Download Geology Module: Seismic Interior Lecture Outline

Geology Module: Seismic Interior Lecture Outline
I. Earth's layered structure
A. Most of our knowledge of Earth’s interior comes from the study of P and S
earthquake waves
1. Travel times of P and S waves through Earth vary depending on the properties
of the materials
2. S waves travel only through solids
B. Layers defined by composition
1. Crust
a. Thin, rocky outer layer
b. Varies in thickness
1. Roughly 7 km (5 miles) in oceanic regions
2. Continental crust averages 35–40 km (25 miles)
3. Exceeds 70 km (40 miles) in some mountainous regions
c. Two parts
1. Continental crust
a. Upper crust is composed of granitic rocks
b. Lower crust is more akin to basalt
c. Average density is about 2.7 g/cm3
d. Up to 4 billion years old
2. Oceanic crust
a. Basaltic composition
b. Density about 3.0 g/cm3
c. Younger (180 million years or less) than the continental crust
2. Mantle
a. Below crust to a depth of 2900 kilometers (1800 miles)
b. Composition of the uppermost mantle is the igneous rock peridotite
(changes at greater depths)
3. Outer core
a. Below mantle
b. A sphere having a radius of 3486 km (2161 miles)
c. Composed of an iron–nickel alloy
d. Average density of nearly 11 g/cm3
C. Layers defined by physical properties
1. Lithosphere
a. Crust and uppermost mantle (about 100 km thick)
b. Cool, rigid solid
2. Asthenosphere
a. Beneath the lithosphere
b. Upper mantle
c. To a depth of about 660 kilometers
d. Soft, weak layer
e. Easily deformed
3. Mesosphere (or lower mantle)
a. 660–2900 km
b. More rigid layer
c. Rocks are very hot and capable of gradual flow
1 Geology Module: Seismic Interior Lecture Outline
4. Outer core
a. Liquid layer
b. 2270 km (1410 miles) thick
c. Convective flow of metallic iron within generates Earth’s magnetic field
5. Inner core
a. Sphere with a radius of 1216 km (754 miles)
b. Behaves like a solid
D. Discovery of Earth’s major layers
1. Discovered using changes in seismic wave velocity
2. Mohorovicic discontinuity
a. Velocity of seismic waves increases abruptly below 50 km of depth
b. Separates crust from underlying mantle
3. Shadow zone
a. Absence of P waves from about 105° to 140° around the globe from an
earthquake
b. Explained if Earth contained a core composed of materials unlike the
overlying mantle
4. Inner core
a. Discovered in 1936 by noting a new region of seismic reflection within the
core
b. Size was calculated in the 1960s using echos from seismic waves
generated during underground nuclear tests
E. Discoveriy of Earth’s composition
1. Oceanic crust
a. Prior to the 1960s scientists had only seismic evidence from which to
determine the composition of oceanic crust
b. Development of deep-sea drilling technology made the recovery of oceanfloor samples possible
2. Mantle
a. Composition is more speculative
b. Lava from the asthenosphere has a composition similar to that resulting from
the partial melting of a rock called peridotite
3. Core
a. Evidence comes from meteorites
1. Composition ranges from metallic meteorites made of iron and nickel to
stony varieties composed of dense rock similar to peridotite
2. Iron, and other dense metals, sank to Earth’s interior during the planet’s
early history
b. Earth’s magnetic field supports the concept of a molten outer core
c. Earth’s overall density is also best explained by an iron core
2 Geology Module: Seismic Interior Lecture Outline
II. Continental drift: an idea before its time
A. Alfred Wegener
1. First proposed hypothesis, 1915
2. Published The Origin of Continents and Oceans
B. Wegener's continental drift hypothesis
1. Supercontinent called Pangaea began breaking apart about 200 million years
ago
2. Continents "drifted" to present positions
3. Continents "broke" through the ocean crust
4. Evidence used by Wegener
a. Fit of South America and Africa
b. Fossil matches across the seas
c. Rock type and structure matches
d. Ancient climates
5.Main objection to Wegener's proposal
was its inability to provide a
mechanism
III. Plate tectonics: the new paradigm
A. More encompassing than continental drift
B. Associated with Earth's rigid outer shell
1. Called the lithosphere
2. Consists of several plates
a. Plates are moving slowly
b. Largest plate is the Pacific plate
c. Plates are mostly beneath the ocean
C. Asthenosphere
1. Exists beneath the lithosphere
2. Hotter and weaker than lithosphere
3. Allows for movement of lithosphere
D. Plate boundaries
1. All major interactions among plates occur along their boundaries
2. Types of plate boundaries
