Notes on Relative Ages of Rocks EarthSpace S

Notes on “Relative Ages of Rocks” Earth/Space

S. W. B. A. T. • • • Describe methods used to assign relative ages to rock layers Interpret gaps in the rock record Give an example of how rock layers can be correlated with other rock layers

Relative Ages (pages 576 -579) • • • Law of Superposition – oldest rocks are on the bottom (p. 576) Younger rocks are deposited horizontally & layered progressively upwards Rock layers have to be undisturbed (not folded)

Disturbed or Undisturbed Rock Formation?

Relative Ages • • Relative age – compares ages to something else (p. 575) Example: an event occurring before or after another event

Relative Ages (p. 576) • • If you find a fossil in a rock layer that is on top of another fossil then the top fossil is younger The fossil on the bottom-most layer will be the oldest – in an undisturbed formation

Unconformities (p. 578) • • • Unconformities - missing gaps in the rock layer Erosion can scrape or wash rocks away over time Unconformities can also form by no new deposition of sediments

Q: Can you find the area of the Unconformity? A:

Unconformities • • • Angular Unconformities – horizontal rock layers that are tilted & eroded (p. 578, Figure 1) A “jagged” line usually indicates an erosion line Tilting of layers are caused by plate tectonics (movement of Earth’s crust)

Angular Unconformity – New Mexico

Unconformities • • Disconformity – missing erosional gaps in non-tilted rock layers (p. 578, Figure 1) A “smooth” line usually separates the layers

Disconformity

Unconformities • • Nonconformity – igneous rocks are uplifted & eroded (p. 578, Figure 1) Sedimentary rocks are then deposited on the eroded igneous rock surface

Nonconformity – Chapman’s Peak, South Africa Video (3: 58) “Explaining Unconformities”

Nonconformity – Grand Canyon Video (3: 42) “Origin of the Rocks of the Grand Canyon: A Sandbox Demonstration”

Matching Up Rock Layers • • Correlation – identifying a rock sequence & matching it to another sequence far away (p. 578) Geologists can correlate rock layers over great distances

Matching Up Rock Layers • • Fossil evidence is also used to match up rock layers Correlation is important in finding oil, natural gas, coal & other resources in rock layers

Matching Up Rock Layers • • Sometimes determining relative ages of rock is not enough and other methods are used Example: numerical dates are needed to determine the age of the Earth

Video Summary (4: 49) “Relative Dating”

Class Activity • • • In the video, relative age dating was compared to taking photographs Create your own analogy for relative age dating and write a short paragraph explaining your analogy Review this lesson at Kahoot. it! And/or use the online Quizizz “Relative Ages of Rock” quiz