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The Earth’s crust is dynamic which means constantly changing.
Some of these changes can be directly observed such as the results of: Taken from http://www.physicalgeography.net/fundamentals/10h.html on 8/18/06
Earthquakes Taken from http:// cse.ssl.berkeley.edu/img/earthquakes/Railroad.gif on 8/15/06 An apartment building in San Francisco’s Marina District shows heavy damage from a 1989 earthquake. Taken from http:// www.nationalgeographic.com /eye/earthquakes/ images/ earthquakes_intro_image.gif on 8/15/06
Volcanoes Mount St. Helens shortly after the eruption of May 18, 1980 Taken from http://en.wikipedia.org/wiki/Volcano on 8/15/06 Taken from http://library.advanced.org/17457/volcanoesdb/images/240/v5p42x240.jpg on 8/15/06 Eruption of Mt. Arenal, Costa Rica
Crustal movements along fault zones Both images taken from http://library.thinkquest.org/C003603/english/earthquakes/multimedia.shtml on 8/15/06
Other evidence indicates that parts of the Earth’s crust have been moving to different locations for billions of years.
Describe 7 pieces of evidence that suggest minor changes in the Earth’s crust.
a) Displaced & Deformed Rock Strata and Fossils
Sedimentary rocks appear to form in horizontal layers. However, observations of the Earth’s surface indicate that the original formations of rock have changed through past movements.
Earth movement resulting in a change in the position of rock layers,
“ rocks at an angle”
Bend in the rock layers produced during the mountain building process
Movement of rock along a crack (fault) in the crust
b) Displaced Fossils
Displaced means “moved.”
Marine fossils- remains or imprints of once living ocean organisms such as coral, fish, etc. found in sedimentary rock
Where would you expect to find marine fossils?
Taken from http://orerockon.com/coal_ck5.jpgon 8/18/06
Marine fossils found in layers of sedimentary rock in mountains, often thousands of feet above sea level .
These marine fossils found at high elevation suggest past uplift of rock strata.
Marine Fossils on Top of the Andes Mountains. More than 500 giant fossilized oysters were found 3000 meters (about 2 miles) above sea level in Peru in 2001 Taken from http://antiquity.ac.uk/ProjGall/jeck/images/fig1.jpg on 8/18/06
Sinking or settling of rock strata
Taken from http://geochange.er.usgs.gov/sw/changes/anthropogenic/subside/fig0.jpg on 8/18/06
Example: finding fossils of shallow water organisms in rocks found deep in ocean floor.
Early in the 20th Century, land near the Alviso Marina in Santa Clara, CA, top, sank nearly 13 feet due to overpumping of local groundwater, below Taken from http://www.valleywater.org/media/Alviso_subsidence.jpg on 8/18/06
d) Displaced Strata
Rock layers that have been moved.
Horizontal Displacement (Faulting)
Earth shifts sideways along a transform fault in the crust
The fence was offset 2.6 m by the magnitude 8.2 earthquake of April 18, 1906, San Francisco, California, a section of the San Andreas fault. Both images taken from http://www.ngdc.noaa.gov/seg/hazard/slideset/10/10_slides.shtml on 8/15/06 Offset of a cement-lined ditch by the Motagua fault resulting from the earthquake of February 4, 1976, in Guatemala.
Vertical Displacement (Faulting)
Portion of Earth’s surface is either uplifted or subsides along a fault or crack.
Taken from http://www.rcep.dpri.kyoto-u.ac.jp/~sato/taiwan/twpics/tw/tw004.JPG on 8/18/06 Collapsed bridge and waterfall created by vertical faulting on river northeast of Fengyuen in Taiwan
Permanent cement or brass marker in ground indicating a measured elevation.
Taken from http://www.adirondackjourney.com/images/Great_Range/Benchmark%20on%20Gothics.jpg on 8/18/06 Brass benchmark: Summit of Gothics Mountain in Adirondacks
Condition of balance or equilibrium in Earth’s crust.
Since the upper mantle acts like a very dense fluid, the crustal plates float on top of it.
Any change in one part of the crust is offset by a corresponding change in another part of the crust.
