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Continent

Primeval Continents

Over four billion years ago, before the first continents developed, the mantle was much hotter than today. Accordingly, the rate of plate tectonics was much faster. This meant that plates were smaller, thinner and much warmer when subducted. The associated crust was nearer to its melting point and so partially melted along with the mantle. Early subduction zone volcanism therefore produced magmas with higher silica content, since partially melting basalt produces an intermediate magma composition. The early Earth consequently developed volcanic island arcs with relatively low density, which resisted subduction. These formed Earth's earliest micro-continents and built the first cratons when sutured together at subduction zones. Throughout the 1990s, research focused on early continent development and the questions of when continents first appeared, how they originated and at what rate growth occurred.


Resources

Books

Trefil, James. Meditations At 10,000 Feet: A Scientist In The Mountains. New York: Macmillan Publishing Company. 1986.

Vogel, Shawna. Naked Earth: The New Geophysics. New York: Penguin Books. 1995.

Periodicals

Taylor, S. Ross, and Scott McLennan. "The Evolution of Continental Crust." Scientific American (January 1996).

Weiss, Peter. "Land Before Time." Earth (February 1998).


Clay Harris

KEY TERMS

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Basalt

—A dense, dark colored igneous rock, with a composition rich in iron and magnesium (a mafic composition).

Continental shelf

—A relatively shallow, gently sloping, submarine area at the edges of continents and large islands, extending from the shoreline to the continental slope.

Felsic

—A term applied to light-colored igneous rocks, such as rhyolite, that are rich in silica. Felsic rocks are rich in the minerals feldspar and quartz.

Granite

—A felsic igneous rock that composes the bulk of the upper continental crust.

Mafic

—Pertaining to igneous rocks composed of silicate minerals with low amounts of silicon and abundant iron and magnesium.

Peridotite

—An ultramafic rock that composes the bulk of the mantle.

Silicate

—Any mineral with a crystal structure consisting of silicon and oxygen, either with or without other elements.

Specific gravity

—The weight of a substance relative to the weight of an equivalent volume of water; for example, basalt weighs 2.9 times as much as water, so basalt has a specific gravity of 2.9.

Ultramafic

—Pertaining to igneous rocks composed of silicate minerals with very low amounts of silicon and abundant iron and magnesium.

Additional topics

Science EncyclopediaScience & Philosophy: Condensation to CoshContinent - Crusts Compared, Continental Margins, Crustal Origins, Growing Pains, Primeval Continents - Structure of a continent