Download The Dynamic Crust Topic 4 Topic 12 in Review Book

The Dynamic Crust Topic 4 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
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
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
PA and SA: read seismogram
Lag time: subtract SA – PA
Distance: measure lag time and slide and fit (use ESRT)
PT : go up from the distance to the P line and over to P travel time
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
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 cross-bracing 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
3. 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 (sun’s rays)
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
III. 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
IV. 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