2 Plate Tectonics

CI 5995 Engineering SeismologyLecture 2 : Plate Tectonics

Class website:

http://richter.uprm.edu/~jclinton/CI5995.html

Geologic Time

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Adapted from http://www.ucmp.berkeley.edu/geology/ http://pubs.usgs.gov/publications/text/dynamic.html

• Plate tectonics is the theory that Earth's outer layer is made up of plates, which have moved throughout Earth's history. • The theory explains the how and why behind mountains, volcanoes, and earthquakes, as well as how, long ago, similar animals could have lived at the same time on what are now widely separated continents. • 225 million years ago, all the major continents formed one giant supercontinent, called Pangaea.• Perhaps initiated by heat building up underneath the vast continent, Pangaea began to rift, or split apart, around 200 million years ago. Oceans filled the areas between these new sub-continents. The land masses continued to move apart, riding on separate plates, until they reached the positions they currently occupy. • These continents are still on the move today.• Exactly what drives plate tectonics is not known.

•One theory is that convection within the Earth's mantle pushes the plates, in much the same way that air heated by your body rises upward and is deflected sideways when it reaches the ceiling.•Another theory is that gravity is pulling the older, colder, and thus heavier ocean floor with more force than the newer, lighter seafloor

•.Whatever drives the movement, plate tectonic activity takes place at four types of boundaries: • divergent boundaries, where new crust is formed; • convergent boundaries, where crust is consumed; • collisional boundaries, where two land masses collide; and • transform boundaries, where two plates slide against each other.

•The main features of plate tectonics are: • The Earth's surface is covered by a series of crustal plates.• The ocean floors are continually, moving, spreading from the center, sinking at the edges, and being regenerated.• Convection currents beneath the plates move the crustal plates in different directions.• The source of heat driving the convection currents is radioactivity deep in the Earths mantle.

Introduction

Historical Perspective (1)•Close examination of a globe often results in the observation that most of the continents seem to fit together like a puzzle: the west African coastline seems to snuggle nicely into the east coast of South America and the Caribbean sea; and a similar fit appears across the Pacific. The fit is even more striking when the submerged continental shelves are compared rather than the coastlines.

•In 1912 Alfred Wegener (1880-1930) noticed the same thing and proposed that the continents were once compressed into a single protocontinent which he called Pangaea (meaning "all lands"), and over time they have drifted apart into their current distribution. He believed that Pangaea was intact until the late Carboniferous period, about 300 million years ago, when it began to break up and drift apart.

•However, Wegener's hypothesis lacked a geological mechanism to explain how the continents could drift across the earths surface as he proposed.Searching for evidence to further develop his theory of continental drift, Wegener came across a paleontological paper suggesting that a land bridge had once connected Africa with Brazil. This proposed land bridge explained paleontological observation that the same fossilized plants and animals from the same time period were found in South America and Africa. The same was true for fossils [Europe / North America], [Madagascar / India]. Many of these organisms could not have traveled across the vast oceans that currently exist. Wegener's drift theory seemed more plausible than land bridges connecting all of the continents. But that in itself was not enough to support his idea.

•Another observation favoring continental drift was the presence of evidence for continental glaciation in the Pensylvanian period. Striae left by the scraping of glaciers over the land surface indicated that Africa and South America had been close together at the time of this ancient ice age. The same scraping patterns can be found along the coasts of South America and South Africa.Wegener's drift hypothesis also provided an alternate explanation for the formation of mountains (orogenesis).

•The theory being discussed during his time was the "Contraction theory" which suggested that the planet was once a molten ball and in the process of cooling the surface cracked and folded up on itself. The big problem with this idea was that all mountain ranges should be approximately the same age, and this was known not to be true. Wegener's explanation was that as the continents moved, the leading edge of the continent would encounter resistance and thus compress and fold upwards forming mountains near the leading edges of the drifting continents. The Sierra Nevada mountains on the Pacific coast of North America and the Andes on the coast of South America were cited. Wegener also suggested that India drifted northward into the asian continent thus forming the Himalayas.Wegener eventually proposed a mechanism for continental drift that focused on his assertion that the rotation of the earth created a centrifugal force towards the equator. He believed that Pangaea originated near the south pole and that the centrifugal force of the planet caused the protocontinent to break apart and the resultant continents to drift towards the equator. He called this the "pole-fleeing force". This idea was quickly rejected by the scientific community primarily because the actual forces generated by the rotation of the earth were calculated to be insufficient to move continents. Wegener also tried to explain the westward drift of the Americas by invoking the gravitational forces of the sun and the moon, this idea was also quickly rejected. Wegener's inability to provide an adequate explanation of the forces responsible for continental drift and the prevailing belief that the earth was solid and immovable resulted in the scientific dismissal of his theories

