Download The Dynamic Crust

The Dynamic Crust
Topic 12
Topic 12 in Review Book
I. Evidence of Crustal
Motion
Original horizontality
A concept that assumes
that sedimentary rocks
(and some igneous rocks)
form in horizontal layers
parallel to Earth’s surface
These horizontal rock layers
are called
strata
Rock layers that are not
horizontal are inferred to be
deformed by crustal motion
1. Folded rock layers are bent
or curved by pressing forces
2. Faulted rock layers are
offset along a zone of
weakness called a fault
3. Tilted rock strata are
slanted or tipped away
from horizontality
4. Displaced rock and fossils
found hundreds of meters
above sea level
indicate that the land has
been uplifted (raised up)
II. Consequences of
Crustal Motion
1. Mountain building
2. Earthquakes
An earthquake is a natural,
rapid shaking of the
lithosphere caused by
the release of energy stored
in rocks
Some earthquakes are
caused by faulting,
 some are associated with
lithospheric motion and
 some are associated with
movements of magma


The potential energy stored
in rocks is given off in
seismic waves
which travel outward from
the point of motion in all
directions
The focus of an earthquake
is the starting point from
which the seismic waves are
emitted
 The epicenter of an
earthquake is the location on
the earth’s surface directly
above the focus

epicenter
focus
lithosphere
a. Analyzing Earthquake
Data

There are three types of
seismic waves:
–P waves (primary waves)
–S waves (secondary waves)
–Surface waves
Properties of Seismic
Waves
 P waves are faster than any
other seismic wave when
traveling through the same
material
 Therefore, P waves will
arrive at a seismic station
first
In general, as the density of
the material increases,
the velocity of the seismic
waves increases
 As waves travel through
areas of differing densities,
they are
refracted (bent)

As pressure increases, the
velocity of seismic waves
increases
 P-waves will travel through
solids, liquids and gases
 S-waves will only travel
through solids

Based upon this knowledge,
seismic data has led to our
understanding of the interior
of the earth.
 Based on the change in
direction of the p and s
waves, we believe that the
outer core of the earth is
liquid


Because seismic waves
reflect off dense rock within
the earth, they can be used
to locate
valuable rock and mineral
resources
Locating the Epicenter
of an Earthquake

Epicenters are located by
using the velocity differences
(lag time) between P and S
waves.
Information from 3 stations
is needed
 The epicenter is where the
circles drawn for all 3
stations intersect

A
B
C
The farther an observer is
from the epicenter, the
longer it takes the seismic
waves to travel there
 The longer they travel, the
farther apart they get…
 The greater the lag time

Analyzing Epicenter
Information
1.
2.
3.
4.
5.
PA and SA: read seismogram
Lag time: subtract SA – PA
Distance: measure lag time
and slide and fit (use ESRT)
PT : go up and over from
distance
OT : subtract PA - PT
An earthquake intensity
(Mercalli) scale can be used
to measure the effects on
humans and/or their
surroundings.
 As distance from the
epicenter increases, the
amount of damage
decreases

An earthquake magnitude
(Richter) scale measures the
strength of an earthquake…
 The amount of energy
released by the crustal
motion

Risk Prevention

Proper planning can greatly
reduce damage, death and
injury from earthquakes
An individual should
remember to
drop, cover and hold…
 Drop down under a strong
object, cover your eyes. Hold
onto the strong object.
 DO NOT run out of the
building


a.
b.
Community planning
includes:
Inspecting the soil and
bedrock to ensure building
on solid ground
Retrofitting older buildings
to make them safer…such
as bolting buildings to their
foundations and crossbracing walls

Seismic sea waves or
Tsunamis are large ocean
waves formed due to a
disruption on the ocean floor
such as an earthquake,
volcanic eruption or rapid
landslide
b. Movement of Magma
When magma reaches the
surface of the earth, it
becomes lava
 A volcano is a mountain
made of extrusive igneous
rock


A volcanic eruption is the
release of gases, lava and/or
lava rock onto the earth’s
surface or into the
atmosphere

People can be injured and
killed by
flowing lava, falling rock and
gases of extreme
temperature
Volcanic ash mixes with
water to create massive
mudslides and flooding
 Gases can cause immediate
death and/or long term lung
damage
 Volcanic ash in the
atmosphere cools the earth
by blocking insolation

Monitoring methods
Satellites measure infrared
energy
 Tilt meters measure
increases in slope caused by
magma inflating the volcano


