Crust describes the outermost shell of a terrestrial planet. Earth's crust is relatively thin, just 40-kilometers (25-miles) deep. Earth's crust can be divided into two types: continental and oceanic. Compare/contrast the two types of crust.

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primary succession occurs in essentially lifeless areas regions in which the soil is incapable of sustaining life as a result of such factors as lava flows, newly formed sand dunes, or rocks left from a retreating glacier. secondary succession occurs