Transcript Chapter 21 - "Building Earth`s Surface"

• Building Earth’s Surface
• An aerial view from the
south of the eruption of
Mount St. Helens
volcano on May 18,
1980.
• Interpreting Earth’s Surface
• Observations
– Today we noticed all kinds of aspects of Earth that seem
to need an explanation.
– Some have in the past thought that these aspects of the
Earth had always been there
– Others thought that they were a result of some
catastrophic event in the Earths history, causing sudden
change.
• Would you believe that this rock island has "always" existed
where it is? Would you believe it was formed by a sudden,
single event? What evidence would it take to convince you
that the rock island formed ever so slowly, starting as a part
of southern California and moving very slowly, at a rate of
cm/yr, to its present location near the coast of Alaska?
• Principle of Uniformity.
– This is the frame of reference that is used today to
understand changes in the Earth’s surface over time.
– This principle helps us to understand what has shaped the
Earth through those processes that are still occurring
today.
• Diastrophism
• Introduction
– Diastrophism describes all of the movements of the
Earth’s plates.
• Movement relative to other plates
• Isostatic adjustment
• Deforming and changing th Earth’s surface
• Stress and Strain
– Four events associated with increased pressure
• As pressure begins to build up, the pressure is slight
and nothing visible is happening to the surface.
• As the pressure increases, eventually the surface will
begin to deform into a concave surface
• The concave shape builds up to the point where it is
no longer able to return to its original shape.
• The surface will rupture and begin to break at the
point of the increasing pressure, releasing the pressure.
– A stress is some force that can compress, pull apart, or
deform a rock and there are 3 basic types
• Compressive stress is caused when two plates move
together or when one moves and applies a force on
another one that is not moving.
• Tensional stress occurs when one part of a plate
moves away from another part of a plate.
• Shear stress occurs when two plates slide past one
another
– The adjustment to stress is called strain and there are 3
basic types
• Elastic strain occurs when rocks recover to their
original shape.
• Plastic strain occurs when rocks are molded or bent
under the stress and do not return to their original
shape.
• Fracture strain is when the rocks crack or break
under the stress.
• Stress and deformation relationships for deeply buried,
warm rocks under high pressure (A) and cooler rocks near
the surface (B). Breaking occurs when stress exceeds
rupture strength.
– How a rock responds to stress depends on 4 variables.
• Nature of the rock
• Temperature of the rock
• The period of time ever which the stress is applied to
the rock
• The confining pressure on the rock.
• Folding
– Sedimentary rock is usually buried as layers of rocks.
– Stress on these layers of rocks usually uplift the layer
together forming bends in the layer called folds.
– Symmetrical up and down folds can be produced by
widespread horizontal stress on sedimentary rock layers.
– A dome can be produced when a vertical upward stress
exerts on a rock layer
– A basin can be produced when there is a dome produced
– An arch shaped fold is called an anticline
– A trough shaped fold is called a syncline
• A syncline of any great extent is called a geosyncline
– When synclines and anticlines are not horizontal they are
called plunging folds
• (A)Rock bedding on a grand scale in the Grand Canyon. (B)
A closer example of rock bedding can be seen in this
roadcut.
• These folded rock layers are in the Calico Hills, California.
Can you figure out what might have happened here to fold
flat rock layers like this?
• Cross sections of
some types of
folds.
• (A)A sketch of an eroded structural dome where all the rock
layers dip away from the center. (B) A photo of a dome
named Little Sundance Mountain (Wyoming), showing the
more resistant sedimentary layers that dip away from the
center.
• An anticline, or
arching fold, in
layered
sediments. Note
that the oldest
strata are at the
center.
• A syncline, showing the reverse age pattern.
• (A)This sketch of a plunging fold shows an anticline on the
left and right and a syncline in the center. (B) A photo of a
plunging fold in Utah.
• Faulting
– Cooler rocks near the surface of the earth do not always
react to pressure by folding.