a. Divergent plate boundaries (constructive margins)
1. Two plates move apart
2. Mantle material upwells to create new seafloor
3. Ocean ridges and seafloor spreading
a. Oceanic ridges develop along well-developed boundaries
1. Represent 20 percent of Earth’s surface
2. Rift valleys may develop along the axis
b. Along ridges, seafloor spreading creates new seafloor
1. Topographic differences are controlled by spreading rates
2. A spreading rate of 5 to 9 centimeters per year is the norm
4. Continental rifts form at spreading centers within a continent
b. Convergent plate boundaries (destructive margins)
3 Geology Module: Seismic Interior Lecture Outline
1. Plates collide, an ocean trench forms, and lithosphere is subducted into
the mantle
2. Types of convergence
a. Oceanic–continental convergence
1. Denser oceanic slab sinks into the asthenosphere
2. Pockets of magma develop and rise
3. Continental volcanic arcs form
a Andes
b. Cascades
c. Sierra Nevada system
b. Oceanic–oceanic convergence
1. Two oceanic slabs converge and one descends beneath the other
2. Often forms volcanoes on the ocean floor
3. Volcanic island arcs forms as volcanoes emerge from the sea
a. Aleutian Islands
Islands
b. Mariana Islands
c. Tonga Islands
c. Continental–continental convergence
1. When subducting plates contain continental material, two continents
collide
2. Can produce new mountain ranges such as the Himalayas
c. Transform fault boundaries
1. Plates slide past one another
a. No new crust is created
b. No crust is destroyed
2. Transform faults
a. Most join two segments of a mid-ocean ridge
b. At the time of formation, they roughly parallel the direction of plate
movement
c. Aid the movement of oceanic crustal material
E. Evidence for the plate tectonics model
1. Paleomagnetism
a. Probably the most persuasive evidence
b. Ancient magnetism preserved in rocks
c. Paleomagnetic records show
1. Polar wandering (evidence that continents moved)
2. Earth's magnetic field reversals
a. Recorded in rocks as they form at oceanic ridges
b. Record of reversals across ocean ridges confirms seafloor spreading
2. Earthquake patterns
a. Associated with plate boundaries
b. Deep-focus earthquakes along trenches provide a method for tracking the
plate's descent
3. Ocean drilling
a. Deep Sea Drilling Project (ship: Glomar Challenger)
b. Age of deepest sediments
1. Youngest are near the ridges
4 Geology Module: Seismic Interior Lecture Outline
2. Older are at a distance from the ridge
c. Ocean basins are geologically young
4. Hot spots
a. Rising plumes of mantle material
b. Volcanoes can form over them
1. Example: Hawaiian Island chain
2. Chains of volcanoes mark plate movement
F. Measuring plate motion
1. By using hot spot “tracks” like those of the Hawaiian Island–Emperor Seamount
chain
2. Using space-age technology to directly measure the relative motion of plates
a. Very Long Baseline Interferometry (VLBI)
b. Global Positioning System (GPS)
G. Driving mechanism of plate tectonics
1. No one model explains all facets of plate tectonics
2. Earth's heat is the driving force
3. Several models have been proposed
a. Slab-pull and slab-push model
1. Descending oceanic crust pulls the plate
2. Elevated ridge system pushes the plate
c. Plate–mantle convection
1. Mantle plumes extend from mantle–core boundary and cause convection
within the mantle
2. Models
a. Layering at 660 kilometers
b. Whole-mantle convection
c. Deep-layer model
5 Geology Module: Seismic Interior Lecture Outline
A Few Notes…
A. The asthenosphere, located between 70 to 700 kilometers deep,
consists of approximately 10 percent melted rock. This zone lies wholly
within the mantle. The lithosphere lies above the asthenosphere and
includes the crust and part of the upper mantle (that part above the
asthenosphere). The asthenosphere behaves plastically, whereas the
lithosphere is rigid.
B. P waves are bent and slowed upon entering the outer core, producing a
shadow zone, and S waves cannot pass through liquids. Thus, geologists
concluded the outer core is molten.
C. Sea-floor spreading refers to the creation of new sea floor at the oceanic
ridges along with its conveyor belt movement away from the ridge crests.
Sea-floor spreading occurs today along the oceanic ridges. Subduction
zones occur in the deep-ocean trenches where slabs of oceanic crust are
descending into the mantle. Subduction zones are associated with
convergent plate boundaries.
D. The Himalayan Mountains formed as a result of a collision between the
Indian landmass and the Asian continent.
6