Relative Ages and Absolute Ages Different sequences of rock layers and dating methods of earths history. Fall 2015 Earth and Space Mrs. Garcias’

Calculated by using the rate of erosion, and is only practical for 10,000-20,000 years. Tree Rings-trees have a distinctive growth pattern. Tree trunks display alternating bands of light-colored, low density summer growth and dark, high density winter growth. Each light-dark band represents one year. By counting tree rings it is possible to find the number of years the tree lived Absolute Dating-numeric age of an event, in years, and calculated by radiometric dating

Ice Cores Ice core section showing annual layers. Lake sediments, especially in lakes that are located at the end of glaciers, also have an annual pattern. In the summer, the glacier melts rapidly, producing a thick deposit of sediment. These alternate with thin, clay-rich layers deposited in the winter. The resulting layers, called varves. Ice Cores

Different methods of measuring Different methods of measuring. Varves, consist of light colored band of coarse particle and dark bands of fine particles.

Radiometric Dating- method of determining the absolute age of an object. Uses parent isotopes and daughter isotopes to compare the relative percentages Isotopes- different amount of neutrons Two different ways of emitted energy (alpha and beta decay)

Time required for half of any amount of isotope to decay is always the same and can be determined for any isotope. Example: if you have 10g of parent isotope, you would have 5g of that isotopes after one half life. Half life- the time it takes half the mass of a given amount of isotope to decay.

Carbon Dating- dating organic material found within the rock. Ages of wood, bones, shells, and other organic remains that are included in the layers and that are less than 70,000 years old.

James Hutton “Father of Geology”- believed that the studying of the present is the key to understanding the past. Known as “uniformitarianism” Uniformitarianism-principle of geologic processes that occurred in the past

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Relative age- indicates that one layer is older or younger than another layer but does not indicate the rocks age in years. Layers of rock are known as strata, the sequence of events that took pace in the past. Determine the relative age of rocks, scientist commonly study the layers in sedimentary rocks.

Law of superposition- law that sedimentary rock layer is older than the layers above it and younger than the layers below it if the layers are not disturbed. Compressed and harden layers are called, beds The area between beds is called, bedding plane

Sedimentary rock structures- Unconformities 3 different types Graded beds- Bottom layer Cross beds- Sandy, curved Ripple Marks- Made by water that hardened. Sedimentary rock structures- Unconformities

Where deposition stopped for a period of time 3 different types 1. nonconformity 2. angular 3. disconformity Unconformities- break in the geological record created when rock layers are eroded or sediment is not deposited for a long period.

3 types

Fault, is a break or crack in Earths crust If a fault intrudes through an unconformity the fault is younger than all the other rocks. Cross cutting Relationships- the fault or igneous intrusion is always younger than the rock layers it cuts through.

Principle of uniformitarianism, the forces that are changings Earths surface Scientists use the law of superposition to determine the relative age Absolute age is the numeric age of an object given in years Nonconformities, angular unconformities, and disconformities are interruptions in the sequence of rock layers, known as unconformities. Summary

In some cases, it is very tricky to determine the sequence of events that leads to a certain formation. Can you figure out what happened in what order in (Figure below)? Write it down and then check the following paragraphs.

A geologic cross section: Sedimentary rocks (A-C), igneous intrusion (D), fault (E). The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. The fault cuts through all three sedimentary rock layers (A, B, and C) and also the intrusion (D). So the fault must be the youngest feature. The intrusion (D) cuts through the three sedimentary rock layers, so it must be younger than those layers. By the law of superposition, C is the oldest sedimentary rock, B is younger and A is still younger. The full sequence of events is: 1. Layer C formed. 2. Layer B formed. 3. Layer A formed. 4. After layers A-B-C were present, intrusion D cut across all three. 5. Fault E formed, shifting rocks A through C and intrusion D. 6. Weathering and erosion created a layer of soil on top of layer A.