the oceanic crust.
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