Transcript Document

Geological Processes
Overview
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Meet the Earth
Development of plate
tectonic theory: what is right
isn’t always popular
Supporting evidence of the
theory of continental drift
and plate tectonics
Three main types of plate
boundaries- fun activity
Volcanoes and earthquakes
Layers of the
Earth
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Continental crust is an average of 3040 km thick (can be up to 100 km
thick on the craton)
Oceanic crust is ~6-7 km thick
The uppermost mantle behaves
rigidly (like a solid), and together with
the overlying crust is referred to as
the lithosphere
Underneath the lithosphere is the less
rigid asthenosphere, which is partially
molten
Below about 200 km the mantle is
fully solid
The Core:
 core is larger than Mars!
 made up of two distinct layers: a
liquid outer layer and a solid
inner core.
 made up of metallic iron-nickel
alloy
What is a plate?
Source:
www.nature.nps.gov/.../
usgsnps/pltec/pltec1.ht
ml
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a massive, irregularly shaped slab of solid rock, generally composed of both
continental and oceanic lithosphere.
Most of the boundaries between individual plates cannot be seen, because they are
hidden beneath the oceans.
Quaking and shaking…
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Humans are curious…..
Why is the Earth so restless?
What causes the ground to shake violently?
Why do volcanoes erupt?
Why are our mountain ranges so tall?
Early thoughts…
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Until the 1700s, most Europeans thought that a Biblical
Flood played a major role in shaping the Earth's
surface.
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“Catastrophism” : the belief that all earthly changes were
sudden and caused by a series of catastrophes.
Mid-19th century: catastrophism =>
"uniformitarianism"
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"Uniformitarian Principle" (James Hutton, 1785):
THE PRESENT IS THE KEY TO THE
PAST
Geologic forces that act on the Earth today are the same as
those that have acted in the geologic past.
Continental Drift
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Predecessor to
modern plate
tectonics
Shape and “fit” of
the continents was
the initial evidence
Snider-Pelligrini
(1858): ideas were
mostly ignored
 Wegner (1915)
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1858: Snider-Pelligrini drawings
Source: http://pubs.usgs.gov/gip/dynamic/historical.html
Continental Drift
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Alfred Wegener (1880-1930)
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Proposed all of the continents
were once part of a large
supercontinent - Pangaea
Based on:
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Source: http://pubs.usgs.gov/gip/dynamic/wegener.html
Similarities in shorelines
Distinctive rock and fossil
groups found in Africa &
South America
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Ideas were not widely accepted
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Froze to death in 1930 during an
expedition crossing the Greenland
ice cap
Evidence for Continental Drift
Source:
http://pubs.usgs.gov/gip/
dynamic/continents.html
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Paleontological: Similarity of fossils on opposite sides of the Atlantic and
Indian Oceans, ex. Mesosaurus, Glossopteris
Problem: Perhaps similar animals had evolved in these areas (convergent
evolution). Perhaps there were floating pieces of crust that rafted
animals and plant seeds across? Transoceanic land bridges?
Evidence for Continental Drift
Source:
http://pubs.usgs.gov/gip/
dynamic/continents.html
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Rock types and structures: Distinct rock type and geologic
structures on both sides of the Atlantic Ocean
Problem: Erosion and weathering breaks down rocks over
time. How could the continents still “fit” together after
millions of years of wear and tear?
Evidence for Continental Drift
Source:
http://pubs.usgs.gov/gip/
dynamic/continents.html
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Paleoclimate
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Evidence of extreme changes in climate as compared to the
present (glacial deposits in present day deserts)
Problem: Perhaps there was a total global cooling event
Development of Plate Tectonic
Theory
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Wegener’s theory was dismissed as being
eccentric, preposterous, and improbable.
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Original evidence for continental drift was
ONLY from continental rocks
Technological advances in the 1950’s and 1960’s
allowed investigation of the sea floor and
oceanic rocks
Geophysics & paleomagnetism provided new
data
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Harry Hess- Sea floor spreading
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From the 1940’s to the 60’s, Harry Hess made many key
intellectual contributions to the coming revolution in
geologic thought:
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Echo-sounding of sea floor revealed deep sea features
and the topography of mid-ocean ridges
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“Father of Plate Tectonics”
Helped provide more evidence to support Wegener’s theories
ridges are areas of high heat flow and volcanic activity
young age of ocean floor, based on thickness of sediment
Hot material rose out of ridges (spreading centers) and
moved across the sea floor, crust was recycled.
