Stress & Strain Elastic, Plastic Ductile or Brittle
THE EARTH’S DYNAMIC CRUST Basic concepts of plate tectonics
Plate tectonics Plate tectonic theory- lithosphere is divided into rigid plates that move over the asthenosphere. Seven major and about 20 minor Plate boundaries rub together & are geologically active. Earthquakes & Volcanoes
Convergent –A. Compression causes shortening of the rock layers through folding & faulting. Divergent –B. Tension lengthens and causes faulting. Transform –C. Shear stress results in displacement (with a horizontal component) along a plane. 3 types of plate boundaries:
Convergent boundaries Associated with structures of compression High frequency of earthquakes May be marked by oceanic trenches and subduction zones Suture zones are formed when continental masses are welded together
Divergent boundaries Associated with tensional structures New crust is added at the trailing edges as basaltic lava wells up from the mantle Many shallow earthquakes associated with rifting Characterized by a mid-oceanic ridge split by a deep rift
Filling the Students “Plates” PBS.org has an excellent on line tutorial on the moving plates. Simple introductive text, interactive Plate Tectonics Graphic, & an “Egg-cellent” hands on activity. http://www.pbs.org/wgbh/aso/tryit/tectonics/#
What mechanism causes plate tectonics? Most theories involve large, rolling convection cells in the asthenosphere –Density Differences –Mantle Convection Try using a Lava Lamp.
Plate tectonics has its roots in two older ideas Continental Drift Hypothesis Seafloor Spreading Theory
Continental Drift Main ideas set forth by : –Alfred Wegener (presented in Europe in 1912, U.S. 1924) Introduction to Alfred Wegener and Continental Drift http://www.ucmp.berkeley.edu/history/wegener.html http://www.ucmp.berkeley.edu/history/wegener.html “Wrath of Science & Obligation to be a Hero” http://pangaea.org/wegener.htm http://pangaea.org/wegener.htm
Alfred Wegener Argued for a much larger supercontinent which he called Pangaea Pangaea was surrounded by a universal ocean (Panthalassa)
Continental Drift Primary Evidence: –Fit of Continents –Evidence of Fossils –& Paleoclimatology
Geographic fit of continents See west Africa and eastern South America Especially true if the continental shelves are included Mismatch is minimal with proper alignment
World’s largest Jigsaw Puzzle Find and print a map of the world’s continents. Specifically, show North America, South America, Europe & Africa. Have the students cut the continents out and reassemble them to a single landmass. They should use arrows to show the plate movements from the single landmass to fragmented continents. Have the students write a descriptive narrative explaining the events that have separated the landmasses. Require them to use proper terminology for the boundaries.
Rocks and sediments Paleozoic tillites in all parts of “Gondwanaland” –Without Gondwanaland, the center of glaciation would have been the ocean –Boulders in tillites had sources on other continents Mountain ranges which would be continuous, but for Atlantic Ocean Evaporite deposits in cold areas
Paleontological evidence Tropical plants in Antarctic coal beds Coral reef deposits in cold climates Glossopteris flora- fossil plants found throughout Gondwanaland Mesosaurus fauna- aquatic reptiles whose remains have been found in many sites in southern hemisphere
The Flaw of Wegener’s Hypothesis Many professionals criticized Wegener because he had no plausible mechanism which could account for the movement of continents (???plowing ship or sliding over???)
Wegener Forgotten "Utter, damned rot!" said the president of the prestigious American Philosophical Society. "If we are to believe [this] hypothesis, we must forget everything we have learned in the last 70 years and start all over again," said another American scientist. *quotes & earlier biographical sketch from, http://earthobservatory.nasa.gov/Library/Giants/Wegener
Technology developed during World War II led to an explosion of knowledge concerning the ocean basins during the post war years.
Examples of this technology: Magnetometers Devices used in monitoring nuclear tests Sonar SCUBA and deep-diving submarines
EARTH as a MAGNET? Earth can be thought of as a giant dipole magnet New rock is created at divergent boundaries as oceanic crust (igneous rock) moves upward to fill the void as crust pulls apart. When the molten rock cools, iron bearing minerals align themselves with Earth’s Magnetic field. The lava records the direction & strength of the magnetic field. CURIE POINT
Magnetic Patterns After WWII we began mapping the magnetic fields on the continents and the sea floor. The newest continental & ocean rock showed patterns of today’s magnetic field, but older rocks did not, they showed different orientations! If you trace the path the continents have moved since Pangaea split, then the orientation of Fe- bearing minerals on the continent do show the magnetic field to be aligned north to south.
