GEOLOGIC HISTORY THE RULES OF THE GAME © Copyright 2005 - 2006  M. J. Krech. All rights reserved.

Time What is time?

Time is… Either relative or absolute Time is: a measured period during which an event, process, or condition exists or continues.

One Year = the time it takes the Earth to complete one orbit around the Sun.

Geologic Time Based on the same time units Generally much greater periods of time 100,000 years 1,000,000 years

Dating Techniques One ways in which geologists look at time: Relative Age

Relative Age of a rock… …is its age compared to the ages of other rocks. Example: My niece, Sara, is older than my other niece, Mariam. What happened first? What happened next?

Absolute Age of a rock… …is the number of years since the rock formed. Example: My niece, Sara, is 3 years old and my niece, Mariam, is 3 months old.

Relative and Absolute Age

Law of Superposition The youngest rocks are on the top. The oldest rocks are on the bottom.

Principle of Superposition Q#1 Which rocks are older? Which rocks are younger? Younger Rocks Older Rocks

Principle of Cross-Cutting Fault is a break or cut in the rock. You can't cut something until that something exists, therefore the cut has to be younger. So faults are younger than the rock it cuts through. Volcanic vents cutting across sedimentary rock layers - Which is youngest?

Principle of Cross-Cutting Q#2 What is younger? The rocks or the fault line?

Intrusions

Principle of Inclusions The inclusion is always younger than the rock layers around and beneath it.

Principle of Inclusions Q#3 Which is older? The rock layer or the rocks included in the layer?

Principle of Unconformities Unconformities are surfaces that represent gaps in the geologic record.

Example of Unconformity http://www.classzone.com/books/earth_science/terc/content/visualizations/es2902/es2902page01.cfm?chapter_no=visualization

Principle of Unconformities missing rock layer = missing time

Where is the youngest rock? Q#4 How do you know?

Where is the unconformity? Q#5 How do you know?

What rock layer is the oldest? Q#6 How do you know?

Which is younger? The fault or the rock layers? Q#7 FAULT How do you know?

Which is younger? The rock layer or the inclusion? Q#8 How do you know? inclusion

Using Fossils to Date Rocks Give relative age to a layer of rock Match layers of rock at other locations Use index fossils to match rock layers Index fossils are distributed in different areas & only occur in one area Ex. pg. 117