Elevation benchmarks,
latitude and longitude
measurements and
topographic maps indicate
increases in elevation and
width associated with
eruptions

Measurements allow enough
warning to develop
emergency action plans
including rescue and
evacuation routes

The regions surrounding the
Pacific Ocean contains many
features associated with
crustal activity and is
referred to as the
Ring of Fire
c. Earth’s Interior

Scientists infer most of the
properties of the earth’s
interior by studying seismic
waves
The crust is the outermost
part of the earth which
includes the soil and
weathered and eroded rock
 The mantle is the mostly
solid region. It makes up
~80% of the earth’s volume


The interface between these
two regions is called the
Moho
which is short for the
Mohorovicic discontinuity

The lithosphere is the
combined area of crust and
rigid mantle. This is divided
into sections called plates

Another portion of the upper
mantle is the asthenosphere
which is believed to be made
of a plastic-like material that
is partly magma
Much of the magma and lava
is thought to originate here

Below the asthenosphere is
the stiffer mantle
The earth’s core is divided
into two parts.
 Because s-waves cannot
pass through the outer core,
it cannot be a solid nor a
gas; therefore it is believed
to be a liquid


Because of the great
pressure as well as the
increase in p-wave velocity,
the inner core is believed to
be solid
The crust is divided into two
divisions:
 The continental crust makes
up the continents and
 The oceanic crust makes up
the crust beneath the oceans


Continental crust is
thicker and
less dense
than oceanic crust
Therefore, continental crust
is
granitic rock and oceanic
crust
is basaltic rock

It is believed that the inner
and outer cores are made
mostly of iron and nickel.
This is believed based on the
composition of meteorites
and the presence of the
earth’s magnetic field
d. Plate Tectonics

A unifying model that
explains most major features
and events of the earth is
plate tectonics
which states that the earth’s
lithosphere is broken into
sections called plates. Their
movement and interaction
produce the major changes
in the earth’s surface
 The plates move at a rate of
~3 cm/year

Plate boundaries
A divergent plate boundary
occurs when two plates
move apart.
 At this boundary, magma
rises up to fill in the space
created
 This separation is sometimes
called seafloor spreading

A convergent plate boundary
occurs when two plates
collide
 Subduction occurs when one
plate sinks under another
plate
 This can result in ocean
trenches and volcanic island
arcs

A long, steep, narrow
depression is called an ocean
trench and forms at
convergent boundaries
 Very deep earthquakes occur
at subduction zones

Magma formed from
subduction can create island
arcs or young mountains
 When two continental plates
collide, the plate edges
bunch up together creating
thickening of the crust and
lithosphere

Orogeny refers to times of
mountain building
 A transform boundary occurs
when two plates slide past
one another. This dragging
builds up potential energy
which is eventually released
as kinetic energy as
earthquakes


The San Andreas Fault is an
example of this type of
sliding boundary
Convection Currents
Convection currents drag or
push plates creating plate
boundaries
 The energy source for this
motion is the heat of the
earth’s interior


Hot spots are regions of
volcanic activity located
away from plate boundaries
It is believed that hot spots
occur where rising magma
stays stationary and the
plate moves over it.
 The intense heat melts its
way to, or near, the surface
becoming sites of volcanic
activity.


Because the plates move, a
series of volcanic mountains
form for miles.
Continental Drift

The outlines of the
continents appear to
fit together like pieces of a
jigsaw puzzle

The ancient supercontinent
called
Pangaea
began splitting apart ~250
million years ago

In 1912, a German scientist
named Alfred Wegener
proposed that the continents
have moved from one
location to another
throughout time.
Supportive evidence

Similarities in
minerals, rocks, fossils, age
and structural features
are found where the
landmasses were once
together
One example is the fossilized
remains of Mesosaurus – a
small freshwater reptile.
 Fossils of this reptile are
found in both South America
and Africa


Evidence of hot climates at
the poles
and cold climates at the
equator
implies that plate
movements have changed
the positions of Earth’s
landmasses

The farther from the center
of an ocean ridge a sample
is taken,
the older the sample is found
to be

Heat measurements show
that
as distance from an ocean
ridge increases
temperature
decreases

A process called
reversal of Earth’s magnetic
polarity
tells us that the earth’s
magnetic field has reversed,
or flip flopped, hundreds of
times throughout Earth’s
history
Normal polarity
is when magnetic north is
near geographic north
 Reversed polarity
is when magnetic north is
near geographic south


There is a pattern of
corresponding stripes of
normal polarity alternating
with reversed polarity
located on either side of the
mid ocean ridge