Geologic Time NotesSI

A) Relative age: the age of rock compared to the ages of other rocks above or below it in a sequence of rock layers

Determining the Age of Rocks

B) Principles to follow to determine relative age1. Law of Superposition

a. Sedimentary rocks form as horizontal layers.

b. Law states that if rock layers are undisturbed, younger rocks lie

above older rocks, and the oldest rocks are at the bottom.

(Law of Superposition cont.)c. Geologists use the law of superposition to determine the relative ages of rocks from the sequence of rock layers and the fossils within each year.d. They examine sedimentary rocks from locations around the world to develop a relative time scale of important events on Earth.

2. Principle of Original Horizontality: When sedimentary rock layers are being deposited, gravity forces them to be deposited as flat, horizontal layers. Once the sediment has solidified and becomes rock, they can be tilted or folded.

3. Principle of Cross-cutting relations: A rock that cuts across another rock must be younger than the rock it cuts across. In other words, the other rock must have already been there for the new rock to cut across it.

4. Principle of Inclusion: A rock that spreads into one layer of preexisting rocks must be younger than the rock it has spread into.

C) Index Fossils and Relative Dating1. Can determine relative ages of rocks by examining fossils that are found in them2. Index fossils: fossils of organisms that are easily identified, occurred over a large area and lived for a well defined period of time3. Use these index fossils to determine relative age of rocks because the rocks must have formed at the same

time that those organisms lived

D) Radioactive Dating1. Geologists use radioactive dating to determine the absolute ages of rocks

a. Absolute age: the time that has passed since the rock formed2. Measure the radioactive isotopes that are being emitted from the rock and use the half-life of this isotope to determine the absolute age3. A valid form of measurement because radioactive isotopes have constant decay rates that measurable and unique to the isotope itself.

A) Geologic time scale is based on the relative ages of rock layers and the use of radioactive dating to find the absolute ages of rocks

1. Eras are major stages in Earth’s history 2. Periods are smaller stages within an era

3. With each subdivision of the time scale, more information is known.

II.) A Brief History of Earth

B) Age of the Earth1) The oldest know rocks are about 3.8 billion years old.2) The Earth is approximately 4.6 billion years

old. How do we know?

3) Samples from meteor showers and the moon that have landed on Earth have been used for radiometric dating. These samples are more than 4.5 billion years old.

II.) A Brief History of Earth