travel within the upper few kilometers of the Earth’s surface; responsible for most surface earthquake damage
transmitted in all directions through the interior of the Earth
P (primary) body waves
compressional in nature;
Particles move back and forth (longitudinal compressional) in the same direction as the wave is traveling
can travel through solids, liquids, and gases
travel faster than S waves and arrive earlier at the seismic station; difference in arrival times between the P and S waves allow geophysicists to determine the focus of the earthquake
refracted at density boundaries
S (secondary) body waves
transverse in nature;
Particles move at right angles to the direction of wave movement
will only travel through solids; do not travel through the liquid outer core
solid and is composed of iron (85%) and nickel
same composition as the inner core, but is liquid
composition is distinctly different from the outer core
The thin solid outer layer where we live
Mohorovicic discontinuity (Moho)
A sharp compositional boundary that exists between the base of the crust and the upper mantle
outermost rigid, brittle layer
Composed of the entire crust and uppermost mantle
Most faults and earthquakes occur here
lies beneath the crust, extending down to approximately 70 km.
Due to its high temperature this layer is plastic, mobile, and is essential for tectonic plate motion
Continental Drift Theory
Alfred Wegener (early 1900s) proposed that about 200 million years ago all the continents were together in a supercontinent he called Pangea, which broke apart and the newly formed continents slowly drifted apart
was not widely accepted at the time: couldn't explain how continental crust could “move” through the oceanic crust
continental drift evidence
Biological evidence: Present-day biological species on widely separated continents have similarities that suggest that these land masses were once together;
Identical fossil plants and animals have been found on a number of continents
Continuity of geologic features: if continents of North America, Europe, South America, and Africa were all put back together the continuity of several geologic features would become evident
Glacial evidence suggests that the southern areas of South America, Africa, India, and Australia were covered with glaciers 300 million years ago
Harry Hess (1960) suggested a viable mechanism that could explain continental drift: the seafloor slowly spreads by moving sideways away from the mid-ocean ridges; New magma wells up and cools as each side of the mid-ocean ridge slowly splits apart; The entire ocean floor can be viewed as a giant conveyor belt where the new seafloor moves away from the ridges, and eventually descends back into the mantle at the trenches; ridge spreading rates are in the range of a few centimeters per year
magnetite (Fe3O4) crystallizes (at cooling) it becomes magnetized in the direction of the Earth’s prevailing magnetic field
the Earth’s magnetic field has abruptly and frequently reversed itself during geologic time
Remanent magnetism of the ocean crust reveals long, narrow, symmetric bands of them on either side of the Mid-Atlantic Ridge
indicate that the Mid-Atlantic Ridge has been continuously spreading and that the Earth’s magnetic field has reversed itself many times
modern theory of plate tectonics (supporting theories)
two ideas have now been merged:
Wegener’s original evidence (biologic, paleontologic, geologic, and glacial) supports the theory of continental drift.
Hess’s theory and evidence (remanent magnetism) supports the idea of seafloor spreading.
We now know that both the oceanic and continental crusts are carried as part of a thicker layer called the lithosphere
modern theory of plate tectonics (processes)
We now visualize ocean basins to be in a constant cycle with new crust being created at the mid-ocean ridges and old crust descending along the ocean trenches.
We also know that the lithosphere is composed of a series of solid segments called plates.
These plates are constantly moving and interacting with other plates.
The lithosphere is divided into approximately 20 plates.
divergent plate boundaries
two plates are moving apart (spreading)
creates new crust: new magma buoyant --> cools and sinks (rift zones)
convergent plate boundaries
zones along which two plates are driven together, one plate is consumed (subduction)
ocean-ocean: trenches, volcanic island arcs (Japan)
boundaries along which two plates slide horizontally past one another
faults: San Andreas, Dead Sea
plates not consumed
the concept that the solid lithosphere floats in gravitational equilibrium (buoyancy) on the plastic asthenosphere (iceberg)
Continental plates float higher because they are less dense than oceanic plates.
At any given time, all of the plates are in isostatic equilibrium.
Mountain ranges simply represent thicker masses of continental material and therefore float higher
Unequal temperature distribution within the asthenosphere and upper mantle results in the hot, less dense material rising, and the cooler, more dense material sinking
one of the plates descends beneath the other plate
Two oceanic plates will have essentially the same density
When two oceanic plates collide one is eventually subducted beneath the other: Long narrow deep see trenches mark the zones where the plate is subducted.
The plate subducted begins to melt as it comes in contact with the asthenosphere.
Molten material begins to rise, forming a volcanic island arc on the overriding plate (Japan)
continental crust is less dense, the oceanic crust that is always subducted
A trench will develop along the zone where the oceanic crust is subducted.
As the oceanic crust descends toward the asthenosphere it begins to melt.
Magma rises up through the overriding continental plate forming volcanic mountain ranges at the surface (Andes and Cascades)
Continental plates have a relatively low density, Subduction of continental crust is minimal
During convergence the plate edges are intensely deformed to construct fold-mountain ranges.
Continents can increase in size during this process by suturing themselves together along fold-mountain systems
Alps, Himalayas, Appalachians
transform plate boundaries
Linear zones where adjacent plates slide past each other in opposite directions: zone of shearing, or transform motion; Crust is not destroyed or created along a transform boundary since neither subduction nor magma upwelling occur.
