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Physical Science, 8e
Chapter 19
Building Earth’s Surface
Start 18
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Core Concept
The surface of Earth
is involved in plate
tectonic processes
that result in an
ongoing building-up
of the surface.
Interpreting Earth’s Surface
• Principle of uniformity
– “The present is the key to the past.”
– Rocks are changed today by the same
processes that changed them in the past.
– Replaced catastrophic models of previous
thinkers
– People didn’t want to accept that Earth is
over 4 billion years old
– Catastrophic events contribute nonetheless
• Volcanoes, earthquakes, meteorite impacts, …
Diastrophism
• The process of deformation that
changes the Earth’s surface
• Produces structures such as plateaus,
mountains and folds in the crust
• Related to volcanism (the movement of
magma) and earthquakes
• Basic working theory is plate tectonics
Stress and Strain
Stress
Strain
•
•
•
•
Force tending to
compress, pull apart or
deform a rock
Three stress forces
1. Compressive stress
• Plates moving together
2. Tensional stress
• Plates moving apart
3. Shear stress
• Plates sliding past each
other
Adjustment to stress
Three strain types
1.
2.
3.
Elastic strain
• Returns to original
shape
Plastic strain
• Molded or bent
• Do not return to original
shape
Fracture strain
• Rock cracks or breaks
Stress and Deformation
• Possible material responses to stress
1. No change
2. Elastic change with recovery
3. Plastic change with no recovery
4. Breaking from the pressure
• Rock variables
1. Nature of the rock
2. Temperature of the rock (cold rocks tend
to break)
3. Speed of stress application
4. Confining pressure
5. Temperature & confining pressure
increase with depth
1. Rocks under these conditions undergo
plastic strain
6. Can better withstand compressional than
pulling apart
Folding
• Sedimentary rocks
– Originate from flat sediment
deposits
– Layers usually horizontal
• Folds
– Bends in layered bedrock
– Result of stress produced plastic
strain
– Widespread horizontal stress can
produce domes and basins
– Anticline: arch-shaped structure
– Syncline: trough-shaped
Folding
• Folds
– Bends in
layered bedrock
– Result of stress
produced plastic
strain
– Widespread
horizontal stress
can produce
domes and
basins
– Anticline: archshaped
structure
– Syncline:
trough-shaped
Faulting
• Fault
– Produced by relative
movement on opposite
sides of a crack
– Footwall: mass of rock
below the fault
– Hanging wall: mass of
rock above the fault
– Fault plane: surface
between the footwall and
hanging wall
• Joints are fractures with
no appreciable
displacement
Classes of Faults
• Normal fault
– Hanging wall has moved
down relative to the
footwall
– Caused by tension forces
– Related features
• (B) Graben
– Block surrounded by
normal faults drops down
• (A) Horst
– Block surrounded by
normal faults is uplifted
– Many fault block
mountains of west US
Other Faults
• Reverse fault
– Hanging wall moved upward
relative to footwall
– Result of horizontal
compressive stress
• Thrust fault
– Reverse fault with a lowangle fault plane
• Faults provide information on
the stresses producing the
formation
Earthquakes
• Quaking, shaking, vibrating or upheaval
of the ground
• Result from sudden release of energy
from stress on rocks
• Vibrations are seismic waves
• Most occur along fault planes when one
side is displaced with respect to the
other
Causes of Earthquakes
• Elastic rebound
theory
– Two plates press
tightly together
– Friction restricts
motion
– Stress builds until
friction or rock
rupture strength is
overcome
– Stressed rock
snaps suddenly
into new position
Locating and Measuring
Earthquakes
•
Focus
–
•
Epicenter
–
•
Actual origin of seismic waves
Location on Earth’s surface
directly above the focus
Seismograph
–
–
Instrument used to detect and
measure earthquakes
Detects three kinds of waves
1.
2.
P-wave (longitudinal)
S-wave (transverse)
3.
Surface wave (up and down)
Seismic Data
• P-waves travel
faster than S-waves
• Difference in arrival
times correlates to
distance from
earthquake
• Triangulation used
to pinpoint epicenter
and focus
Classification of Earthquakes
•
Based upon depth of focus
1. Shallow-focus earthquakes
•
•
Down to 70 km deep (within the depth of the continental crust)
85% of all earthquakes (surface rocks more brittle; more plate
friction near surface)
2. Intermediate-focus earthquakes
•
•
70 to 300 km deep
Upper part of the mantle
3. Deep-focus earthquakes
•
•
•
350 to 700 km deep
Lower part of upper mantle
About 3% of all earthquakes
Measuring Earthquake
Strength
• Effects: structural
damage to buildings,
fires, landslides,
displacement of land
surfaces, tsunami (tidal
wave)
• Mercalli scale
– Relative intensity
– I (not felt) to XII (total
destruction with visible
ground waves)
Measuring Earthquake
Strength
• Richter scale
– Based on swings
in seismograph
recordings
– Logarithmic
scale
– 3 (not felt);
9 (largest
measured so far)
Earthquake Safety
During the Shaking
• Don’t panic
• If indoors, stay there.
Stay away from glass.
Do not use any other
flames.
• If outside, move away
from buildings. Stay in
open.
• If in car, bring to a stop
as soon as possible but
stay in car.
After the Shaking
• Check but do not turn
on utilities.
• Turn on radio or TV.
• Stay off telephone
unless to report
emergency.
• Stay out of damaged
buildings.
• Don’t go sightseeing.
Origin of Mountains
•
Mountains
– Elevated parts of Earth’s crust rising
abruptly above the surrounding surface
– Created by folding and faulting of crust
– Three basic origins
1. Folding
2. Faulting
3. Volcanic activity
Folded and Faulted Mountains
• Domed mountains
– Begin as a broad
arching fold
– Overlying
sedimentary rocks
weather away,
leaving more
resistant granite
peaks
– Black Hills, SD
– Adirondacks, NY
Folded and Faulted Mountains
• Folded
sedimentary
rocks form
mountains
such as some
areas of the
Rockies
Folded and Faulted Mountains
• Fault block
mountains
– Rise sharply
(upthrust) along
steeply inclined fault
planes
– Weathering erodes
sharp edges
– The Tetons & Sierra
Nevadas
Volcanic Mountains
Volcano
• A hill or mountain
formed by the
extrusions of lava or
rock fragments from
magma below
• Structure: vent,
crater, lava flow
Types of Volcanoes
• Shield volcano
– Constructed of solidified lava flows
– Broad, gently sloping cones
– Hawaiian Islands
• Cinder cone volcano
– Constructed of rock fragments
(cinders)
– Steeper and smaller than shield
volcanoes
– Sunset Crater, Flagstaff AZ
• Composite volcano
– Alternating layers of cinders, ash
and lava flows with volcanic mud
– Cascades Mountains
Other Features
• Most magma remains
underground
• Cools and solidifies to form
intrusive rocks
• Batholith
– Large amount of crystalized
magma
– Stock: small protrusion from
a batholith
– Batholith intrusions can
cause hogbacks
• Related processes: dikes,
sills, laccoliths,…
Overall Picture
• Mountain ranges are
composites of many
different processes,
each uniquely
structured
– Folding
– Faulting
– Volcanic activity
• Especially apparent
along converging plate
boundaries