The Geologist as Detective

Chapter 6 : Interpreting Geologic History

How does a Geologist tell how old a rock is?

• They look for clues in the rocks

• They follow a set of rules or principles that helps them determine the sequence of events leading to the formation of a particular rock outcrop

• Think of it as rock detective work!!!

Fundamental Principles of Geologic History

• Uniformitarianism• Superposition• Original Horizontality

or Cross Cutting

Uniformitarianism

• The idea that the processes that shaped the Earth today are the same processes that occurred in the geologic past

• “The Present is the key to the past”.

Superposition

• States the oldest rock layer is on the bottom, the youngest on top

• This occurs in an undisturbed sequence of rock layers only

Original Horizontality or Cross Cutting

• States the rock is always older than the processes that changed it.

• Rocks are laid down in horizontal layers.

• After rocks form, they fold or fault.

• After rocks form, magma will well up between the layers or onto the surface (igneous intrusions or extrusions).

Sequencing Events in Rocks: Terms to use

• Unconformity: a buried erosional surface or gap in rock record

• Emergence/uplift• Submergence/sinking• Deposition• Tilting• Faulting• Intrusion/extrusion• Melting and solidification

Keep the three basic principles in mind when deciding the order of

events in rock formation!!!