[GEOL 101] - Midterm Exam Guide - Ultimate 23 pages long Study Guide!

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GEOL 101
MIDTERM EXAM
STUDY GUIDE
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Plate Tectonics | Chapter 2
Continental Drift: large-scale horizontal movements cause Earth’s major topographical features such as
mountains and oceans basins
Alfred Wegener (1880-1930)
o Proposed in 1912 based on observation of drifting ice sheets
Geographic fit of the continents, one of the first pieces of evidence used to argue for continental drift.
Once Pangea: Supercontinent
Fossil match across the Atlantic
Paleontologists: animals and plants diverge in evolution after the postulated breakup time
Rejection and Acceptance of Continental Drift
Initially rejected by geologists
o New data after WWII led to 1960’s plate tectonic revolution
o Ocean drilling
No fully embraced and textbooks rewritten
Seafloor Spreading: Convection in the Earth’s mantle could break up and push continents apart.
Arthur Holmes (1890-1965)
Harry Hess (1906-1969): got the credit for Holmes’ idea. He used the hypothesis of mantle
convection to explain seafloor spreading, island arcs, gravity, anomalies and serpentine rocks.
Rifting: When the continental rock heats up and spreads apart, is continental rifting, creates a new ocean
basin between.
New Ocean crust forms at mid-ocean ridge
Iceland: an opening rift on the Mid-Atlantic ridge
Earthquakes and Volcanoes
Their locations are predicted by plate tectonics.
The Pacific “Ring of Fire”
Lithosphere: plates comprises top of the mantle, oceanic crust and/or continental crust
Cold, outer rigid (brittle) shell of the Earth (approx. 100km) containing the plates
Plates slide over partially molten weak mantle region called asthenosphere
Heat-softened (viscous) part of the mantle beneath the lithosphere
Plate Boundaries
1. Transform: transform-fault boundaries plates slide horizontally past each other. Parallel
a. San Andreas Fault
2. Divergent: plates move apart and create new lithosphere
a. Mid-Atlantic Ridge
b. East African Rift Valley
3. Convergent: plates collide and one is pulled into the mantle and recycled.
a. Andes Mountains: Ocean-Continent Convergence.
b. Trenches, the plate going underneath the thicker plate, are the deepest topographical
points on Earth.
c. With ocean plates, normally the older plate sinks, such as the Pacific plate sinking
bellowing the Eurasian plate, creating the Japanese Islands
d. Fold-and-thrust mountains: continent-continent boundaries, one rises over the other.
Seafloor as a Magnetic Recorder
Produced by the convection of the liquid Fe-Ni alloy in the outer core
Similar to the ones produced by a bar magnet or a current coil
No Geomagnetic Field
When lavas crystalize each magnetic moment orientation is preserved. Mineral magnetite is ‘magnetic.’
Random orientation of the magnetic minerals
Geomagnetic Field Present (Normal Direction)
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When lavas crystallizes each magnetic mineral remains aligned
Magnetic minerals orient their magnetization in the direction of the geomagnetic field
Geomagnetic Field Present (Reversed Direction)
New lavas preserve magnetization the new ‘reversed’ direction
Paleomagnetism in a Nutshell
1. Geologists examine magnetization of volcanic rocks. Their findings show reversals of Earth’s
magnetic field
2. Geologists then determine the absolute age of the rock. Every sample is characterized by its
magnetization and age
3. Large number of rock samples allow geologists to construct a reversal time scale by studying
many volcanoes and lava flows (age vs. magnetization)
4. Measure magnetic field offshore using ship-borne instruments. Match the magnetic patterns to
those determined on land and assign its age
The Grand Reconstruction…
Isochron map of the seafloor - Seafloor is very young, the oldest being approx. 200ma
Geologists use plate motions, and the study of different types of rocks in continents to reconstruct
geological past…
Read: Assembly of Pangaea and the Breakup of Pangaea
Mantle Convection
a) Whole-mantle convection
a. Plate recycling extends to the mantle-core boundary
b) Stratified convection
a. Plate recycling is confined to the upper mantle
b. The lower mantle convects more sluggishly than the upper
Mechanism for Plate Tectonics
Plastic upper mantle asthenosphere
Gravity field as the driving force
Slab pull away from the ocean ridge
Hotspot is a location on the Earth’s surface that has experienced active volcanism for a long period time
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