Earth’s magnetic field provides more
evidence
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The Earth has an invisible
magnetic field (all free-floating
magnets at the Earth’s surface
point to magnetic north)---Think
about a compass.
Iron-rich minerals crystallizing
from molten rock will orient
towards magnetic north when they
cool below the Curie point, the
temperature above which
permanent magnetism is
impossible (580 degrees C for
magnetite).
Thus lavas lock in the record of
Earth’s magnetic field when
they form.
Magnometers
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Adapted from airborne devices
developed during World War II to
detect submarines
Used to recognize odd magnetic
variations across the ocean floor.
Discovered magnetic “stripes” on
the sea floor
 Rocks formed over time will
record either a “normal”
magnetic orientation or
“reversed”
 Magnetic reversals are global
phenomena and can be used for
global stratigraphic correlation
http://pubs.usgs.gov/gip/dynamic/developing.html
Evidence for plate tectonics
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Demonstration of the ruggedness and youth of the
ocean floor
Confirmation of repeated reversals of the Earth
magnetic field in the geologic past
Emergence of the seafloor-spreading hypothesis and
associated recycling of oceanic crust
Precise documentation that the world's earthquake and
volcanic activity is concentrated along oceanic trenches
and submarine mountain ranges
So…why do the plates move?
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Plate tectonics is:
 a geological theory
 that pieces of Earth’s
lithosphere are in constant,
slow motion, driven by
convection currents in the
mantle.
 explains the formation,
movement, and subduction of
Earth’s plates.
The plates of the lithosphere float
on top of the asthenosphere.
Convection currents rise in the
asthenosphere and spread out
beneath the lithosphere.
HEAT
Types of Plate Boundaries
http://cimss.ssec.wisc.edu/sage/geology/lesson2/images/concepts_fig8.gif
Plate boundaries
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Divergent: plates are pulling away from each
other
Convergent: Plates are colliding and one is
subducted under the other
Transform: Plates are sliding past each other
Snickers Bar Activity
Snickers Bar Activity- Step 1
Obtain a Snickers bar. Unwrap the bar and make a few
cracks across the top middle portion of its top.
What does the chocolate on each side of the cracks
represents.
a tectonic plate
The Earth is covered with several tectonic plates. We
live on the North American Plate
What does the nougat/ caramel portion represent?
The asthenosphere
Snickers Bar Activity- Step 2
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Hold each end of the candy bar with your
thumb and forefinger and SLOWLY stretch the
candy bar, pulling it no more than 1/2 to 1 inch
apart.
What kind of plate boundary have you made?
A Divergent Plate boundary
Snickers Bar Activity- Step 3
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Push the candy bar back together. Continue
adding pressure until one piece of chocolate
moves beneath another.
What kind of plate boundary have you made?
A Convergent Plate boundary
Snickers Bar Activity- Step 4
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Pull the bar back to its normal shape. Push one
side forward and pull one side back. What kind
of plate boundary have you made?
A Transform Plate boundary
The Consequences of
Plate
Movement…volcanoes
and earthquakes
What is a volcano?
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A place on the earth’s surface
(or any other planet) where
molten rock and gases are
erupted.
There are, however, many
kinds of volcanoes.
Volcano does not have to be
a beautiful snowcapped
conical peak (Mt St Helens).
It can be a hole in the
ground, or a crack in the
earth’s surface
Most volcanoes occur at
convergent boundaries
Types of Volcanoes
Why do volcanoes erupt?
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Hot, molten rock (magma) is buoyant (has a lower density than
the surrounding rocks) and will rise up through the crust to
erupt on the surface.
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When magma reaches the surface it depends on how easily it
flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it
as to how it erupts.
Large amounts of gas and a high viscosity (sticky) magma will
form an explosive eruption!
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Same principle as hot air rising, e.g. how a hot air balloon works
Think about shaking a carbonated drink and then releasing the cap.
Small amounts of gas and (or) low viscosity (runny) magma will
form an effusive eruption
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Where the magma just trickles out of the volcano (lava flow).
