Plate Tectonics
Alfred Wegener
1912- Wegener proposed several hypothesis
a. Continental drift- stated that continents had moved
b. Continents once formed a single land mass called Pangaea
c. There was once a huge sea called Panthalassa
d. About 200 mya Pangaea broke up into smaller pieces and drifted to current locations over time.
e. Movements of crust may have formed Mountain ranges
Evidence
oSimilarities in coastlines- parts of the continents seemed to fit together like a puzzle
ofossil remains-
oMesosaurus- a small extinct reptile of 270 mya has been found on both E. coast of South America and W. coast of Africa
oClimate- coal deposits indicate that continents were once in a more tropical area, glacial deposits in areas that are now too warm for glaciers
- Rockformations-Mountains such as the Appalachians seem to extend to Scotland, Greenland and Europe
Geologic Evidence
Wegener’s ideas met with strong opposition. He died in 1930 without finding an explanation for why continents move.
Seafloor Spreading
o1947- Harry Hess suggested a new hypothesis
oWhile mapping the Mid-Atlantic Ridge (an undersea mountain chain)
oScientists discovered that rock samples from the ocean floor were much younger (150my) than the continental crust 4 by
What makes it younger?
oThe center of the ridge is actually a break in the crust and magma wells up pushing crust apart
omagma solidifies to new rock
oRobert Dietz named this seafloor spreading
oIf the ocean is moving, then the continents must be moving too.
The Proof- Paleomagnetism
oThe earth acts like a giant magnet
oWhen magma cools and solidifies iron bearing rock becomes magnetized and the orientation becomes permanent
oIt was once thought that the orientation was always north
oIn the mid 1900’s rocks were found that oriented south
- Scientist concluded that the magnetic fields had reversed
- All the rocks with magnetic fields pointing north fell within a given time frame
called normal polarity
- At many times the polarity has reversed which show up as patterns of alt. Bands
- Other evidence- Patterns of Volcanoes and Earthquakes
Kinds of Plate Boundaries
Three Main types of plate boundaries
oDivergent- where new crust is generated as the plates pull away from each other
oConvergent- where crust is destroyed as one plate dives under another
oTransform boundaries- where crust is neither produced nor destroyed as the plates slide horizontally past each other
Continental Margins
oThe boundary between the continental crust and the ocean crust
oActive margins- occur along plate boundaries and result in earthquakes, volcanoes and mountain building ex. West coast of South America
oPassive Boundaries- do not occur at plate boundaries. Ex. East coast of NorthAmerica
Diverging Boundaries
oCalled spreading centers
oMost on the sea floor
oBest known mid-Atlantic Ridge
o~2.5 cm a year spreading rate
oMolten rock from asthenosphere rises and fills the space between plates
- Rock on either side of the rift is the same age and polarity
oRift valley- narrow valley formed as plates separate ex. Mid ocean ridge and East coast of Africa
oOther rift valleys form where continents are separated by plate movement
oex. Red sea formed by the separation of the African and Arabian Plate
- Madagascar separated from Africa
Iceland
oStraddles Mid-Atlantic ridge
oSplitting along a spreading center between the North Americana and Eurasian Plates
Convergent Boundaries
oTwo plates coming together or converging
- Ring of fire- volcanoes that encircle most of the Pacific Ocean
oThree main types of collisions
Oceanic- continental
1) Oceanic- Continental Collision
oOceanic is denser- it subducts or plunges under continental crust
oSubduction- where one plate moves under another.
oForms deep ocean trench ex. Peru-Chile trench, Mariana Trench
oOceanic plate melts, magma rises the continental crust to
form volcanic mountains ex. Andes Mountains
- Features- earthquakes, volcanic mountains, trenches, subduction
2) Continental-Continental Collision
oNeither plate is subducted because the plates are the same density and very thick
oColliding edges are crumpled and uplifted producing
mountain ranges ex. Himalayas
- Features-Mountains and earthquakes only
3) Oceanic-Oceanic Collision
- Forms from the collision of two oceanic plates
oForms deep Ocean trench
oIsland Arc- part of the subducted material melts
resulting magma rises to the surface along the trench to form a chain of volcanic islands
- Features- subduction, earthquakes, volcanic islands, trenches
Transform Fault Boundaries
oTwo plates grinding past each other
oDo not slide smoothly, scrape together and move in sudden spurts of activity
ex.San Andreas fault,
oBreaking of the rock (faults) cause earthquakes but NOT volcanic activity
oAreas that have not moved are the areas likely for strong earthquakes
Plate-boundary Zones
oUsually include two large plates and one or more micro plates
oHave complicated geologic structures and earthquake patterns
Rates of Motion
oRates vary greatly
- Average rate of spreading is 2.5 cm per year
, 15 cm/yr.
