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.
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
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
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.
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
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
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.
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
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
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
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
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.
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
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.
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
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.
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
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.
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.