ESCI_1012_06

ESCI_1012_06 - Time Scales of Hazards and their Associated...

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ESCI 1012: Natural Hazards & Disasters The Nature of Geologic Time Relative Time vs. Absolute Time Time Scales of Hazards and their Associated Processes Murck et al., 1997 Volcano lifespan Time (years) Geologic Time Scale Today we’re going to talk about geologic time in two ways: Relative Time Absolute Time
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Principles of Relative Time 1. Superposition - Younger sediments on top of older 2. Original Horizontality - Rocks were deposited horizontally 3. Cross-Cutting Relationships - Any feature that cuts across another rock or structure is younger that what it cuts 4. Inclusions - Rocks that include bits of another rock are younger than the inclusions. 5. Unconformities - Unconformities are surfaces that represent gaps in the geologic record. Relative Age 1 Superposition If one layer is on top of another, then it came later (it’s younger). Younger Older Relative Age 1 Superposition An exception! Layers can be completely upside down, and you need something like ripple marks to tell which way the layers are “facing” Youngest Oldest Youngest Oldest If layers are inclined at an angle, then something tilted them - they didn’t form that way. Relative Age 2 Original Horizontality Sedimentary rocks are formed in layers ( strata ) which were originally horizontal. (within 0-5˚)
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Relative Age 3 Cross-Cutting Relationships Crosscutting igneous rocks are younger than what they intrude. Oldest Intermediate Youngest An outcrop by a lake. Hammer for scale. Relative Age 3 Cross-Cutting Relationships Faults are younger than what they cut. younger older younger older Relative Age 4 Inclusions Rock layers that include bits of another came later (they’re younger). The sandstone incorporated bits of underlying eroded granite The granite intruded into the sandstone, pieces of which fell into the magma as inclusions. Similar Appearance, But Very Different Interpretations! Relative Age 5 Unconformities Unconformities are surfaces that represent gaps in the geologic record. Missing time means missing sediment Three Types: 1.Disconformities 2.Nonconformities 3.Angular Unconformities
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Relative Age 5 Unconformities Unconformities are surfaces that represent gaps in the geologic record. Missing time means missing sediment Disconformity An unconformity between parallel layers usually marked by an irregular surface. An outcrop by in the canyonlands of Arizona Relative Age 5 Unconformities Unconformities are surfaces that represent gaps in the geologic record. Missing time means missing sediment Nonconformity StratiFed rocks upon unstratiFed rocks (Sedimentary rocks overlying metamorphic or plutonic rocks) A nonconformity between the metamorphic Vishnu Schist and Tapeats ±ormation. Grand Canyon.
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This note was uploaded on 01/28/2012 for the course ESCI 1012 taught by Professor Joshfeinbergbrucemoskowitz during the Spring '11 term at Minnesota.

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ESCI_1012_06 - Time Scales of Hazards and their Associated...

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