Unit 6 – The Geosphere Vocabulary Uniformitarianism Relative Dating Law of Superposition Principle of Original Horizontality Principle of Cross-Cutting Relationships Unconformity Correlation Fossil Index Fossil Radioactivity Half Life Radiometric Dating Radiocarbon Dating Geologic Time Scale

Discovering Earth’s History

Rocks Record Earth’s History 1. Rocks record geological events and changing life forms of the past. 2. Scientists have learned that the Earth is much older than previously thought and the Earth’s surface and interior have been changed by the same geological processes that go on today.

A Brief History of Geology 1. In the mid 1600’s James Ussher made a timeline of both human and Earth history and determined the Earth was more than 5,000 years old.

2. In the late 1700’s, James Hutton, a Scottish physician, published a book called Theory of the Earth and established the principle of uniformitarianism.

Uniformitarianism means that the forces and processes that have shaped Earth in the past are essentially the same as those operating today.

Relative Dating - Key Principles 1. Relative dating is a process by which rocks are placed in their proper sequence or order. Only the chronological order of events are determined, not the absolute age in years.

2. Nicolaus Steno proposed the Law of Superposition which states in an undeformed sequence of sedimentary rocks, each layer is older than the one above it and younger than the layer below it. A B C D Youngest Oldest

3. Steno also posed the Principle of Original Horizontality which says the layers of sediment are generally deposited in a horizontal position.

4. The Principle of Cross-Cutting Relationships states that a rock or fault is younger than any rock or fault through which it cuts. Youngest Oldest

5. No place on Earth is geologically complete 5. No place on Earth is geologically complete. Unconformities are surfaces that represent a break in the rock record, caused by erosion or lack of deposition. The Great Unconformity

Correlation of Rock Layers 1. To develop a geologic time scale that can be applied to the entire Earth, rocks of similar age in different regions must be matched up- a task called correlation. 2. By correlating rocks, it is possible to create a more complete view of the geologic history of the region.

Relative Ages of Rocks WS

Can you find the man in the beans?

Dating With Radioactivity Basic Atomic Structure There are protons and neutrons in an atom’s nucleus. The number of neutrons in an atom can vary - these are called isotopes. Radioactivity When nuclei are unstable, they break apart, or decay, in a process known as radioactivity.

An unstable or radioactive isotope is called a parent. The isotopes that result from the decay of the parent are called daughter products. Half-Life A half-life is a common way of expressing the rate of radioactive decay. A half-life is the time required for one half of the atoms of a radioactive substance to decay.

Say what!?!? A 100g sample of Borchiksonium has a half-life of 5 years. After 5 years - 50 grams remain After 10 years - 25 grams remain After 15 years - 12.5 grams remain After 20 years - 6.25 grams remain

Radiometric Dating Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes. Dating with Carbon-14 Radiocarbon dating is the method for determining age by comparing the amount of Carbon-14 to the amount of Carbon-12 in a sample.

So what! Importance of Radiometric Dating Radiometric dating has been used to date thousands of events in Earth’s history. Rocks on Earth have been dated to be as much as 4 billion years old.