Photo of baby on waterbed taken from http://thepeacock.com/Spiritual/Photo_of_Baby_Jennifer_on_Waterbed_for_Why_were_we_Born.jpg on 8/18/06 Taken from http:// www.platetectonics.com /book /images/Convection.gif on 8/18/06
Example of Isostasy
If a piece of crust loses some of its material due to erosion, it becomes lighter and floats higher in the mantle. When the eroded material gets deposited, the crust is weighted down causing that area to sink lower into the mantle.
Taken from http://tlacaelel.igeofcu.unam.mx/~GeoD/isostasy/figs/isost4.jpg on 8/18/06
Another isostatic example.
Occurred in NYS and created seismic activity. The deposition of 2 miles thick ice on NY during a glacial ice age caused the area to subside slightly. This forced other areas to rise higher in response to the settling under the ice. Later after the ice receded or melted, the region responded with gradual uplift causing minor seismic activity or earthquakes.
Give examples of crustal activity and explain where the zones of crustal activity are located. (ESRT p.5)
II. Areas of Crustal Activity
Crustal activities such as earthquakes and volcanoes occur for the most part in specific zones or regions of the Earth.
These regions are usually along the borders of continents and oceans . These zones mark boundaries or edges of large pieces of the Earth’s crust called crustal boundaries.
What is an earthquake? Explain the difference between an epicenter and a focus of an earthquake.
Describe properties of the 3 types of earthquake waves and tell the difference between a seismograph and a seismogram.
Sudden trembling or shaking of ground usually caused by movement along a break or a fault releasing built up stress
When an earthquake occurs, seismic waves are created and move out in all directions from the focus or point of origin.
The earthquake’s focus or point of origin is usually deep below the Earth’s surface.
The point on the Earth’s surface directly above the focus is called the epicenter.
Describe properties of the 3 types of earthquake waves and tell the difference between a seismograph and a seismogram.
II. Earthquake Waves
Seismograph: Instrument that detects and records seismic waves.
Earthquakes generate several kinds of seismic waves that can be detected by a seismograph.
3 types of seismic waves are p , s , & l waves.
Do not pass through the Earth.
Ripple along the surface of the Earth
Create the damage associated with earthquakes
Also called compressional because they cause the material through which they pass to vibrate back and forth (compress) in the same direction as the wave is traveling.
Called primary because they move quickly through the Earth with a greater velocity than secondary waves and therefore are the first waves to reach a distant location.
Taken from http://www.lamit.ro/images/earthquake-p-waves-passage.jpg on 8/18/06
Also called shear waves because they cause the material through which they pass to vibrate at right angles (up & down) to the direction in which the wave is traveling
Taken from http://www.lamit.ro/images/earthquake-s-waves-passage.jpg on 8/18/06 http://www.thetech.org/exhibits_events/online/quakes/waves/p&s_waves.html
III. Velocities of Waves
When traveling in the same material, primary waves travel at a greater velocity than secondary waves. So a seismograph will read the primary waves before the secondary waves arrive.
Waves rock house animation
A single seismogram showing the arrival times of p & s waves may be used to determine the distance to the earthquake and its time of origin .
The greater the difference in arrival times of the primary and secondary waves, the greater the distance to the earthquake epicenter .
Eureka, CA seismogram of earthquake Elko, NV seismogram of earthquake Both seismograms taken from http://www.sciencecourseware.org/VirtualEarthquake/php/Seismograms.php on 8/18/06 Animation showing waves arriving at seismograph
From a seismogram, be able to find the distance to epicenter, origin time and epicenter location by using ESRT p.11.
a) Finding the Distance to an Earthquake’s Epicenter
To find out how far an epicenter was away from a location, a seismograph reading or seismogram is necessary that shows the arrival of both p and s waves.
Then follow 4 steps using the Earthquake Graph in the ESRT on page 11 .
Taken from http://upload.wikimedia.org/wikipedia/id/a/a6/Seismogram.gif on 8/18/06
4 Steps for Finding Epicenter Distance
Calculate the difference between p and s arrival times.
Using the y-axis of the graph, mark the time difference between p and s waves on a scrap edge of paper.
Take the scrap edge and find the gap between the p and s waves on graph that equals the time difference.
Follow the scrap edge from gap straight down to the epicenter distance.
