Geologic Time and the Rock Cycle-CA
Study Guide
Rock Cycle…
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All of the energy that drives Earth’s rock cycle comes from Earth’s interior and the
sun.
Igneous rocks—formed by…cooling magma
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Examples of: granite, basalt, gabbro, pumice, obsidian
Sedimentary rocks— formed by…sediments…weathering, erosion, deposition, compaction,
cementation
 Sedimentary rocks with ripple marks suggest that the rocks formed along a beach or
stream bed.
 Examples of: sandstone, halite, limestone, shale
Metamorphic rocks— formed by…heat and pressure
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Examples of: marble, slate, gneiss, schist
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contact metamorphism—being heated by nearby magma
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regional metamorphism—pressure built up in rock that’s buried deep below
other formations or crust collides
B. Sedimentary
E. Compaction and cementation
H. Metamorphic rock
I. Melting
F. Sedimentary rock
Geologic Time and the Rock Cycle-CA
Study Guide
Earth’s Formation:

What may have caused oceans to form as Earth’s atmosphere changed--The Earth had
cooled enough for heavy rain to fall.
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Much of the water of the early oceans may have been condensed from volcanic gases.

Scientists think that Earth’s early atmosphere was made up of carbon dioxide and water
vapor.
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Early life-forms released oxygen into Earth’s atmosphere through the process of
photosynthesis.
Relative Dating:
 Uniformitarianism— geologic change happens gradually
o Examples of: processes that shape the earth…weathering, erosion, deposition
 Catastrophism—geologic change occurs rapidly
o Examples of: asteroids, volcanic eruptions
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How does a geologist know that rock layers are undisturbed? Rock layers are horizontal.
How are rock layers arranged in the geologic column? Oldest rocks are on the bottom.
An example of a geologic event that could be recorded in rocks is a volcanic eruption.
Geologic Time and the Rock Cycle-CA
Study Guide
Absolute Dating

Isotopes--atoms with the same number of protons but a different number of neutrons

During radioactive decay? Radioactive isotopes break down into stable isotopes of the same or
other elements

What does a scientist need to know to figure out the absolute age of a rock? the rate of decay for a
radioactive element in the rock
Time it takes for 50% of the nuclei in a radioactive sample to decay to its stable isotope is called half-life.


To determine the most accurate and precise age of Earth’s oldest rocks, geologists would use the
uranium-lead method

Which method is most useful in dating the remains of plants and animals that lived within the last 50,000
years? carbon-14
Terms to know…
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half-life the time it takes for one-half of a radioactive sample to decay
radioactive-unstable isotopes
radioactive decay-the process in which radioactive isotopes break down into stable isotopes
radiometric dating-the most common method of absolute dating
potassium-argon method-radiometric dating method that dates rocks older than 100,000 years old
Fossils and Geologic time scale

How many years of the history of Earth do geologists study? 4.6 billion years

What does the rock and fossil record represent? geologic time

Where are most fossils preserved? in sedimentary rock
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A use for fossils found in sedimentary rocks-- interpreting past environments, indicating when the
rock formed, matching rocks of the same age found in different places
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Why is the fossil record incomplete? Most organisms never became fossils.
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What did marine fossils discovered on a Canadian mountaintop tell scientists?
Fossils found on mountain tops means that that area was at one time below the ocean.
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What do scientists think may have caused dinosaurs to become extinct? climate change
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Which of the following do geologists use to date rock layers? index fossils
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Index fossils allow geologists to match rocks of the same age in different regions
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Which of the following was known as the Earth's most severe extinction event, occurring 251.4 million
years ago? Permian-Triassic
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K-T extinction-dinosaurs, occurring 65 million years ago
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During what Epoch of the Cenozoic Era was the most recent ice age on Earth? Pleistocene

During the Pleistocene Ice age, the most significant events included the growth of ice caps, valley
glaciers, large ice sheets of the following except icebergs which item?