Earth History

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Uniformitarianism Noun

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[yoo-nuh-fawr-mi-tair-ee-uh-niz-uhm]

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Uniformitarianism

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The principle that geologic processes observed today are the same as those that have always been at work.

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Example: The river seen today deposits sediment, forming new land. This same slow process, happening over millions of years, created the rock layers seen underground.

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Principle of Original Horizontality Noun

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[prin-suh-puhl uv uh-rij-uh-nl hawr-uh-zon-tal-i-tee]

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Principle of Original Horizontality

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The principle stating that layers of sediment are originally deposited flat and horizontally under the action of gravity.

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Example: This image shows how layers of sediment, like soil and rock, are originally deposited in flat, horizontal layers due to gravity.

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Principle of Superposition Noun

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[prin-suh-puhl uv soo-per-puh-zish-uhn]

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Principle of Superposition

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The principle that in an undisturbed sequence of rock layers, lower layers are older than the upper layers.

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Example: This diagram shows layers of soil and rock stacked on top of each other, where the deepest layer (bedrock) is the oldest.

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Lithosphere Noun

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[lith-uh-sfeer]

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Lithosphere

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The rigid outer part of the Earth, consisting of the crust and solid upper mantle, broken into tectonic plates.

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Example: This diagram shows the Lithosphere as the solid, rocky outer layer of the Earth, which includes the crust and the upper part of the mantle.

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Asthenosphere Noun

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[as-then-uh-sfeer]

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Asthenosphere

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The soft, fluid layer of the upper mantle beneath the lithosphere on which the tectonic plates move.

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Example: This diagram shows the Asthenosphere as the semi-solid layer of the upper mantle located directly beneath the rigid lithosphere, allowing tectonic plates to move.

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Plate Tectonics Noun

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[pleyt tek-ton-iks]

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Plate Tectonics

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The scientific theory that Earth's outer shell is divided into several large, moving plates that glide over the mantle.

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Example: This diagram shows an example of plate tectonics where two of Earth's plates collide, causing one to slide under the other, forming a volcano.

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Pangaea Noun

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[pan-jee-uh]

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Pangaea

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The name given to the supercontinent that existed millions of years ago when all continents were one landmass.

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Example: This map shows the supercontinent Pangaea, with labels indicating how today's continents like North America and Africa were once joined together in one landmass.

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Seafloor Spreading Noun

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[see-flawr spred-ing]

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Seafloor Spreading

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The process by which new oceanic crust forms at mid-ocean ridges and moves sideways away from the ridge.

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Example: This diagram shows how magma rises at a mid-ocean ridge, cools to form new oceanic crust, and pushes the older crust away.

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Subduction Noun

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[sub-duhk-shuhn]

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Subduction

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The process where one tectonic plate slides under another and sinks into the mantle at a convergent boundary.

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Example: This diagram shows subduction, where a dense oceanic crust plate slides underneath a lighter continental crust plate, forming a deep ocean trench at the boundary.

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Weathering Noun

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[weth-er-ing]

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Weathering

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The process of breaking down rocks, soil, and minerals into smaller pieces through contact with Earth's atmosphere and water.

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Example: This image shows biological weathering, where a tree's roots grow into a crack and physically break the rock apart over time.