Let’s Do an Example following the 4 steps
b) Calculating the Origin Time of an Earthquake
Use ESRT p.11
Find the epicenter distance on the x-axis & go straight up to the p wave graph line.
Take that point on the p graph line & go straight across to the time it takes a p wave to go that distance (y-axis).
Take the p travel time you just found & subtract it from the time the p wave arrived to get the origin time.
Let’s Do an Example following the 3 steps
c) Determining the Exact Location of an Earthquake’s Epicenter
Epicenter location is found by the comparison of differences in travel time of p & s seismic waves.
Knowing the separation time between arrival of both p & s waves gives the distance to the point on the Earth’s surface directly above the earthquake called the epicenter.
Taken from http://www.stkc.go.th/LOEarthScience/OFFLINE/LO504/1_en.htm on 8/18/06 Taken from http://www.worsleyschool.net/science/files/earthquake/epicentre.html on 8/18/06
Since only the distance to epicenter and not direction is known, a circle is drawn with the radius equal to the distance.
The epicenter is on the circle.
To find the actual location of the epicenter you must find the distance from 3 different seismograph stations. Why not 2?
Images taken from http://www.worsleyschool.net/science/files/earthquake/epicentre.html on 8/18/06
Draw 3 circles around the 3 seismograph stations and where they intersect is the earthquake’s epicenter.
The earthquake occurred at a point somewhere below the epicenter and that internal point is called the focus.
Scientists wanting to improve accuracy of finding the true epicenter will find the distance from more than 3 seismograph stations.
Try these exercises on the computer with Virtual Earthquake! Review animation
Compare and contrast the 2 scales for determining the strength of an earthquake.
a) The Modified Mercalli Scale
Based upon the damage inflicted by an earthquake.
This intensity scale ranges from I to XII with I being felt by few people to XII resulting in total devastation .
Taken from http://data.gns.cri.nz/geoatlas/images/mercalli.jpg on 8/18/06
Modified Mercalli Scale Continued
Although this scale is still used, it is not very precise. Why?
Damage inflicted by earthquakes depends on many factors besides the strength of the earthquake such as location, type of land, building design & structure, etc.
Taken from http://www.ericandsylvia.com/pictures/2000/11-25-2000/cvt/img_4538.jpg on 8/18/06 The building that has straps on it is supposed to be very earthquake proof, because the straps hold the building suspended on a central cement column. Alaskan tundra in fall foliage Taken from http://www.galleryone.com/images/cook/cook_-_alaska_tundra_in_autumn_glory_denali_highway_central_alaska.jpg on 8/18/06
b) The Richter Scale
A Magnitude scale used to describe the amount of energy released by an earthquake.
Richter scale magnitudes range from 0 to 9.
Each number step up the scale indicates a release of 32 times more energy than the previous step.
Taken from http://en.wikipedia.org/wiki/Richter_scale on 8/18/06 estimate for a 10 km rocky bolide impacting at 25 km/s 1 teraton 10 2004 Indian Ocean earthquake 32 gigatons 9 Anchorage, AK Quake, 1964 5.6 gigatons 8.5 San Francisco, CA Quake, 1906 1 gigaton 8 Landers, CA Quake, 1992 178 megatons 7.5 Largest thermonuclear weapon 32 megatons 7 Northridge quake, 1994 5.6 megatons 6.5 Double Spring Flat, NV Quake, 1994 1 megaton 6 Little Skull Mtn., NV Quake, 1992 178 kilotons 5.5 Nagasaki atomic bomb 32 kiloton 5 Average tornado (total energy) 5.6 kilotons 4.5 Small atomic bomb 1 kiloton 4 Chernobyl nuclear disaster, 1986 178 metric tons 3.5 Massive Ordnance Air Blast bomb 32 metric tons 3 WWII blockbuster bomb 5.6 metric tons 2.5 late WWII conventional bombs 1 metric ton 2 WWII conventional bombs 178 kg (392 lb) 1.5 Construction site blast 32 kg (70 lb) 1 Hand grenade 5.6 kg (12.4 lb) 0.5 Seismic Energy Yield Magnitude Example Approximate TNT for Richter
Earthquakes that are less than 2.5 are not usually felt by people.