• This is due to the fact that they are cooler and
therefore more brittle.
– These cooler more brittle rocks can react to pressure by
faulting.
– A joint is when there is a break in the rock, but the rock
is not displaced on either side of the break
– A fault is when there is a break in the rock and the rock
on either side of the break is displaced.
• Can you see the recumbent folds in this mountain in
the Canadian Rockies?
• Columnar jointing forms at right angles to the surface as
basalt cools. (A) Devil's Post Pile, San Joaquin River,
California. (B) The Devil's Tower, Wyoming.
– The fault plane is the movement of the rocks on one side
of the fault relative to rocks on the other side of the fault.
– We describe faults in terms of:
• The steepness of the fault plane
• Direction of the relative movement
– Three basic ways in which rocks can move
• Dip is the movement of rock up and down
relative to the fault plane
• Strike is the horizontal or sideways movement
of rock relative to the fault plane.
• Oblique is the movement of rock both up and
down and sideways relative to the fault plane.
– When two rock planes move relative to one
another one is called the footwall as it has
dropped below and the other is called the hanging
wall as it is above the plane
– In a normal fault the hanging wall has moved
down relative to the footwall
– In a reverse fault the hanging wall has moved up
relative to the footwall.
– A reverse fault with a low angle fault plane is
called a thrust fault.
• (A)This sketch shows the relationship between the hanging
wall, the footwall, and a fault. (B) A photo of a fault near
Kingman, Arizona, showing how the footwall has moved
relative to the hanging wall.
• How tensional stress could produce (A) a normal
fault, (B) a graben, and (C) a horst.
• How compressive stress could produce (A) a reverse
fault, and (B) a thrust fault.
• Earthquakes
• Causes of Earthquakes
– An earthquake is a quaking, shaking, vibrating, or
upheaval of the ground.
• They occur as a result of the sudden release of
energy due to stress on rock that occurs beneath the
Earth’s surface.
– If the rock fractures as a result of the stress if produces
waves or vibrations that move out from the source as
seismic waves.
• Seismic waves are produced when a mass of rock
breaks and slides into a different position.
• These lines show fault
movement that has occurred
along the San Andreas and other
faults over the last 2 million
years.
– Most earthquakes occur along a fault plane and near
the Earth’s surface since this is where the rocks are
coolest and the most brittle.
– Elastic rebound is when the stress causes the rock to
break and snap into a new position.
• The rocks are displaced to new positions and the
released energy causes an earthquake.
• The elastic rebound theory of the cause of
earthquakes. (A) Rock with stress acting on it.
• (B) Stress has caused strain in the rock. Strain builds
up over a long period of time.
• (C) Rock breaks suddenly, releasing energy, with rock
movement along a fault. Horizontal motion is shown; rocks
can also move vertically.
• (D) Horizontal offset of rows in a lettuce field, 1979, El
Centro, California. (D) Photo by University of Colorado;
courtesy National Geophysical Data Center, Boulder,
Colorado.
• Locating and Measuring Earthquakes
– Focus
• This is the place where the seismic waves originate
below the surface of the Earth
– Epicenter
• The point on the surface of the Earth directly above
the focus.
– Seismograph
• An instrument used to measure the seismic waves.
• Simplified diagram of a fault, illustrating component
parts and associated earthquake terminology.
• A schematic of a seismograph that records horizontal
motion. The suspended mass remains motionless when the
earth vibrates, and the motion detector moves with the
seismic waves. The movements are recorded on a remote
recording drum.
– P-wave
• A compressional seismic wave that moves in a
longitudinal fashion
– S-wave
• A shear seismic wave that moves in a transverse
fashion.
– Surface wave
• A seismic wave that moves in an up and down fashion
with crests and troughs.
• Use of seismic waves in
locating earthquakes. (A)
Difference in times of
first arrivals of P-waves
and S-waves is a
function of the distance
from the focus. (B)
Triangulation using data
from several
seismograph stations
allows location of the
earthquake.
