Week 7: Geological Time

What is relative dating and numerical/absolute dating?

Relative:
Establish a sequence of events without establishing exactly when they occurred

Numerical/Absolute:

Establishes when an event took place or when a feature formed

Assigned a specific age in years

What is the principle of uniformitarianism?

Things that happen today occurred through similar ways previously

What is the principle of original horizontality and continuity?

Strata (Sedimentary rock layers) often form laterally extensive horizontal sheets

Flat-lying rocks layers are unlikely to have been disturbed

What is the principle of superposition?

In an undeformed sequence of layered rock each bed is older than the one above

What is the principle of cross-cutting relations?

Younger features cut across older features

What is the principle of inclusions?

Inclusions: A rock fragment within another

Igneous xenoliths: Rocks that existed before magma intruded fell into magma

Weathering rubble: Debris from preexisting rock (sediment)

What is the principle of baked contracts?

Thermal metamorphism occurs when country rock is invaded by a plutonic igneous intrusion

The baked rock must have been there first (it is older)

What is an unconformity?

A time gap in the rock record due to non-deposition or erosion

What is angular unconformity?

Represents a huge time gap

Horizontal marine sediments deformed by orogenesis

High mountains are eroded to below sea level

Sediments deposited horizontally on the erosion surface

What is nonconformity?

Metamorphic or igneous rocks overlain by sedimentary strata

Crystalline igneous rocks or metamorphic rocks were exposed by erosion and uplift

Sediment was deposited on this eroded surface

What is disconformity?

Parallel strata bracketing non-deposition
Occurs due to an interruption in sedimentation

What is a fossil?

Remnant or trace of ancient life

Examples:

Body fossils
Petrified remains

Molds and casts

Carbonization

Impression

Amber preservation
Trace fossils (Footprints, burrows)

Chemical fossils

What is the process of fossil preservation?

Primarily found in sedimentary rocks, though also sometimes in volcanic ash

Fossil form by:

Rapid burial

Energy of depositional environment

Low oxygen content

Possession of hard parts

Fossil records are biased towards organisms that lived in sedimentary environments and that possessed hard parts

What is the principle of fossil succession?

Fossil range: first and last appearance

Each fossil has a unique range

Overlapping ranges provide distinctive time markers

Define mass extinction?

Abrupt global decreases in biodiversity

Causes:

Global climate change

Tectonic activity

Asteroid or comet impacts

Flood basalt eruptions

What is radiometric age?

The time a mineral began to preserve all atoms of parent and daughter isotopes

Requires cooling below a closure temperature (When everything is locked in place in a closed system)

Igneous rocks are best for geochronological work