Approximately 20 major earthquakes in the magnitude 7.0-7.9 occur every year and each 5-10 years an earthquake of 8.0 or more will devastate a portion of Earth.
Table taken from http://www.factmonster.com/ipka/A0763403.html on 8/18/06 8.5 Feb. 3, 1923 Kamchatka 10. 8.6 Aug. 15, 1950 India-China border 9. 8.7 March 28, 2005 Northern Sumatra, Indonesia 8. 8.7 Feb. 4, 1965 Rat Islands, Aleutian Islands 7. 8.8 Jan. 31, 1906 Off the coast of Ecuador 6. 9.0 Dec. 26, 2004 Off western coast of Sumatra, Indonesia 5. 9.0 Nov. 4, 1952 Kamchatka 4. 9.1 March 9, 1957 Andreanof Islands, Aleutian Islands 3. 9.2 March 28, 1964 3 Prince William Sound, Alaska 2. 9.5 May 22, 1960 Chile 1. Magnitude Date Location
Give examples of dangers to humans from volcanic and earthquake activity.
VI. Dangers to Humans from Earthquakes and Volcanoes
Tell at least 4 of these hazards.
Fires (Ruptured gas or power lines)
Collapsing buildings/Falling Debris
Broken bridges and roads
Tsunamis (Seismic Sea Waves)
Lava flows melt and burn
Volcanic ash & poisonous gases make it difficult to breathe
Large submarine (under water ) earthquakes or those that occur along a coastline may result in tsunamis or seismic sea waves.
Describe differences between p and s wave transmission through the Earth and how it creates a shadow zone.
VII. Transmission of Earthquake Waves
The velocity of an earthquake wave varies according to density of the material through which it is traveling.
The greater the density of the material, the greater the velocity.
Taken from http://www.earthscrust.org/earthscrust/science/historic/img/moho.gif on 8/18/06
As seismic waves travel through materials of different densities, the velocity of the seismic waves will change.
This change in velocity of the wave causes the wave to be bent or refracted.
Since the density of the Earth gradually increases with depth, seismic waves tend to increase in their velocity and continually refract (bend) as they travel down into the Earth.
Difference in P and S Wave Transmission
Compressional or p waves are transmitted through all phases of matter; solid, liquid or gas.
However, shear or s waves are only transmitted through solids .
This difference provides valuable information for scientists about the composition and interior structure of the Earth.
S waves that penetrate the Earth to the depth of the outer core disappear.
Since these waves are not transmitted by the outer core, the material of the outer core is assumed to be liquid .
Taken from http://www.geo.cornell.edu/geology/classes/ Geo101/graphics/seismic_interior.jpgon 8/18/06
Earthquakes generate p & s waves that move out from the earthquake through the Earth in all directions.
Seismographs that are located within 102 degrees from the epicenter record both p & s waves.
Those seismograph stations that are farther away than 102 o do not record any s waves because they are not transmitted through the core.
Taken from http://www.astronomynotes.com/solarsys/seismicb.gif on 8/18/06
A band that runs approximately 102 o to 143 o away from the epicenter records neither p nor s waves.
This is because p waves get refracted out of that region.
This region is called the shadow zone .
Describe a model of the Earth’s crust and interior. Be able to use ESRT p.10 to describe characteristics of both the crust and interior.
C. Crust & Interior Properties
There are 4 major Earth zones, three solid ones and one liquid.
The 3 solid zones are the crust, mantle and inner core .
The only liquid zone is the outer core .
See ESRT p. 10
I. Crustal Thickness
The crust of the Earth compared to other zones is relatively thin , only a few kilometers in average depth.
The average thickness of the continental crust is greater than the average thickness of the oceanic crust.
Taken from http://www.deafhoosiers.com/sci/SOARHIGH/lithosphere/lithosphere.jpg on 8/18/06
II. Crustal Composition
The continental crust is composed mainly of felsic igneous rock like granite that is low in density.
The oceanic crust is composed mainly of mafic igneous rock like basalt that is high in density.
Black sand beach of Hawaii Taken from http:// ruby.colorado.edu/~smyth / Research/Images/ Volcanix/Blacksand.jpg on 8/18/06 Granitic beach of Lake Ontario Taken from http:// www.dec.state.ny.us /website/ environmentdec/2006a/greatlakesagreement20106.jpg on 8/18/06
Crust sits on top of mantle.
