The question is misleading. There are not four main types of crust. There are two main types of crust: oceanic crust and continental crust. The continental crust then consists of different types of igneous, metamorphic, and sedimentary rocks.
Here's a breakdown:
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Oceanic Crust: This type of crust is primarily composed of mafic igneous rocks (rocks rich in magnesium and iron). It is thinner and denser than continental crust.
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Continental Crust: This type of crust is made up of a wide variety of igneous, metamorphic, and sedimentary rocks. Its average composition is granitic, which is less dense than the mafic rocks of oceanic crust. Because of the variety of rocks that make up continental crust, it isn't possible to define four main types.
It's possible the question intended to ask about the types of rocks found within the continental crust. If so, here's a clarification:
Types of Rocks Found in Continental Crust
While continental crust isn't categorized into four distinct types of crust, it is composed of three main types of rocks:
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Igneous Rocks: Formed from the cooling and solidification of magma or lava. Examples include granite and basalt (though basalt is more common in oceanic crust).
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Sedimentary Rocks: Formed from the accumulation and cementation of sediments, such as sand, silt, and clay. Examples include sandstone, shale, and limestone.
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Metamorphic Rocks: Formed when existing rocks are transformed by heat, pressure, or chemical reactions. Examples include gneiss, schist, and marble.
It's important to differentiate between types of crust and types of rocks found within the crust.