Periodic movements along these faults result in sudden energy release and repeated earthquakes.
These zones are said to be seismically active;locations of many of the world’s longest continuous faults
can refer to either a vent from which hot molten material escapes or a mountain created by solidified volcanic rock
vast majority along plate boundaries (mainly convergent)
sudden release of energy due to a sudden movement in the Earth’s crust or mantle, resulting from stresses
cause the Earth’s surface to vibrate and sometimes result in violent movements, depending on the amount of energy released
occur when rocks grind past each other along plate boundaries; vibrations radiate out in all directions from the disturbance.
The major danger is the human-made structures that collapse
the study of earthquakes
causes of earthquakes (I)
Most earthquakes are caused by movements of the lithospheric plates;
can also result from explosive volcanic eruptions or by human-caused explosions.
Movements of lithospheric plates generally cause faults in the crustal material
a fracture in rock along which there has been visible movement of the two sides relative to each other
causes of earthquakes (II)
most likely to occur along plate boundaries.
Stresses are exerted on the rock formations in adjacent plates, as movement occurs; Since rock possess elastic properties, energy is stored until the stresses can overcome the friction between the two plates; At the moment of energy release, the rocks along the fault suddenly move, the energy is released, and an earthquake occurs.
san Andreas fault
the master fault of an intricate fault zone that runs along the coastal area of south and central California.
Many earthquakes have occurred along this fault
In about 10 million years Los Angeles will move far enough north to be adjacent with San Francisco
The point of the initial movement, or energy release, along the fault
generally located underground; from a few miles to perhaps several hundred miles in depth
The point on the Earth’s surface directly above the focus
the surface position that receives the greatest impact from the earthquake
When an earthquake occurs: the energy released from the focus propagates outward in all directions
monitors and measures the seismic waves.
The greater the energy released in the quake, the greater the amplitude (height) of the traces (lines) on the record
During a quake, the spool vibrates and the light beam is relatively still
measures the amount of absolute energy released during a quake by calculating the seismic wave energy at a standard distance
scale correlates the largest seismogram peak during a given quake to the amount of energy released by the quake
logarithmic: Each whole number increment represents a 10-fold increase in amplitude tracings.
Each whole number increment represents a 31-fold increase in energy release
modified Mercalli scale
describes the results of the earthquake in terms of felt and observed effects
Richter scale drawback
the magnitude of the earthquake gives no indication of the damage it may cause
related earthquake damage
When a submarine earthquake occurs: some of the energy may be release into the water to form huge waves
the buckling, fracturing, or shifting of rock units
folding and faulting
buckling of the rock layers into anticlines (arches) and synclines (toughs)
occurs when slow compressive forces apply extreme pressures on the rock layers
occurs mainly during the early stages of mountain building
If the compression is vertical uplifts are produced.
If the compression is horizontal the crust will be shortened
Tension causes the crust to lengthen
an approximately planar surface along which the actual movement takes place
this is the fault block that is on the uppermost side of an inclined fault plane
this is the fault block that is on the lowermost side of an inclined fault plane
The fault block that has moved up relative to the other side
the hanging wall (uppermost side) moves down with respect to the footwall
Tensional forces (pull-apart) cause normal faults
the footwall (lowermost side) moves down with respect to the hanging wall
Compressive forces cause reverse faults
strike-slip (transform) fault
stresses are parallel to the fault plane (horizontal motion)
volcanic, fault-block, fold
volcanic mountains (ocean-ocean)
primarily formed through a series of volcanic eruptions.
Most are located along convergent boundaries, since that is where most volcanoes occur.
Along an oceanic-oceanic convergent boundary, chains of volcanic islands will form on the plate overlying the subduction zone.
The Aleutian Islands, Japan, and the Lesser Antilles
volcanic mountains (ocean-continent)
continental plate always overlies the subduction zone and this is where they will form.
Andes Mountains of South America were formed as the oceanic Nazca plate is subducted beneath the continental South American plate.
The Cascade Mountains
Normal faulting can produce tilting and uplift of large crustal blocks.
results in dramatic mountains rising abruptly above the surrounding lowlands.
The Grand Tetons of Wyoming, the Sierra Nevada Mountains of California, and the Wasatch range of Utah
characterized by prolific folding of the rock strata.
The Alps, Himalayas, and Appalachians
also characterized by thick packages of marine sedimentary strata that was originally deposited below sea level and then uplifted and incorporated into them
Marine fossils are regularly found high in a fold mountain range
formation of the Himalayas (I)
During the breakup of Pangea (200 m.y.a.) the subcontinent of India broke away from Africa.
As the Indian plate moved north toward Asia, oceanic lithosphere was continually subducted beneath Eurasia.
During the time before continental collision, sediments were deposited in the marine waters between India and Eurasia
formation of the Himalayas
The sedimentary strata that was deposited between Eurasia and India was eventually uplifted and folded into the mountains.
When the continental crust portions finally collided, subduction was significantly slowed.
The edge of the Eurasian plate was uplifted as the continental Indian plate wedged under it.
The Indian plate continues to move north today, resulting in continued uplift of the Himalayas
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