Mantle accounts for the greatest part of the volume of the Earth.
The crust-mantle boundary is called the Mohorovicic Discontinuity or the Moho.
Below the mantle is the liquid outer core and the solid inner core.
Taken from http://earthnet-geonet.ca/images/glossary/mantle.jpgon 8/18/06
Evidence from the behavior of seismic waves and metallic meteorites suggests that the inner portion of the Earth is a high density combination of the metallic elements iron (Fe) and nickel (Ni).
Taken from http://buhlplanetarium3.tripod.com/CSC-Meteorite.JPG on 8/18/06
III. Characteristics of Earth’s Interior
The density, temperature and pressure of the Earth’s interior increases with depth. (ESRT p. 10 ). The density ranges from 2.7g/cm 3 for the continental crust and 3.0g/cm 3 for the oceanic crust to 12.7 g/cm 3 -13.0g/cm 3 for the inner core.
Compare theories of continental drift and plate tectonics. Give evidence that support the idea that continents have moved.
I. Plate Tectonics Theory
Theory that Earth’s lithosphere is made of a number of solid plates that move in relation to each other.
Theory that continents are now, as well as in the past, shifting positions.
Although plate tectonics is a recent idea, it incorporates the earlier idea of Continental Drift put forth by Alfred Wegener in 1915. Photo taken from http://www.uni-marburg.de/profil/Geschichte/wegner on 8/18/06
Wegener noted that the present continents appear to fit together as fragments of an originally larger landmass, much the same way the pieces of a jigsaw puzzle fit together.
This is especially true if the edges of the continental shelves are used as the boundaries.
However, over the years new evidence has been collected that indicates that approximately 200 million years ago, the major continents were connected and since that time the continents have been moving generally apart.
The following diagrams show the Inferred Positions of the Continents over the last 458 million years.
Label the Geologic Period for each diagram. Diagrams found in ESRT on page 9 .
II. Evidence to Support Idea that Continents Have Moved
Many rock layers and fossils can be correlated across ocean basins. Rock types along with mineral composition and the fossils found in those rocks match up.
A good example of this are rocks and fossils found on the east coast of South America match those found along the west coastline of Africa.
Diamonds found in eastern Brazil are very similar to those found in western Africa.
More Evidence for Continental Movement
Some mountain chains appear to be continuous from continent to continent.
Example: Appalachians and Caledonian
More Evidence for Continental Movement
Rock and fossil evidence indicates ancient climates much different from those of today.
Examples: glacial deposits in tropical regions or coal deposits in Arctic
More Evidence for Continental Movement
Rocks of the ocean basins are much younger than continental rocks.
The most conclusive evidence comes from the ocean basins.
Explain evidence for sea floor spreading from both igneous ocean rocks and the reversal of magnetic polarity .
III. Evidence to Suggest Sea Floor Spreading
There is much evidence to indicate that the ocean floors are spreading out from the mid-ocean ridges . The two major pieces of evidence are related to the age of igneous ocean materials and the reversal of magnetic polarity.
Taken from http://www2.nature.nps.gov/geology/usgsnps/animate/A48.gif on 8/18/06
a) Igneous Ocean Rocks
The ocean crust is made up mainly of basalt that is formed when magma (molten rock) rises, cools, solidifies and crystallizes into igneous rocks of the mid-ocean ridges.
Evidence shows that igneous rocks along the center of the mid-ocean ridge is younger (more recently formed) than the igneous rock found farther from the mid-ocean ridge.
The age of igneous rock has been accurately determined using radioactive dating techniques.
This suggests that as new ocean crust is generated at mid-ocean ridges, the ocean floor widens.
Taken from http://www.physicalgeography.net/fundamentals/images/seafloor_spreading.gif on 8/18/06
b) Reversal of Magnetic Polarity
The strips of basaltic rock that lie parallel to the mid-ocean ridge show matched patterns of magnetic reversals. Check out this animation!
Over thousands of years, the magnetic poles of Earth reverse their polarities . The magnetic north pole changes to the magnetic south pole and vice versa.