Magnetic Reversals Based on land rock we know: In the past the magnetic field has completely reversed, it keeps its north to south alignment, but the poles of the magnet have reversed. Today’s field is considered “normal magnetism” and when the poles have swapped is called “reversed magnetism” If new crust is constantly being formed from lava at divergent boundaries in the ocean shouldn’t we see the reversals there?
Harry Hess & Seafloor Spreading Hess proposed the continents do not move across the oceanic crust, but rather they move together. Like a conveyor belt that originates at the divergent boundaries. Samples taken confirm that indeed, not only does the oceanic crust become older as one moves away from the ridge but the ocean floor is striped by parallel & symmetric magnetic reversals that have occurred in the past.
-Continued... Apparently the polarity of Earth’s magnetic field “flips” from time to time. This led Vine and Matthews, in 1963, to suggest that all sea floor was formed at the mid-oceanic ridges, from which point it spread outward. This is the concept of seafloor spreading.
More evidence of seafloor spreading Rocks get progressively older as one moves away from the midoceanic ridges. Sediments over the rocks are youngest near ridges. Sediments are thicker toward the continents. Seafloor spreading has been measured using lasers and moon-based reflectors.
The Graffiti Conveyor Belt Construct a divergent boundary (rift zone). Use a cardboard box, coat hanger, corks, two pieces of adding machine tape, & some spray paint Assign two persons to be tectons (Greek for builder) and let them move the plates. Assign two persons to record the polarity of the new rock being formed. 1 normal & 1 reverse. One person will be the unpredictable, whimsical mother nature.
Hot spots Evidence suggests that all the Hawaiian Islands the adjoining Emperor Seamounts were formed over the same “hot spot” in the mantle and were transported to their present positions by moving plates. Yellowstone is also over a hot spot and can be used as an example as well. *image taken from http://volcano.und.edu/vwdocs/vwlessons/hot_spots/int roduction.html
Measuring the rate of plate movement Plates move about as fast as my fingernails grow?* How can we measure plate movement? Use the age of the Hawaiian Islands and their distance from the hot spot to determine the rate of movement. The average growth rate for nails is 0.1 mm each day (or 1 centimeter in 100 days). The exact rate of nail growth depends on numerous factors including the age and sex of the individual and the time of year. Fingernails generally grow faster in young people, in males, and in the summer. Taken from www.medicinenet.com
Measuring the rate of plate movement Use the following pages to find printable maps to use when determining the rate http://www.bsu.edu/csh/geology/dept/faculty/teri/ec2b.html http://volcano.und.edu/vwdocs/vwlessons/hot_spots/introd uction.htmlhttp://volcano.und.edu/vwdocs/vwlessons/hot_spots/introd uction.html
Measuring the rate of plate movement Measure the distance between island, divide by the difference in years. Convert to cm/yr & compare to the rate of growth for average fingernails.
Where are earthquakes occurring? Recent earthquakes http://earthquakes.usgs.gov/recenteqsUS/ http://www.iris.edu/ http://earthquakes.usgs.gov/recenteqsUS/ http://www.iris.edu/ Seismic Hazard maps for the U.S. http://eqhazmaps.usgs.gov/ http://eqhazmaps.usgs.gov/ Close to Home: New Madrid Information http://www.hsv.com/genlintr/newmadrd/ http://www.hsv.com/genlintr/newmadrd/
Frequency & Distribution Shallow 300km 90% are less than 100km deep, shallow are the most destructive 80% of all earthquakes along the ring of fire, only 5% intra-plate 150,000/year strong enough to be felt; 900,000 recorded by seismograph, but too small to catalogs
Intensity Intensity-subjective measure based on damage & people’s reaction to the quake. Measure using the Modified Mercalli Intensity Scale other factors include distance from epicenter, focal depth, population density, & local geology, duration, & building types
I. People do not feel any Earth movement. II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring. IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock. V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers. VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. Modified Mercalli Intensity Scale, from FEMAFEMA
Modified Mercalli Intensity Scale, from FEMAFEMA VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings. VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change. IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage. X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly. XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.
Modified Mercalli Scale in TN Accounts of 1811- 1812 New Madrid Earthquakes. http://www.hsv.com/genlintr/ne wmadrd/accnt3.htmhttp://www.hsv.com/genlintr/ne wmadrd/accnt3.htm
Magnitude Quantitative measure of the total amount of energy released at its source using the Richter magnitude scale. Highest peak recorded Largest ever upper 8, >9 possible, but not likely since rocks can’t store that much energy Base 10 logarithmic scale that converts the largest recorded seismic wave to a numerical magnitude, (10 fold increase in magnitude). Each magnitude increase represents a 30 fold increase in the energy released