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Geological Processes Overview Meet the Earth Development of plate tectonic theory: what is right isn’t always popular Supporting evidence of the theory of continental drift and plate tectonics Three main types of plate boundaries- fun activity Volcanoes and earthquakes Layers of the Earth Continental crust is an average of 3040 km thick (can be up to 100 km thick on the craton) Oceanic crust is ~6-7 km thick The uppermost mantle behaves rigidly (like a solid), and together with the overlying crust is referred to as the lithosphere Underneath the lithosphere is the less rigid asthenosphere, which is partially molten Below about 200 km the mantle is fully solid The Core: core is larger than Mars! made up of two distinct layers: a liquid outer layer and a solid inner core. made up of metallic iron-nickel alloy What is a plate? Source: www.nature.nps.gov/.../ usgsnps/pltec/pltec1.ht ml a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Most of the boundaries between individual plates cannot be seen, because they are hidden beneath the oceans. Quaking and shaking… Humans are curious….. Why is the Earth so restless? What causes the ground to shake violently? Why do volcanoes erupt? Why are our mountain ranges so tall? Early thoughts… Until the 1700s, most Europeans thought that a Biblical Flood played a major role in shaping the Earth's surface. “Catastrophism” : the belief that all earthly changes were sudden and caused by a series of catastrophes. Mid-19th century: catastrophism => "uniformitarianism" "Uniformitarian Principle" (James Hutton, 1785): THE PRESENT IS THE KEY TO THE PAST Geologic forces that act on the Earth today are the same as those that have acted in the geologic past. Continental Drift Predecessor to modern plate tectonics Shape and “fit” of the continents was the initial evidence Snider-Pelligrini (1858): ideas were mostly ignored Wegner (1915) 1858: Snider-Pelligrini drawings Source: http://pubs.usgs.gov/gip/dynamic/historical.html Continental Drift Alfred Wegener (1880-1930) Proposed all of the continents were once part of a large supercontinent - Pangaea Based on: Source: http://pubs.usgs.gov/gip/dynamic/wegener.html Similarities in shorelines Distinctive rock and fossil groups found in Africa & South America Ideas were not widely accepted Froze to death in 1930 during an expedition crossing the Greenland ice cap Evidence for Continental Drift Source: http://pubs.usgs.gov/gip/ dynamic/continents.html Paleontological: Similarity of fossils on opposite sides of the Atlantic and Indian Oceans, ex. Mesosaurus, Glossopteris Problem: Perhaps similar animals had evolved in these areas (convergent evolution). Perhaps there were floating pieces of crust that rafted animals and plant seeds across? Transoceanic land bridges? Evidence for Continental Drift Source: http://pubs.usgs.gov/gip/ dynamic/continents.html Rock types and structures: Distinct rock type and geologic structures on both sides of the Atlantic Ocean Problem: Erosion and weathering breaks down rocks over time. How could the continents still “fit” together after millions of years of wear and tear? Evidence for Continental Drift Source: http://pubs.usgs.gov/gip/ dynamic/continents.html Paleoclimate Evidence of extreme changes in climate as compared to the present (glacial deposits in present day deserts) Problem: Perhaps there was a total global cooling event Development of Plate Tectonic Theory Wegener’s theory was dismissed as being eccentric, preposterous, and improbable. Original evidence for continental drift was ONLY from continental rocks Technological advances in the 1950’s and 1960’s allowed investigation of the sea floor and oceanic rocks Geophysics & paleomagnetism provided new data Harry Hess- Sea floor spreading From the 1940’s to the 60’s, Harry Hess made many key intellectual contributions to the coming revolution in geologic thought: Echo-sounding of sea floor revealed deep sea features and the topography of mid-ocean ridges “Father of Plate Tectonics” Helped provide more evidence to support Wegener’s theories ridges are areas of high heat flow and volcanic activity young age of ocean floor, based on thickness of sediment Hot material rose out of ridges (spreading centers) and moved across the sea floor, crust was recycled. Earth’s magnetic field provides more evidence The Earth has an invisible magnetic field (all free-floating magnets at the Earth’s surface point to magnetic north)---Think about a compass. Iron-rich minerals crystallizing from molten rock will orient towards magnetic north when they cool below the Curie point, the temperature above which permanent magnetism is impossible (580 degrees C for magnetite). Thus lavas lock in the record of Earth’s magnetic field when they form. Magnometers Adapted from airborne devices developed during World War II to detect submarines Used to recognize odd magnetic variations across the ocean floor. Discovered magnetic “stripes” on the sea floor Rocks formed over time will record either a “normal” magnetic orientation or “reversed” Magnetic reversals are global phenomena and can be used for global stratigraphic correlation http://pubs.usgs.gov/gip/dynamic/developing.html Evidence for plate tectonics Demonstration of the ruggedness and youth of the ocean floor Confirmation of repeated reversals of the Earth magnetic field in the geologic past Emergence of the seafloor-spreading hypothesis and associated recycling of oceanic crust Precise documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges So…why do the plates move? Plate tectonics is: a geological theory that pieces of Earth’s lithosphere are in constant, slow motion, driven by convection currents in the mantle. explains the formation, movement, and subduction of Earth’s plates. The plates of the lithosphere float on top of the asthenosphere. Convection currents rise in the asthenosphere and spread out beneath the lithosphere. HEAT Types of Plate Boundaries http://cimss.ssec.wisc.edu/sage/geology/lesson2/images/concepts_fig8.gif Plate boundaries Divergent: plates are pulling away from each other Convergent: Plates are colliding and one is subducted under the other Transform: Plates are sliding past each other Snickers Bar Activity Snickers Bar Activity- Step 1 Obtain a Snickers bar. Unwrap the bar and make a few cracks across the top middle portion of its top. What does the chocolate