Volcanic Hazards
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Pyroclastic flow
Lahars/Mud flows
Pyroclastic and ash fall
Lava flow
Noxious Gas
Earthquakes: Large
volumes of magma
moving through the
shallow crust can cause
large earthquakes.
What is an earthquake?
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a tremor of the earth's
surface usually triggered
by the release of
underground stress along
fault lines. This release
causes movement in
masses of rock
Epicenter and Focus
Earthquake
waves
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Body Waves
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Primary or Compressional: P- waves
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Secondary or Shear: S- waves
Surface Waves (won’t discuss these)
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Raleigh (large vertical displacements)
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Love (shear)
How do we measure earthquakes
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Richter scale
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invented by Charles F. Richter in 1934.
calculated from the amplitude of the largest seismic wave recorded for
the earthquake, no matter what type of wave was the strongest.
based on a logarithmic scale (base 10). What this means is that for each
whole number you go up on the Richter scale, the amplitude of the
ground motion recorded by a seismograph goes up ten times.
Mercalli scale.
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invented by Giuseppe Mercalli in 1902
uses the observations of the people who experienced the earthquake to
estimate its intensity.
isn't considered as scientific as the Richter scale
witnesses of the earthquake might exaggerate just how bad things were
during the earthquake and you may not find two witnesses who agree on
what happened; everybody will say something different.
Seismograph
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How we measure
earthquake waves
Determines depth and
magnitude of quakes
Earthquake fact or
fiction
Mega- Quakes, as seen in “10.5”
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The truth: THEORETICALLY, YES.
REALISTICALLY, NO.
The magnitude of an earthquake is related to the length of the
fault on which it occurs
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-- the longer the fault, the larger the earthquake.
No fault long enough to generate a magnitude 10.5 earthquake is
known to exist.
The largest earthquake ever recorded was a magnitude 9.5 on
May 22, 1960 in Chile on a fault that is almost 1,000 miles long.
We can’t rule out a Mega-Quake because diagnostic equipment
has only been around for about 100 years, insignificant when
compared to geologic time.
California has the most earthquakes
in the US
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The truth: Alaska registers the most earthquakes in a
given year.
California= the most damaging earthquakes because
of its greater population and extensive infrastructure.
Alaska’s large earthquakes occur in remote locations
such as along the Aleutian Island chain.
Florida and North Dakota have the fewest earthquakes
each year.
The ground can open up during an
earthquake.
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The truth: NO WAY!!! Gaping faults exist only
in movies and novels. The ground moves across
a fault during an earthquake, not away from it,
due to friction. If the fault could open, there
would be no friction. Without friction, there
would be no earthquake. Shallow crevasses can
form during earthquake induced landslides,
lateral spreads, or other types of ground failures,
but the fault line itself does not open.
California will eventually fall into the
ocean.
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Sorry Lex Luthor… NO WAY!! It’s absolutely
impossible that California will be swept out to sea.
Instead, southwestern California is moving horizontally
northward towards Alaska as it slides past central and
eastern California. California sits on the Pacific Plate
which is moving to the northwest with respect to the
North American Plate at approximately 46 millimeters
(two inches) per year (the rate your fingernails grow).
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In about 15 million years, Los Angeles and San Francisco
will be next-door neighbors
In about 85 million years, Los Angeles residents will find
themselves with an Alaska zip code!
People can cause earthquakes.
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A glimmer of hope for Lex Luthor…PARTIALLY
FACT: Earthquakes induced by human activity have
been documented in a few locations in the United
States, Japan, and Canada. The cause was injection of
fluids into deep wells for waste disposal and secondary
recovery of oil, and the filling of large reservoirs for
water supplies. Most of these earthquakes were minor.
Deep mining can cause small to moderate quakes and
nuclear testing has caused small earthquakes in the
immediate area surrounding the test site, but other
human activities have not been shown to trigger
subsequent earthquakes.
We can predict earthquakes.
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IF ONLY… Earthquake prediction is the holy
grail for earthquake scientists, but there
currently is no accepted method to accomplish
the goal of predicting the time, place and
magnitude of an impending quake. Most
research is focused on long range (next 30 years)
, not 1-5 years. We can predict the type of
ground motion to expect based on the geology
and the history of earthquake activity of the
region, but not when the motion will occur.