Earth Sciencestudy Guide Plate Tectonics & Earthquakes

Earth ScienceStudy Guide – “Plate Tectonics & Earthquakes”

1. The Theory of Continental Drift

• Francis Bacon (1620) proposed that the continents once fit together
• Alfred Wegener (1912), a German meteorologist, came up with The Theory of Continental Drift which stated that:
• All the continents were once connected and have since moved apart
• Giant land mass (supercontinent) is called PANGAEA
• Giant ocean surrounding the supercontinent is called PANTHALASSA
• 200 million years ago the supercontinent began to break apart

• Evidence for Continental Drift
• shapes of continents look like they fit together
• Fossil remains of the same species found on multiple landmasses (same fossils found on east coast of South America and west coast of Africa)
• mountain ranges that were connected separated over time; geologic features formed and ended up on different continents (Appalachian Mtns. in North America and mountains in England and Norway)
• remains of glacial material fit together (pattern of Pangaea)
• coal found in areas today that are not tropical climates (meaning that they must have formed near the equator in the past)

B. Sea Floor Spreading

• echo sounding devices gave scientists insight as to what the ocean floor looked like
• ocean floor contained mountain ranges with steep, narrow valleys running down the center
• underwater ranges were 65,000 km long when connected
• ocean crust is younger than continental crust

1. ocean crust is newer lava that has cooled
2. continental crust used to be the ocean crust before it was raised up

• Dr. Harry Hess (1960), a geologist, came up with hypothesis

1. magma spreads outward because it is less dense than the water
2. it forms a growing crust and comes from a rift
3. sea floor spreading – the ocean floor and continents are moving
4. Hess could not prove his theory

• Evidence to support Hess’s theory of sea floor spreading
• magnetic clues – iron particles that formed
• rock on sea floor is younger than that of the continents

• as sea floor spreading occurs, new ocean floor is made

1. new ocean floor comes from the rift
2. old ocean floor moves toward the continental crust

• scientists found that the ocean floor had many magnetic reversals (poles reversed magnetism)
• iron minerals samples in basalt on ocean floor aligned itself with the magnetic orientation at the time the rock was formed

C. Plate Tectonics

• theory of plate tectonics combines theories of Continental drift and Sea floor spreading
• earth’s crust is divided into about 12 major “plates” that ride on the asthenosphere
• tectonics – the study of formation of crustal features

• convection currents are believed to drive plate motions

1. temperature increases, density decreases and currents rise
2. as currents get further from heat source, the temperature decrease, density increases and the currents fall

D. Types of Plate Boundaries

DIVERGENT CONVERGENT TRANSFORM

E. TYPES OF CONVERGENT PLATE BOUNDARIES

• There are three types of CONVERGENT PLATE BOUNDARIES that all have subduction.

Earthquakes

1. Stress – force that causes pressure in the rocks of the crust
2. Compression

• rocks of the crust are squeezed together
• reduces volume of rocks
• pushes rocks higher up or deeper down into crust
• Tension
• the force that pulls rocks apart
• when pulled apart, rocks tend to become thinner
• Shearing
• pushes rocks in opposite horizontal directions
• rocks bend, twist, or break apart as they slide past each other

1. Strain – change in shape or volume of rocks that results from the stress of being twisted, squeezed or pulled apart
2. Folding – rock responds to stress by becoming permanently deformed without breaking
3. Anticlines
4. upcurved folds in the layers
5. looks arch like
6. Syncline
7. downcurved folds in the layers
8. warping downward
9. Monocline
10. gently dipping bends in the horizontal rock layers

1. Breaks in rocks – caused by cooler temperatures and lower pressures near the Earth’s surface
2. Fracture – no movement
3. Fault – breaks when there is movement

a. Normal fault

• results form tension or divergent forces which causes the hanging wall to slip down past the footwall

1. Strike-slip fault
2. occurs when rocks on either side of faults are moving past each other or transforming without much upward or downward movement
3. Reverse fault
4. results from compression or convergent forces which cause the hanging wall to move upwards
5. Horst fault
6. results from compression or convergent forces which cause two reverse faults and the hanging walls to move upwards.
7. Graben fault

• results from tension or divergentforces which cause two normal faults. The hanging walls to move downwards. An example is a rift valley.
  

1. Seismic Waves
2. Body Waves (travel through the body of Earth)
3. Primary waves (P waves)

• fastest moving waves; cause little damage
• recorded first by seismograph
• can travel through solids and liquids of the earth
• in an earthquake this is the up and down motion first felt
• Secondary waves (S waves)

• slower moving waves
• also known as shear waves
• can only travel though solid rock
• moves rocks up and down, or side to sidecausing more damage than P waves
• Surface Waves (cause the most damage)
• Can only move along the surface, move the slowest
• Rayleigh waves
• Love waves

1. Epicenter

• point on the Earth’s surface directly above the focus of an earthquake
• focus is the origin or starting point of the earthquake (location of plates shifting)
• can be found using threeseismograms at the point where the circles intersect
• Steps to locate an epicenter include:

1. determine time difference between the arrival of the P & S waves at seismogram station
2. use P & S wave chart to translate time difference to distance station is from epicenter
3. using compass, draw circles from three stations to locate the epicenter
4. mark epicenter

1. Tsunami’s

• Are a series of waves (wave train) that are caused by an underwater earthquake with vertical movement of the crust.
• The danger from a tsunami can last for several hours after the arrival of the first wave.
• A tsunami wave train may come as a series of surges that are five minutes to an hour apart.
• If you are in a tsunami, make way to higher ground.
• The waves can be 30 meters in height.
• A tsunami is not the same as a tidal wave. A tidal wave is caused by the moon’s gravitational pull.

1. What should you do if you are in an earthquake?

• If inside, go under a table for protection
• Stand in a door frame if table or other protection is not available
• If outside, go to an open area and protect your head
• Stay away from water or electrical wires
• If you are driving, stop the car, duck and cover your head.
• If you are in school, get under a desk or lab station

THERE WILL BE A LARGE FOCUS ON USING THE TRAVEL TIME CURVE GRAPH AND LOCATING EPICENTERS OR FINDING DISTANCES TO EPICENTERS.