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The continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called ''sial'' because its bulk composition is more felsic, or granitic, in contrast to the oceanic crust, called ''sima'', whose bulk composition is more mafic or basaltic rock. (Based on the change in velocity of seismic waves, it is thought that at a certain depth sial becomes to sima close in its physical properties. This line is called the Conrad discontinuity.) Consisting mostly of granitic rock, continental crust has a density of about 2.7 g/cm3 and is less dense than the material of Earth's mantle, which consists of ultramafic rock and whose density is about 3.3 g/cm3. Continental crust is also less dense than oceanic crust, whose density is about 2.9 g/cm3. Continental crust is considerably thicker than oceanic crust, mostly 25 to 70 km versus the average thickness of the oceanic crust of around 7–10 km. About 40% of Earth's surface is now overlaid by continental crust. Continental crust makes up about 70% of the volume of Earth's crust. ==Importance== Because the surface of continental crust mainly lies above sea level, its existence allowed land life to evolve from marine life. Its existence also provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time, in what is now called the Cambrian explosion. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Continental crust」の詳細全文を読む スポンサード リンク
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