GEOL 120: CHAPTER 9
22 Cards in this Set
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Uniformitarianism
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The present is the key to the past (physical processes we observe operating today also operated in the past, at roughly comparable rates)
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Original Horizontality
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At the time of deposition, layers of sediment are fairly horizontal.
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Superposition
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The layer at the bottom of a sequence is the oldest, the layer at the top is youngest.
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Original Continuity
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If today you find a sedimentary layer cut by a canyon, then you can assume that the layer once spanned the canyon but was later eroded by the river that formed the canyon.
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Cross-cutting Relations
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If one geologic feature cuts across another, the feature that has been cut is older.
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Inclusion
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A rock fragment within another.
Igneous xenoliths – Country rock that fell into magma. Weathering rubble – Debris from pre-existing rocks.
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Baked Contacts
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Metamorphism occurs when rock is invaded by an igneous intrusion.
The baked rock must have been there first - it is older.
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Fossil Succession
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Fossil species occur in a predictable order.
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Unconformity
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A buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous.
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Angular Unconformity
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An angular difference between originally horizontal strata.
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Nonconformity
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Metamorphic or igneous rocks overlain by sedimentary strata.
Crystalline igneous /met rocks were exposed by erosion. Sediment was deposited on this eroded surface.
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Disconformity
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Parallel strata bracketing nondeposition.
Due to an interruption in sedimentation. May be difficult to recognize.
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Geologic column
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Constructed from incomplete sections across the globe.
It brackets almost the entirety of Earth history.
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Time scale subdivisions
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Named based on:
The nature of life. A characteristic of the time period. A specific locality.
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Numerical Age
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Based on radioactive decay of atoms in minerals.
Radioactive decay proceeds at a known, fixed rate. Radioactive elements act as internal clocks.
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Phanerozoic
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Eon - “Visible life” (542 Ma to the present).
Started 542 Ma at the Precambrian / Cambrian boundary. Marks the 1st appearance of hard shells. Life diversified rapidly afterwards.
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Proterozoic
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Eon - “Before life” (2.5 to 0.542 Ga).
Development of tectonic plates like those of today. Buildup of atmospheric O2; multicellular life appears.
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Archean
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Eon - “Ancient” (3.8 to 2.5 Ga).
Birth of continents. Appearance of the earliest life forms.
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Hadeon
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Eon - “Hot as Hell!!” (4.6 to 3.8 Ga).
Internal differentiation. Formation of the oceans and secondary atmosphere.
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Cenozoic
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Era - “Recent life.”
65.5 Ma to present. The “Age of Mammals.”
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Mesozoic
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Era - “Middle Life.”
251 to 65.5 Ma. The “Age of Dinosaurs.”
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Paleozoic
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Era - “Ancient Life.”
542 to 251 Ma. Life diversified rapidly.
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GEOL 120: CHAPTER 9