Why Do Plates Move?
oThe earth’s crust consists of two parts oceanic and continental crust
oOceanic crust is made of basalt
oContinental crust is composed of granite and is
less dense than basalt
oThe oceanic crust and continental crust and the ridged upper mantle make up the lithosphere. It is broken up into 30 identified plates
Asthenosphere
oFound beneath the lithosphere
oPartially molten and flowing
oForm convection currents
oHot rising currents push the plates apart and form divergent boundaries
oCool sinking currents push the crust together and form convergent boundaries
Hot Spots
- Not a plate boundary
- Magma works it way up through lithosphere
- Hot spot remains stationary, but plate drifts
- Volcano on the surface is carried away from the hot spot
- A new volcano forms where new crust has moved over it
- ex. Hawaiian islands and YellowstoneNational Park
Volcanic Cones
Shield cones-
Low silica and low dissolved gas (low viscosity)
broad at the base and have gently sloping sides.
cover wide areas
quiet eruptions
composed mostly of lava
ex. Hawaiian Islands
Cinder cones-
Low silica high dissolved gas
made from solid material ejected from the volcano.
very steep slopes
composed mostly of tephra
explosive eruptions
Composite cones also known as strato volcanoes-
High silica and dissolved gas (high viscosity)
Very explosive eruptions
cone formed of alt. layers of lava flow and tephra
some of the best known volcanoes such as: Mount Fuji, Mount Rainier, Mount Shasta, Mount St. Helen's
Viscosity-resistance to flow. High viscosity= more explosive
More dissolved gas and silica the more explosive a volcano
Craters and calderas
Crater- funnel shaped pit at the top of a volcanic vent
formed when material is blown out of the volcano by explosions
Caldera- large, basin shaped depression, formed when explosions completely destroy the upper part of the cone or when magma chamber below is emptied and cone collapses.
ex Krakatau
Ring of Fire- 90% of volcanos occur on the rim of the Pacific ocean
Earthquakes
Vibrations of the earth’s crust
Produced by rapid release of energy
Occur when rocks under stress suddenly shift or break along a fault
Seismology- the study of earthquake waves
Elastic Rebound Theory
The rocks on each side of a fault are moving slowly and bending under great stress
When the stress becomes too much, the rocks fracture at their weakest point
they then spring back or rebound to their original shape
As they fracture they release energy in the form of seismic waves (earthquakes)
Aftershocks
The release of energy often causes other rocks to fracture
major earthquakes are usually followed by a series of smaller tremors called aftershocks
Parts of an earthquakes
Focus- the area along a fault where slippage (break) first occurs.
Epicenter- the point on the earth’s surface directly above the focus
seismic waves radiate outward in all directions
Earthquake depths
Shallow focus- within 70km of the surface
90% of all continental earthquakes
Intermediate focus, depth between 70 km and 300km.
Deep focus- depths of 300 km - 650 km They usually occur the farthest inland
Major Earthquake Zones
Most occur along or near the edges of plates
Major Zones
1. Pacific Ring of Fire-subducting plates produce stress
2. Mid-ocean ridge- spreading motion creates stress
Seismograph
A device to detect seismic waves
has three devices
1 records vertical movement
2 records horizontal north south motion
3 records horizontal east west
traces wave motion on paper or by translating the motion into electronic signals
Types of Seismic Waves
Primary or P waves-
fastest
first recorded
move through solids or liquids
cause objects to move compress and expand (push or pull)
S waves
Secondary waves
can only move through solid material
can not be detected on the opposite side of the earth (can not penetrate the liquid)
second to be detected by seismograph
move objects up and down
Shadow Zones
Areas where neither S or P waves, or only P waves are detected
Occur on the side of the earth opposite where the earthquake occurred.
Occurs because S waves can not pass through liquid and P waves change speed and direction as the waves pass through different medium.
Surface Waves
L and R waves
slowest- moving
last recorded on seismograph
cause the greatest damage
Locating an Earthquake
Epicenter found by analyzing the difference in arrival times of the P and S waves.
If the S waves occur soon after the P waves epicenter close to seismograph station
If the S waves occur a long time after the P waves then the epicenter is far from the seismograph station
Need information from at least three stations
Distance from earthquake is calculated
The station is placed in the center of a circle with the distance to the earthquake being the radius.
Where the three circle intersect is the epicenter
Earthquake measurement
Richter scale- used to express the magnitude of an earthquake
Magnitude- a measure of the energy released by an earthquake
Each increase in magnitude releases 31.7 times more energy
ex. A 8 magnitude has 31.7 x 31.7 or 1000 times more energy than a 6 magnitude
Magnitude scale
Largest ever recorded was a 9.6 mag.
Major earthquake is 7 mag or more
moderate- 6-7 mag
minor- 2.5 - 6 mag
microquakes less than 2.5 and are not usually felt
Mercalli scale
Expresses intensity of the earthquake ( the amount of damage)
expressed in Roman numerals I- XII and a description
ex. XII - total destruction
Earthquake damage
Causes by:
Collapse of buildings, most are not designed to withstand swaying
Buildings built on loose soil are more likely to be damaged than those on solid ground
Flying objects and flying glass
Landslides, fires, explosions caused by broken electric and gas lines
Tsunamis
Epicenters on the ocean floor sometimes cause giant ocean waves called tsunamis
ex. 1964 an Alaskan earthquake killed 9 people, the tsunami that resulted caused 107 deaths.
SSWWS- Seismic Sea Wave Warning System seismic stations in the Pacific Ocean that alert scientists to potential tsunamis