AimAim: How do we know the age of the Earth?

I. Earth’s History

A. UniformitarianismUniformitarianism – the principle that most of the geologic events of the past are similar to the processes that we can observe in the present.

B. The Law of SuperpositionThe Law of Superposition – says that the rock layers on the bottom of an undisturbed rock exposure are usually the oldest.

1. From this knowledge, geologists can date the relative ages of the strata (layers) from bottom to top (oldest to youngest).

youngest

oldest

“Rock is always older than the processes that changed it.”

C. Igneous Intrusions and ExtrusionsIgneous Intrusions and Extrusions

1. IntrusionIntrusion – created when magma is injected into older rock layers in the crust.

2. ExtrusionExtrusion – igneous rock that formed from lava at the surface of the crust.

D. Folds and FaultsFolds and Faults

1. Folds are bends in rock strata. Sometimes folding can overturn rock strata so that older rock lies on top of younger rock.

2. FossilsFossils – any naturally preserved remains or impressions of living things.

a. Sometimes the remains of the whole organism are found in sap (insects), tar pits, ice.