Earth’s History How do we know about it?

What is Uniformitarianism? Uniformitarianism: –Is a theory developed by the Scottish geologist, James Hutton, in the late 1700’s –Is the idea that the Earth is an always- changing place –Is the idea that the same forces of change at work today were at work in the past

How are the order of events that occurred in the Earth’s past known? Using evidence from rocks and fossils, scientists can determine the order of events that occurred in the past. Relative and Absolute age are used to determine WHEN the events occurred.

Relative Age The age of an object compared to the ages of other objects.

Basic Rules for Determining Relative Age Layers of Rock (Law of Superposition & Principle of Horizontality) Igneous Rock (Law of Cross-cutting Relationships) Faults (Law of Cross-cutting Relationships) Unconformities Index Fossils

Law of Superposition In an undisturbed layer of rock, the oldest rocks are on the bottom and the rocks become younger toward the top. Principle of Horizontality Layers of sediment are generally deposited in a horizontal position

Ordering the Grand Canyon’s History

Igneous Rock Igneous rock that cuts across layers of sedimentary rock is ALWAYS younger than the rock layers it goes through. This is called the Law of Cross- Cutting Relationships.

Intrusions & Extrusions Intrusions are igneous rock layers formed when MAGMA hardens beneath the Earth. Extrusions are igneous rock layers formed when LAVA flows onto the Earth’s surface and hardens

Intrusion

Extrusion

Faults A fault is also ALWAYS younger than the rock layers it cuts through. This is also the Law of Cross- Cutting Relationships.

Applying Cross-Cutting Relationships

Unconformities Gaps in a sequence of rock layers (missing layers), resulting from erosion or faults. Click me and then click Molten Rock in Sedimentary Layers

Formation of an Angular Conformity

A Record of Uplift, Erosion, and Deposition

Discovering Earth’s History Correlation: Establishing the age of rocks in different locations using fossil evidence and rock layers

Correlation of Strata at Three Locations

Index Fossils (Guide Fossils) These fossils are used to determine the age of rock layers.

THREE REQUIREMENTS FOR A FOSSIL TO BE A GOOD INDEX FOSSIL Must be found in large numbers. Must be found in large numbers. Must have existed for a short period of time. Must have existed for a short period of time. Must be found over a large area of the Earth’s surface. Must be found over a large area of the Earth’s surface. Relative & Absolute Age