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Study Guide Exam II Spring 2008 |
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Plate Tectonics: A
Scientific Theory Unfolds______________________________________________________ |
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I. 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 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 II. 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) 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 b.
Cascades c.
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.
b.
c.
c.
Continental–continental convergence 1.
When subducting plates contain continental material, two continents collide 2.
Can produce new mountain ranges such as the 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 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: 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 |
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Earthquakes and
Earth’s Interior_______________________________________________________________ |
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I.
Earthquakes A.
General features 1.
Vibration of Earth produced by the rapid release of energy 2. Associated with movements along faults a.
Explained by the plate tectonics theory b. Mechanism for earthquakes was first
explained by H. F. Reid in the early 1900s 1.
Rocks "spring back" a. Phenomenon called elastic rebound b.
Vibrations (earthquakes) occur as rock elastically returns to its original
shape c. Fault creep 3.
Often preceded by foreshocks 4.
Often followed by aftershocks B.
Earthquake waves 1.
Study of earthquake waves is called seismology 2.
Earthquake-recording instrument (seismograph) a.
Records movement of Earth b.
Record is called a seismogram 3.
Types of earthquake waves a.
Surface waves 1.
Complex motion 2.
Slowest velocity of all waves b.
Body waves 1.
Primary (P) waves a.
Push-pull (compressional) motion b.
Travel through 1.
Solids 2.
Liquids 3.
Gases c.
Greatest velocity of all earthquake waves 2.
Secondary (S) waves a.
"Shake" motion b.
Travel only through solids c.
Slower velocity than P waves C.
Locating an earthquake 1.
Focus—the place within Earth where earthquake waves originate 2. Epicenter a.
Point on the surface directly above the focus b.
Located using the difference in the arrival times between P and S wave
recordings, which are related to distance c.
Three station recordings are needed to locate an epicenter 1.
Circle equal to the epicenter distance is drawn around each station 2.
Point where three circles intersect is the epicenter 3.
Earthquake zones are closely correlated with plate boundaries a. Circum-Pacific belt b.
Oceanic ridge system D.
Earthquake intensity and magnitude 1.
Intensity a.
A measure of the degree of earthquake shaking at a given locale based on the
amount of damage b.
Most often measured by the Modified Mercalli Intensity Scale 2.
Magnitude a.
Concept introduced by Charles Richter in 1935 b.
Often measured using the Richter scale 1.
Based on the amplitude of the largest seismic wave 2. Each unit of Richter magnitude equates
to roughly a 32-fold energy increase 3.
Does not estimate adequately the size of very large earthquakes c.
Moment magnitude scale 1.
Measures very large earthquakes 2.
Derived from the amount of displacement that occurs along a fault zone E.
Earthquake destruction 1.
Factors that determine structural damage a.
Intensity of the earthquake b.
Duration of the vibrations c.
Nature of the material upon which the structure rests d.
The design of the structure 2.
Destruction generated a.
Ground shaking b.
Liquefaction of the ground 1.
Saturated material turns fluid 2.
Underground objects may float to surface c.
Tsunami, or seismic sea waves d.
Landslides and ground subsidence e.
Fires F.
Earthquake prediction 1.
Short-range—no reliable method yet devised for short-range predictions 2.
Long-range forecasts a.
Premise is that earthquakes are repetitive b.
Region is given a probability of a quake II. 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 |