When the basaltic magma flows up in the middle of the ridge and begins to cool , crystals of magnetic minerals align themselves with the Earth’s magnetic field. This alignment of minerals in the rock leaves a recording of magnetic polarity for the Earth at the time of rock formation.
When the Earth’s magnetic field is reversed, the new igneous rocks formed during the reversed polarity period have their minerals aligned in an opposite direction from the previously formed rocks.
These changes in magnetic orientation are found in rock on both sides of the mid-ocean ridge, indicating that the development of the ocean floor is form the center of the mid-ocean ridges outward.
Check out this animation!
Describe the 3 types of plate motion. Use ESRT p.5 to identify plate boundaries.
IV. Lithospheric Plates and Plate Boundaries
Three kinds of plate motion are associated with plate boundaries; convergent, divergent and transform .
Taken from http://www.3villagecsd.k12.ny.us/Murphy/medina/eqs.html on 8/18/06
a) Convergent Plate Boundaries
Convergent Plate Boundaries- plates collide with each other
Ocean Plate Meets Continental Plate
If an oceanic plate collides with a continental plate, the denser ocean plate made of basalt dives down (subducts) into the mantle forming a subduction zone with an ocean trench formed at the surface.
At the subduction zone, old crust is consumed by the mantle to create more molten material. The overriding continental plate made of granite forms mountains. An example is the Andes of South America.
Ocean Plate Meets Ocean Plate
If two oceanic plates converge, the older, denser plate will subduct also forming a trench on the surface along with a chain of islands called an island arc .
An example of this convergent subduction zone is the Northern and Western boundaries of the Pacific Ocean .
Continental Plate Meets Continental Plate
If a continental plate collides with another continental plate, the edge of both plates are crumpled up forming folded mountains .
An example of this type of convergent boundary is the Himalayas of India.
Now look at the two diagrams of convergent boundaries in your note packet. Which one shows a subduction zone? Label the subduction zone in that diagram.
b) Divergent Plate Boundaries
Divergent Plate Boundaries- plates move apart
A divergent boundary allows heat and magma to flow up from below forming parallel ridges made of new crustal material.
An example of a divergent plate boundary like this is any mid-ocean ridge.
In your note packet, label the parallel ridges formed from the upwelling magma in the divergent boundary diagram.
c) Transform Plate Boundary
Transform Plate Boundary- plates grind slowly past each other
At this type of boundary, crust is neither formed nor consumed.
An example is San Andreas Fault in California.
Shallow focus earthquakes are very common at transform boundaries.
In your note packet, label the shallow focus earthquakes in the transform diagram. Also place large arrows on each plate to show the direction of plate motion. Taken from http:// observe.arc.nasa.gov/nasa / earth/tectonics/Tectonics3.html 0n 8/18/06
Plate Tectonic Map (ESRT p. 5 )
Although plate motion is only a few centimeters a year, the interactions of the boundaries result in earthquakes, volcanoes and mountain building on a grand scale showing that the Earth is a dynamic system.
Explain how mantle convection cells are thought to be the method for moving crustal plates.
V. Mantle Convection Cells
Although forces exist within the Earth that are powerful enough to move the lithospheric plates, the scientific community is not in total agreement on the specific mechanism (method) involved.
Convection cell- stream of heated material that is moving due to density differences
Taken from http://www.sunblock99.org.uk/ sb99/people/KGalsgaa/convect.gif on 8/18/06 Taken from http://www.platetectonics.com/book/images/Convection.gif on 8/18/06 Animation
Evidence suggests that convection cells exist within a part of the mantle called the asthenosphere because of the occurrence of heat flow highs in areas of mountain building and heat flow lows in areas of shallow subsiding basins.
These convection cells may be part of the driving force which causes continents to move.
What are hot spots? How are they formed?
VI. Hot Spots
Hot Spots- places on Earth’s surface with unusually high heat flow
Most hot spots occur along active plate margins but some are found within the plates.
Hot spots are thought to be caused by magma rising up from the mantle producing sites of active volcanism.
As a plate passes over a hot spot, a chain of volcanic mountain forms, like the Hawaiian Islands.
The only mountain that remains an active volcano is the one located directly over the hot spot.