Earth History

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Big Bang Theory Noun

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[big bang thee-uh-ree]

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Big Bang Theory

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The scientific theory explaining the universe's origin from an initial point of extremely high density and temperature about 13.8 billion years ago.

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Example: This diagram shows that the universe started from a single point in an event called the Big Bang and has been expanding outward ever since.

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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 geological principle that natural laws and processes operating today have always operated in the past, guiding geological study.

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Example: This diagram shows the geological cycle, where processes like the decay of continents and the formation of new rock happen over and over. Uniformitarianism is the idea that these same slow processes have always been shaping the Earth.

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Relative Dating Noun

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[rel-uh-tiv dey-ting]

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Relative Dating

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The method of determining the age of a rock or fossil by comparing its placement with that of fossils in other layers of rock.

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Example: This cross-section shows rock layers and features. By seeing which layers are on the bottom and which features cut across others, we can determine their relative ages.

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Law of Superposition Noun

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[law ov soo-per-puh-zish-uhn]

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Law of Superposition

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The geological principle that in an undeformed sequence of sedimentary rocks, each layer is older than the one above it.

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Example: This diagram shows rock layers where the oldest layers are at the bottom and the youngest layers are at the top, demonstrating the Law of Superposition.

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Principle of Cross-Cutting Relationships Noun

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[prin-suh-puhl ov kros-kuht-ing ri-ley-shuhn-ships]

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Principle of Cross-Cutting Relationships

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The geological principle stating that a fault or igneous intrusion is younger than the rock layers it cuts through.

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Example: This diagram shows a fault (E) and an igneous intrusion (D) cutting through older rock layers (A, B, C), illustrating they are younger.

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Unconformity Noun

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[uhn-kuhn-fawr-mi-tee]

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Unconformity

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A buried surface of erosion or non-deposition that represents a significant gap of time in the geologic record.

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Example: This diagram shows an unconformity, a boundary where older, tilted rock layers were eroded before younger, horizontal layers were deposited on top.

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Inclusions Noun

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[in-kloo-zhuhns]

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Inclusions

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Pieces of one rock unit that are contained within another, where the inclusions are older than the rock that contains them.

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Example: This diagram shows that rock fragments, called inclusions, are older than the rock layer they are found inside, helping geologists determine relative rock ages.

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Absolute Dating Noun

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[ab-suh-loot dey-ting]

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Absolute Dating

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The method of determining the actual numerical age of a rock or event, often by using the decay of radioactive elements.

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Example: This graph shows radioactive decay. After each 'half-life' on the x-axis, the amount of a substance decreases by half, a predictable rate used to find the absolute age of rocks.

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Radioactive Decay Noun

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[rey-dee-oh-ak-tiv dih-key]

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Radioactive Decay

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The process by which an unstable atomic nucleus loses energy by radiation, serving as a 'clock' for dating geological materials.

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Example: A large, unstable radioactive atom releases smaller particles and energy (radiation) to become more stable. This process is called radioactive decay.

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Half-life Noun

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[haf-lahyf]

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Half-life

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The time required for half of the radioactive parent atoms in a sample to decay into stable daughter atoms.

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Example: This graph shows that after each half-life passes, the amount of a radioactive substance (like Co-60) decreases by half, from 10g to 5g, then 2.5g.