Geologic TimeWe know it’s BIG…but HOW do we know that?

BIG QUESTIONS:

1) What methods do we use to figure out the age of our earth?

2) How do fossils help

us to learn things about the past?

UniformitarianismUniformitarianism is the assumption that the same

processes that operate on the earth now have always operated on the earth in the past.

For example: Given a long period of time, a river can erode a valley. Based

on the principle of uniformitarianism, all valleys in the past have been formed by

rivers eroding them and all future valleys will be formed

this same way

Relative Dating

Relative dating refers to the relative order in which things occurred, without knowing the actual date of

each event.

Relative dating relies on several principles.

Principle of Original HorizontalityThis principle states that layers of sediment are

originally deposited horizontally under the action of gravity.

This means that any “sloping” of layers

must have occurred AFTER the layers

formed

Principle of SuperpositionLayers of earth are deposited in a time sequence, with the oldest on the bottom and the youngest on the top.

Cross-Cutting Relationships

The principle of cross-cutting relationships states that the geologic feature which cuts another is the younger of the two features.

Younger

InclusionsThis principle states that fragments included in a

host rock are older than the host rock itself.

This rock (inclusion) must have existed BEFORE the lava (host rock) formed…

therefore it is older.

Practice…Identify the layers from oldest to youngest

Absolute Dating

Absolute dating refers to the exact age in years of an object

Several methods exist for absolute dating.

Radiometric Dating

Technique used to date materials such as rocks,

using radioactive materials contained in the

rocks/fossils

This exact age can then help us with some aspects

of relative dating

FossilsThe preserved remains or traces of animals, plants, and

other organisms from the past

Many fossils can be dated using relative AND absolute dating

Index Fossils

A particular plant/animal that only existed for a very short period of time on the earth

Useful for relative dating purposes.