– Classification
• Shallow-focus
– Occur within a depth of 70 km
• Intermediate-focus
– Occur in the upper part of the Earth’s mantle
• Deep-focus
– Occur in the lower part of the upper mantle
– Most earthquakes are shallow-focus because:
• The rocks at the surface are cooler and more brittle
• There is the most resistance to movement of the plates
at the surface of the Earth.
• Measuring Earthquake Strength
– Mercalli scale
• Expresses relative intensities of earthquakes with
intensities ranging from I to XII.
• Level I is not felt by humans
• Level VI is felt by all
• Levels V through VII are concerned with levels of
damage
• Level XI means that few buildings are left standing
• Level XII means total destruction with waves moving
across the ground being visible to the eye.
– Richter scale
• Assigns numbers based on magnitude of the
earthquake
• Describe the severity of the vibrations and the energy
released during the earthquake.
– Moment magnitude
• A logarithmic scale based on the size of the fault and the
amount of movement along the fault.
– Surface-wave magnitude
• Based on the displacement of the Earth’s surface.
• Origin of Mountains
• Folded and Faulted Mountains
– The Earth’s surface is thickened in the areas where
mountains occur due to the compressional forces that
produce tight almost vertical folds.
– The Appalachian Mountain range was produced by
differential weathering which gives it its parallel features
– The Rocky Mountains have almost upright beds
produced by folding of sedimentary rock layers.
– The Black Hills of South Dakota were produced by a
broad folding arch called a dome.
• A domed mountain
begins as a broad,
upwarped fold, or
dome. The
overlying rock of
the dome is eroded
away, leaving more
resistant underlying
rock as hills and
mountains in a
somewhat circular
shape. These
mountains are
surrounded by
layers of the rock
that formerly
covered the dome.
• Fault block mountains are weathered and eroded as they are
elevated, resulting in a rounded shape and sedimentation
rather than sharply edged fault blocks.
• The folded
structure of the
Appalachian
Mountains,
revealed by
weathering and
erosion, is obvious
in this Skylab
photograph of the
VirginiaTennesseeKentucky
boundary area. The
clouds are over the
Blue Ridge
Mountains.
• Volcanic Mountains
– As volcanic material builds up on the surface of the
Earth, it can build to the point where it produces a
mountain.
– A volcano is a hill or mountain produced by the extrusion
of lava or rock fragments from the magma below the
Earth’s surface.
• This is the top of Mount St. Helens several years after the
1980 explosive eruption.
– 3 major types of volcanoes
• Shield volcanoes
– Broad, gently sloping cones made as magma
solidifies at the surface of the Earth.
– These originate from low viscosity lava which
spreads out quickly from the vent.
• Cinder cone volcano
– Made of cinders.
– These cinders are rock fragments that have
solidified from lava that cooled as it was thrown
into the air.
• Composite volcano
– Built of alternating layers of cinder, ash, and lava.
• (A)A schematic cross section of an idealized shield volcano.
(B) A photo of a shield volcano, Mauna Loa in Hawaii.
• (A)A schematic cross section of an idealized composite
volcano, which is built up of alternating layers of cinders,
ash, and lava flows. (B) A photo of Mount Shasta, a
composite volcano in California. You can still see the shapes
of former lava flows from Mount Shasta.
• The Juan
de Fuca
Plate, the
Cascade
volcanoes,
and the
Columbia
Plateau
Basalts.
– Batholith
• A large amount of lave that has crystallized below the
surface.
• If part of the batholith protrudes above the surface it is
called a stock.
– Dike
• An intrusion that flows into a joint or fault that cuts
across rock bodies.
– Sill
• When the intrusion flowed into the plane of contact
between sedimentary rock layers.
– Laccolith
• Similar to a sill, but with an arched top where the
intrusion has raised the overlying rock into a blisterlike uplift.
• Here are the basic intrusive igneous bodies that form
from volcanic activity.