on each side of the cracks represents. a tectonic plate The Earth is covered with several tectonic plates. We live on the North American Plate What does the nougat/ caramel portion represent? The asthenosphere Snickers Bar Activity- Step 2 Hold each end of the candy bar with your thumb and forefinger and SLOWLY stretch the candy bar, pulling it no more than 1/2 to 1 inch apart. What kind of plate boundary have you made? A Divergent Plate boundary Snickers Bar Activity- Step 3 Push the candy bar back together. Continue adding pressure until one piece of chocolate moves beneath another. What kind of plate boundary have you made? A Convergent Plate boundary Snickers Bar Activity- Step 4 Pull the bar back to its normal shape. Push one side forward and pull one side back. What kind of plate boundary have you made? A Transform Plate boundary The Consequences of Plate Movement…volcanoes and earthquakes What is a volcano? A place on the earth’s surface (or any other planet) where molten rock and gases are erupted. There are, however, many kinds of volcanoes. Volcano does not have to be a beautiful snowcapped conical peak (Mt St Helens). It can be a hole in the ground, or a crack in the earth’s surface Most volcanoes occur at convergent boundaries Types of Volcanoes Why do volcanoes erupt? Hot, molten rock (magma) is buoyant (has a lower density than the surrounding rocks) and will rise up through the crust to erupt on the surface. When magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it as to how it erupts. Large amounts of gas and a high viscosity (sticky) magma will form an explosive eruption! Same principle as hot air rising, e.g. how a hot air balloon works Think about shaking a carbonated drink and then releasing the cap. Small amounts of gas and (or) low viscosity (runny) magma will form an effusive eruption Where the magma just trickles out of the volcano (lava flow). Volcanic Hazards Pyroclastic flow Lahars/Mud flows Pyroclastic and ash fall Lava flow Noxious Gas Earthquakes: Large volumes of magma moving through the shallow crust can cause large earthquakes. What is an earthquake? a tremor of the earth's surface usually triggered by the release of underground stress along fault lines. This release causes movement in masses of rock Epicenter and Focus Earthquake waves Body Waves Primary or Compressional: P- waves Secondary or Shear: S- waves Surface Waves (won’t discuss these) Raleigh (large vertical displacements) Love (shear) How do we measure earthquakes Richter scale invented by Charles F. Richter in 1934. calculated from the amplitude of the largest seismic wave recorded for the earthquake, no matter what type of wave was the strongest. based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the Richter scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Mercalli scale. invented by Giuseppe Mercalli in 1902 uses the observations of the people who experienced the earthquake to estimate its intensity. isn't considered as scientific as the Richter scale witnesses of the earthquake might exaggerate just how bad things were during the earthquake and you may not find two witnesses who agree on what happened; everybody will say something different. Seismograph How we measure earthquake waves Determines depth and magnitude of quakes Earthquake fact or fiction Mega- Quakes, as seen in “10.5” The truth: THEORETICALLY, YES. REALISTICALLY, NO. The magnitude of an earthquake is related to the length of the fault on which it occurs -- the longer the fault, the larger the earthquake. No fault long enough to generate a magnitude 10.5 earthquake is known to exist. The largest earthquake ever recorded was a magnitude 9.5 on May 22, 1960 in Chile on a fault that is almost 1,000 miles long. We can’t rule out a Mega-Quake because diagnostic equipment has only been around for about 100 years, insignificant when compared to geologic time. California has the most earthquakes in the US The truth: Alaska registers the most earthquakes in a given year. California= the most damaging earthquakes because of its greater population and extensive infrastructure. Alaska’s large earthquakes occur in remote locations such as along the Aleutian Island chain. Florida and North Dakota have the fewest earthquakes each year. The ground can open up during an earthquake. The truth: NO WAY!!! Gaping faults exist only in movies and novels. The ground moves across a fault during an earthquake, not away from it, due to friction. If the fault could open, there would be no friction. Without friction, there would be no earthquake. Shallow crevasses can form during earthquake induced landslides, lateral spreads, or other types of ground failures, but the fault line itself does not open. California will eventually fall into the ocean. Sorry Lex Luthor… NO WAY!! It’s absolutely impossible that California will be swept out to sea. Instead, southwestern California is moving horizontally northward towards Alaska as it slides past central and eastern California. California sits on the Pacific Plate which is moving to the northwest with respect to the North American Plate at approximately 46 millimeters (two inches) per year (the rate your fingernails grow). In about 15 million years, Los Angeles and San Francisco will be next-door neighbors In about 85 million years, Los Angeles residents will find themselves with an Alaska zip code! People can cause earthquakes. A glimmer of hope for Lex Luthor…PARTIALLY FACT: Earthquakes induced by human activity have been documented in a few locations in the United States, Japan, and Canada. The cause was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the filling of large reservoirs for water supplies. Most of these earthquakes were minor. Deep mining can cause small to moderate quakes and nuclear testing has caused small earthquakes in the immediate area surrounding the test site, but other human activities have not been shown to trigger subsequent earthquakes. We can predict earthquakes. IF ONLY… Earthquake prediction is the holy grail for earthquake scientists, but there currently is no accepted method to accomplish the goal of predicting the time, place and magnitude of an impending quake. Most research is focused on long range (next 30 years) , not 1-5 years. We can predict the type of ground motion to expect based on the geology and the history of earthquake activity of the region, but not when the motion will occur.