Download - Catalyst

To understand plate tectonics one must understand the dynamics
between the solid, rigid lithosphere that is “floating” on a partially
molten asthenosphere.
The rigid lithosphere is composed of continental and ocean crust
and the uppermost solid mantle.
The plastic asthenosphere is composed of the upper mantle and
is at or near the melting temperature for upper mantle rock.
The plate tectonic system is driven by convection within the
asthenosphere. Convective upwelling, and partial melting of the
asthenosphere, along divergent margins produces new ocean
lithosphere, while it is subducted (consumed) along convergent
margins.
The lithosphere is broken up into seven or eight major tectonic
plates and several minor plates. Plate boundaries are defined
by the relative motion between adjacent plates: divergent,
convergent or transform.
Seismicity and plate tectonics. Note the relationship of
depth of foci of earthquakes and tectonic plate boundary.
Relationship between active volcanism and tectonic plate
boundaries.
The plate tectonic system is driven by convection within the
asthenosphere. Convective upwelling, and partial melting of the
asthenosphere, along divergent margins produces new ocean
lithosphere, while it is subducted (consumed) along convergent
margins.
Divergent margins first form under
continental crust because of the
insulating properties. When the
continental crust is first heated at
its base what will its topographic
expression look like at the
surface? Think of East Africa.
Eventually, the upwelling basaltic
magma will completely melt
through the overlying continental
crust and an incipient ocean basin
will form and the two segments of
continental crust will rift apart.
Note that the ocean floor becomes
wider as the new ocean
lithosphere is produced at the
devergent margin (referred to as
sea floor spreading).
Basalt Flows
The East African rift zone represents a continental rift. The topography of
East Africa is highest in Africa because of heating the base of the continent
by upwelling basaltic magma. The Red Sea is an incipient ocean basin.
Note the dark basalt flows seen along the margins of the Red Sea in the
above satellite image.
Midocean ridge spreading and volcanism in Heimey,
Iceland.
We know that the Earth is not increasing in size due to the creation of new ocean
lithosphere along seafloor spreading zones; thus it stands to reason that the ocean
lithosphere must be consumed along convergent boundaries.
Ocean-Ocean lithosphere
convergence (subduction).
Examples: Japan and Phillipines
Ocean-Continent lithosphere
convergence (subduction).
Examples: Andes Mountains and
Cascade Mountains.
Continent-Continent lithosphere
collision (no subduction).
Examples: Himalaya Mountains
and European Alps.
Ocean-Ocean convergence. Formation of an island arc (Japan). What is the
source of the rising magma (What components are partially melting)?
Ocean-Continental Convergence and formation of a continental volcanic arc. What is
the source of magma produced along a continental arc? Is this consistent with the
similar composition of volcanic rocks collected from Japan and the Cascades?
Cascade volcanoes related to subduction
zone tectonics along the coastline of
northern Californial to southern British
Columbia.
What is the relationship between the depth of earthquake foci and
the distance from the deep sea tranch along the subduction zone?
Why do earthquakes cease at 600 km?
Continental collisions are
preceded by ocean-continental
convergence.
As the continental lithosphere
arrives at the subduction zone,
it cannot be subducted because
of its low density.
The “collision” cause
upwarping and deformation of
the ocean floor and ocean
lithosphere. Even segments of
the upper mantle can be
squeezed to the surface along
the suture zone (where the two
continents are stitched
together).
The Indian tectonic plate collided
with the Eurasian plate and caused
the uplift of the Himalaya
Mountains.
Transform boundaries typically form along mid-ocean rift zones, where the spreading
rate differs and rigid crust is offset along the transform fault. Why do you think the
earthquake foci are shallow along transform faults?
San Andreas Fault,
California
Faults form where differential offset occurs between rigid rock or curst. Fault
components are shown above. Note the difference between the focus and
epicenter of a fault.
Energy is stored in the rock and then suddenly released when the
stress exceeds the strength of the rock. When an earthquake
occurs four different types of seismic waves are produced.
Seismic waves are energy waves that
can move through the earth’s interior
(Body Waves) or at the earth’s surface
(surface waves).
P- and S- waves are body waves and
have higher frequencies and lower
amplitudes than surface waves.
P-waves have the highest velocities
and are compression waves and
particles move in the same direction as
the wave. S-waves are shear waves
and move particles up and down
relative to wave direction.
Surface waves are slowest of the
seismic waves and have the largest
amplitudes. Surface waves cause
almost all the damage and destruction
from earthquakes.
Differential P- and S- wave
velocities coupled with
differential arrival times at a
given seismic station can be
used to determine the
distance from the seismic
station to earthquake focus.
Note the relationship
between seismic wave
amplitude and wave
frequency.
Wave amplitude and
frequency are components
of the total energy release.
Earthquake magnitude must be shown on a logrithmic scale.
Each increase in magnitude equates to a ~30-fold increase in total
energy release.
The focus (and epicenter) can be located by triangulation using
three different seismograph stations.
Seismic waves are generated naturally by earthquakes and anthropogencally
by detonation of nuclear bombs or other explosives below the ground surface.
Seismic risk in the United States
(%g) acceleration due to force of gravity due to ground shaking.
What is the relationship between seismic risk and tectonic setting
in the United States?
Estimated fatalities from the 10 deadliest earthquakes in history.
1.Shensi, China. Jan. 23, 1556. Magnitude 8 – 830,000 fatalities.
1.Tangshan, China. July 27, 1976. Magnitude 7.5 – 655,000 fatalities.
1.Aleppo, Syria. Aug. 9, 1130. Magnitude ? – 230,000 fatalities.
1.Sumatra, Indonesia. Dec. 26, 2004. Magnitude 9.1 – 227,898 fatalities.
1.Haiti, Jan. 12, 2010. Magnitude 7.0 – 222,570 fatalities.
1.Damghan, Iran. Dec. 22, 856. Magnitude ? - ~200,000 fatalities.
1.Haiyuan, Ningxia, China. Dec. 16, 1920. Magnitude 7.8 - ~200,000 fatalities.
1.Ardabil, Iran. March 23, 893. Magnitude ? - ~150,000 fatalities.
1.Kanto, Japan. Sept. 1, 1923. Magnitude 7.9 – 142,000 fatalities.
1.Ashgabat, Turkmenistan. Oct. 5, 1948. Magnitude 7.3 – 110,000 fatalities.