Geology Study Guide Exam 2
Chapter 7-9

7: Achaean - 9: Precambrian

Chapter 7: Plate Tectonics

How do we categorize Earth’s interior?
Compare and contrast composition and characteristics of the layers of the Earth:
Core: 3740 km radius
Inner Core: Solid Fe (85%) with some Ni
Solid because of the enormous pressure prevents the molecules from moving.
The inner core is slightly larger than the moon.
Outer Core: Molten Fe (85%) with some Ni. (may also contain lighter elements such as Si, S, C, or O.
S-waves can not pass through the outer core, therefore we know the core is liquid (molten)
Convection in liquid outer core plus spin of solid inner core generates Earth’s magnetic field.
Magnetic Field also evidence for a dominantly iron core.
Mantle: 2885 km thick
Average density of mantle is 4.5 g/cm3
Made of rock
Made of oxygen, silicon, iron and magnesium
Composition based on studies of rock from mantle brought up by volcanoes, from density calculations, and composition of stony meteorites (Peridotite, Kimberlit, Eclogite)
The Mantle is not uniform. It has several layers with different properties.
Lithosphere:
Outermost 100 km of Earth. Consists of the crust plus the outermost part of the mantle.
Divided into tectonic or lithospheric plates that cover surface of the Earth.
Asthenosphere:
Low velocity zone at 100-250 km depth of Earth (seismic wave velocity decreases)
Rocks are or near melting point.
Magmas generate here.
Solid that flows (rheid)
Plastic behavior
Convection in this layer moves tectonic plates.
Crust: Two types of Crust:
Oceanic crust: Thin, dense, basaltic
Continental crust: Thick, low density, granitic
How have earthquakes helped us understand Earth’s interior?
P-waves: Primary, Pressure, Push-pull. Travel fastest of the seismic waves (6km/sec in crust; 8 km/sec in uppermost mantle) and travels through solids and liquids
S-waves: Secondary, shaking, shear, side-to-side, slower (3.5 km/sec in crust; 5 km/sec in upper mantle) *Travels through solids only
Mohorovicic and Gutenburg discontinuities (Discontinuities are the boundries between Earth’s layers)
Structures: Faults and Folds
Faults: A crack in the Earth’s crust along which movement has occurred.
Folds: During mountain building or compressional stress, rocks may deform plastically to produce folds. Generally, a series of folds are produced, much as a carpet might wrinkle when you push on one end. The up-folds and the down-folds are adjacent to one another, and grade into one another.
How did the theory of plate tectonics evolve?
Continental Drift Hypothesis
Seafloor mapping: Discovered patterns of mid-ocean ridges and deep sea trenches and provided images of rocks and sediments beneath the ocean
What types of evidence support and have verified the theory of plate tectonics?
Echo sounding: used for seafloor mapping
Magnetometers: Charted the Earth’s magnetic field over large areas of the seafloor
Submarine Detection Devices: Discovered differences in magnetic properties of rocks on seafloor
Global Network of Seismometers: established to monitor atomic explosions. Also provided info on world wide earthquake patterns
Rift valleys: 2 plates pull apart, water comes in and fills the gap and eventually creates a new ocean. Divergent plate boundaries.
What happens at mid-ocean ridges?
Sites of seafloor spreading.
High heat flow
Seismic wave velocity decreases at the ridges, due to high temperatures
A valley is present along the center of ridge
Volcanoes are present along the ridge
Earthquakes occur around the ridge
Paleomagnetism: The branch of geophysics concerned with the magnetism in rocks that was induced by Earth’s magnetic field at the time of their formation. (The Earth’s magnetic field acts as if there were a bar magnet in the center of the Earth.)
Polar Wandering: The slow and erratic, real or apparent, movement of Earth’s rotational or magnetic poles relative to the continents throughout geological time, due