The Rock Record

Section 1: Determining Relative Age Geologists estimate that the Earth is about 4.6 billion years old James Hutton started the idea of uniformitarianism – current geological processes such as volcanism and erosion are the same now as in the past. He encouraged scientists to look at rock layers to determine the Earth’s history.

Relative Age Show the sequence of events that took place in the past. Indicates that one layer is older or younger than another layer but does not indicate the rock’s ages in years.

1. Law of superposition States that an undeformed sedimentary rock layer is older than the layers above it and younger than the layers below it.

2. Principle of Original Horizontality States that sedimentary rocks left undisturbed will remain in horizontal layers.

3. Unconformities Movements in Earth’s crust can lift up rock layers that were buried and expose them to erosion. 1. Nonconformity – the rock has been pushed up from below, eroded and then sediments are deposited on it. 2. Angular unconformity - there is a boundary between tilted rocks and horizontal layers. 3. Disconformity – when rock layers have been pushed up

4. Crosscutting Relationships Law of crosscutting relationships – is a fault or igneous intrusion is younger than the rock layers it cuts through Fault – a break or crack in Earth’s crust along which rocks shift their position Intrusion – a mass of igneous rocks that forms when magma is injected into rocks; cools and solidifies.

Section 8.2 Determining Absolute Age Scientists often need to determine the numeric age called absolute age

1. Rates of Erosion Scientists measure the rate at which a stream erodes a bed and estimates the age of the stream Only works for features that formed in the past 10,000-20,000 years.

2. Rates of Deposition ~ 30 cm of sedimentary rock is deposited over a period of 1,000 years * This number does increase with unusual events such as floods.

3. Varve Count Varve – is a sedimentary deposits annual layer Consists of light colored band of coarse particles and a dark band of fine particles Generally form in glacier lakes

4. Radiometric Dating Scientists use the natural breakdown of isotopes to measure the absolute age Each atom has a decay period that is known They measure the amount of parent isotopes to the new daughter isotopes.

5. Half Life Radioactive decay occurs at a relatively consistent rate that is not changed by temperature, pressure or other environmental conditions. Half-life is the time it takes half the mass of a given amount of radioactive parent isotope to become a daughter isotope.

6. Carbon Dating Younger rock layers may be dated indirectly by dating organic material found in the rock. Only works for rocks less than 70,000 years old.

Geological Time As conditions on Earth’s surface change some organisms flourish and then later become extinct. To describe the sequence and length of these changes scientists have developed a geological time scale

The Geological Column A geological column represents a timeline of Earth’s history. The oldest rocks are on the bottom of the column. Fossils in the upper layer are more modern while those in the lower layer may have been extinct for millions of years. They are used to determine the approximate age of rocks.

Divisions of Geological Time The largest unit of time is an eon Hadean, Archean, Proterozoic, Phanerozoic Each eon is divided into smaller units called eras Eras are divided into periods – characterized by specific fossils and are usually named for the location where the fossils were first discovered The period could then be divided into an epoch and an epoch is divided into ages.