Catastrophism vs Uniformitarianism and Relative Age Dating GEOLOGIC TIME Catastrophism vs Uniformitarianism and Relative Age Dating

Using Biblical chronology, the Irish estimated Earth’s age as 6,000 – 10,000 years old (circa 1658). The “young Earth” idea requires catastrophism.

In 1785, James Hutton proposed recognized that: 1) Earth is dynamic and changing. 2) Present day earth processes operated in the past and can explain changes seen in the rock record. 3) Since most geologic processes are slow, the Earth must be very old.

This concept is called uniformitarianism (opposite of catastrophism). Hutton’s slogan: “The present is the key to the past”. In its most strict form, uniformitarianism implies that rates of earth processes are constant.  This is not strictly true because rates can vary over time.

Actualism is a more balanced view for explaining that: 1) the same processes and natural laws that operate today have operated in the past but... 2) uniformitarian is punctuated by catastrophic events from time to time. Examples?

Relative Age Dating Goal- Determine the sequence of geologic events without knowing actual ages of the events. We use geometric relationships between sedimentary “strata” and other rock bodies to determine a relative chronology.

1) Law of Original Horizontality – Sedimentary beds were deposited in water form as horizontal layers due to gravitational settling.                             

2) Law of Superposition – In undisturbed sedimentary layers, the layers get younger from bottom to top.

3) Law of Lateral Continuity – Sedimentary layers extended laterally in all directions until they taper out.

4) Law of Cross-Cutting Relationships – An event that cuts across existing rock is younger than that disturbed rock.

5) Principle of Inclusion – Fragments of rock included in (contained by) a host rock are older than the host rock.

Relative Age Dating Practice ______Youngest Event ______ ______Oldest Event

Modifications to Horizontal Bedding Faults: Result of brittle deformation (shallow in crust). Folds: Result of plastic deformation (deep in crust).

Unconformity – A contact surface that represents a major gap in the geologic rock record (due to erosion or long periods of non-deposition). 1) Disconformity – A contact representing missing rock strata (layers), separating rock layers that are parallel to each other. They are difficult to see in nature.

2) Angular Unconformity – A contact in which younger strata overlie an erosional surface on tilted or folded rock layers.  This type of unconformity is easy to identify in nature.

3) Nonconformity – A contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.

Important Ideas About Unconformities! Unconformities represent huge gaps in time!  The nonconformity between the Vishnu Schist and overlying sedimentary layers in the Grand Canyon represents 1.3 billion years of missing rock record.

What is an unconformity? Sketch the three main types. WARMUP What is an unconformity? Sketch the three main types.

Correlating Rock Exposures Correlation- The process of determining time relationships between two physically separated rock units.

Methods of Rock Correlation Rock layers can be correlated by physical continuity (by actually following the rock layers).

2) Rock layers can be correlated by rock type. Caution! Just because a region may contain two sandstones, they do not necessarily represent identical times.

3) Rock Layers can be correlated by the fossils they contain. Most of the fossilized species found in rock layers are now extinct. - Fossils near the bottom of rock layers are more different from today’s plants and animals than those near the top.

The best fossils for correlation are index fossils. Some fossilized organisms thrived for a long time (sharks). They are useless for correlation. The best fossils for correlation are index fossils. Good index fossils are: a) short-lived and b) widespread. 23

Fossil assemblages can also be used for correlating rock layers Fossil assemblages can also be used for correlating rock layers.  A fossil assemblage consists of several different species occurring together in a rock layer.  The coexistence of several different species together may represent a specific point of geologic time. 24

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