The Dynamic Crust

Topic 12Topic 12 in Review

Book

I. Evidence of Crustal Motion

Original horizontalityA concept that assumes that sedimentary rocks (and some igneous rocks) form in horizontal layers parallel to Earth’s surface

These horizontal rock layers are calledstrataRock 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 levelindicate that the land has been uplifted (raised up)

II. Consequences ofCrustal Motion

1. Mountain building2. Earthquakes

An earthquake is a natural, rapid shaking of the lithosphere caused bythe 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 inseismic waveswhich 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

focus

epicenter

lithosphere

a. Analyzing EarthquakeData

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 arerefracted (bent)

As pressure increases, the velocity of seismic wavesincreases

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 locatevaluable rock and mineral resources

Locating the Epicenterof 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. PA and SA: read seismogram

2. Lag time: subtract SA – PA

3. Distance: measure lag time and slide and fit (use ESRT)

4. PT : go up and over from distance

5. 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:

a. Inspecting the soil and bedrock to ensure building on solid ground

b. 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

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 byflowing 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 theRing 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 theMohowhich 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 asthenospherewhich is believed to be made of a plastic-like material that is partly magmaMuch 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 isthicker andless densethan oceanic crustTherefore, 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 isplate 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 tofit together like pieces of a jigsaw puzzle

The ancient supercontinent calledPangaea 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 inminerals, rocks, fossils, age and structural featuresare 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 polesand cold climates at the equatorimplies 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 thatas distance from an ocean ridge increasestemperaturedecreases

A process calledreversal of Earth’s magnetic polaritytells us that the earth’s magnetic field has reversed, or flip flopped, hundreds of times throughout Earth’s history

Normal polarityis when magnetic north is near geographic north

Reversed polarityis 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