Geologic Time: How do we know……? Looking at the layers of the Earth and interpreting the evidence.

Creating a Geologic Timeline Relative Dating Radiometric Dating

Review: Radiometric Dating A procedure used to date materials based on the amount of radioactive elements present This gives a specific age (within an error margin)

Radiometric Dating: Some Elements Decay An element always has a specific number of protons They usually have the same number of neutrons, but variations exist called “isotopes” Radioactive Decay is when an unstable isotope loses particles in its nucleus and becomes another element Decay is measured by its half-life

Half Life Decay

Slowly decaying isotopes can be used to date rocks:

Carbon Dating C14 Half-life: 5730 years Used to date organic material up to 60,000 years old

relative dating No.. Don’t ask your cousin out on a date

Relative Dating Before radiometric dating….. The process of placing events in the sequence in which they occurred without using exact dates. Where are the youngest rocks?

Strata and the Principle of Superposition Nicolaus Steno, 1636-1686 The oldest rock layer will be at the bottom and the youngest at the top.

The Principle of Original Horizontality Sediment is always deposited flat If the strata (layers) are not flat, something happened! What might have happened here?

A few other “dating” guidelines:

BEWARE OF INTRUDERS! Magma can “intrude” into sedimentary rock that was already there!

Cross-Cutting An igneous intrusion is always younger than the strata it cuts through. The layers had to be there first!

Cross cutting

Inclusions Embedded Fragments Rocks that are “included” in another rock must be older than the rock in which they are found. Older

Unconformity The layer or layers of rock missing from a strata sequence. Where did they go? Usually caused by erosion

Index Fossils

Rock Layer Correlation Matching of rock layers from one area to another. Gives a more comprehensive view of a region’s geologic history

D.U.D.E Deposition Uplift Down-cutting Erosion

200 Million Years Ago

Pangaea Our continents were part of one landmass: Pangaea Due to continental drift our continents are where they are today, and continue to drift apart.

Evidence to support Pangea Matching reptile fossils found in South America and Africa Identical species of ferns found in all southern continents. Rocks in SW Africa and SE Brazil were distinctively identical, and their age the same. Plate tectonics and continental drift