Geologic History Relative Dating

Birth of Modern Geology Began in the late 1700’s when James Hutton published his Theory of the Earth. He was the first scientist to argue effectively that geologic processes proceed over long spans of time.

Siccar Point, Scotland

Hadrian’s Wall, Scottland Built c. 1500, Hutton worked in 1700s.

Relative Dating Prior to the discovery of radioactivity, geologists had no reliable way of giving specific dates to geologic events and had to rely on relative dating techniques. Relative dating means placing rocks or events in their proper order, based on a comparison to other rocks

Relative Dating Laws The Principle of Uniformitarianism states that the physical, chemical, and biological processes that operate today have also operated in the past. “The present is the key to the past.”

Relative Dating Laws 2) Principle of Original Horizontality states that sediments are deposited in horizontal layers Layer 3 – Sandstone Layer 2 – Limestone Layer 1 – Siltstone

What happened to these rock strata?

What happened to this rock strata?

Relative Dating Laws 3) Principle of Superposition states that in an undeformed sequence of strata, each bed is older than the one above it and younger than the one below it. *The layers on top are youngest. * The layers on the bottom are oldest. Relative dating (nonconformity animation); (Angular Unconformity)

Which layers is youngest? Oldest?

Possible Exceptions to the Law of Superposition Folding of Strata over upon itself.

Possible exceptions to the law of superposition Strata is displaced by a Thrust Fault

Relative Dating Laws 4) Principle of Cross-Cutting Relationships states that an intrusion or fault is younger than the layers it cuts through.

Cross-Cutting Relationships Fault

Cross-Cutting Relationships

Relative Dating Laws 5) Principle of Inclusions states that an inclusion is older than the rock layer it is in.

Unconformities Buried erosional surfaces that are preserved in the rock record Unconformities represent gaps in the rock record. Relative dating (nonconformity animation); (Angular Unconformity) unconformity animation

Angular Unconformity

Put the rock layers in order from oldest to youngest.

1) Rock layers A-E are deposited.

2) Rock layer D (a sill) intrudes.

3) Rock layer F (a dike) intrudes.

Volcanic Column with radiating dikes, exposed after erosion.

4) Layers are tilted and eroded (an angular unconformity).

5) Deposition of rock layers G-K .

6) Upper surface is eroded.