Historical Perspective (2)• In 1929, about the time Wegener's ideas began to be dismissed, Arthur Holmes elaborated on one of Wegener's many hypotheses; the idea that the mantle undergoes thermal convection. This idea is based on the fact that as a substance is heated its density decreases and rises to the surface until it is cooled and sinks again. This repeated heating and cooling results in a current which may be enough to cause continents to move. Arthur Holmes suggested that this thermal convection was like a conveyor belt and that the upwelling pressure could break apart a continent and then force the broken continent in opposite directions carried by the convection currents. This idea received very little attention at the time.Not until the 1960's did Holmes' idea receive any attention. Greater understanding of the ocean floor and the discoveries of features like mid-oceanic ridges, geomagnetic anomalies parallel to the mid-oceanic ridges, and the association of island arcs and oceanic trenches occurring together and near the continental margins, suggested convection might indeed be at work. These discoveries and more led Harry Hess (1962) and R.Deitz (1961) to publish similar hypotheses based on mantle convection currents, now known as "sea floor spreading". This idea was basically the same as that proposed by Holmes over 30 years earlier, but now there was much more evidence to further develop and support the idea.

•Advances in sonic depth recording during World War II and the subsequent development of the nuclear resonance type magnometer (proton-precession magnometer) led to detailed mapping of the ocean floor and with it came many observation that led scientists like Howard Hess and R. Deitz to revive Holmes' convection theory.

•Hess and Deitz modified the theory considerably and called the new theory "Sea-floor Spreading". Among the seafloor features that supported the sea-floor spreading hypothesis were: mid-oceanic ridges, deep sea trenches, island arcs, geomagnetic patterns, and fault patterns.

•Mid-Oceanic Ridges The mid-oceanic ridges rise 3000 meters from the ocean floor and are more than 2000 kilometers wide surpassing the Himalayas in size. The mapping of the seafloor also revealed that these huge underwater mountain ranges have a deep trench which bisects the length of the ridges and in places is more than 2000 meters deep. Research into the heat flow from the ocean floor during the early 1960s revealed that the greatest heat flow was centered at the crests of these mid-oceanic ridges. Seismic studies show that the mid-oceanic ridges experience an elevated number of earthquakes. All these observations indicate intense geological activity at the mid-oceanic ridges.



• Geomagnetic Anomalies Peridically, the Earth's magnetic field reverses. New rock formed from magma records the orientation of Earth's magnetic field at the time the magma cools. Study of the sea floor with magnometers revealed "stripes" of alternating magnetization parallel to the mid-oceanic ridges. This is evidence for continuous formation of new rock at the ridges. As more rock forms, older rock is pushed farther away from the ridge, producing symmetrical stripes to either side of the ridge. In the diagram to the right, the dark stripes represent ocean floor generated during "reversed" polar orientation and the lighter stripes represent the polar orientation we have today. Notice that the patterns on either side of the line representing the mid-oceanic ridge are mirror images of one another. The shaded stripes also represent older and older rock as they move away from the mid-oceanic ridge. Geologists have determined that rocks found in different parts of the planet with similar ages have the same magnetic characteristics.

•Deep Sea Trenches The deepest waters are found in oceanic trenches, which plunge as deep as 35,000 feet below the ocean surface. These trenches are usually long and narrow, and run parallel to and near the oceans margins. They are often associated with and parallel to large continental mountain ranges. There is also an observed parallel association of trenches and island arcs. Like the mid-oceanic ridges, the trenches are seismically active, but unlike the ridges they have low levels of heat flow. Scientists also began to realize that the youngest regions of the ocean floor were along the mid-oceanic ridges, and that the age of the ocean floor increased as the distance from the ridges increased. In addition, it has been determined that the oldest seafloor often ends in the deep-sea trenches.

•Island Arcs Chains of islands are found throughout the oceans and especially in the western Pacific margins; the Aleutians, Kuriles, Japan, Ryukus, Philippines, Marianas, Indonesia, Solomons, New Hebrides, and the Tongas, are some examples.. These "Island arcs" are usually situated along deep sea trenches and are situated on the continental side of the trench.These observations, along with many other studies of our planet, support the theory that underneath the Earth's crust (the lithosphere: a solid array of plates) is a malleable layer of heated rock known as the asthenosphere which is heated by radioactive decay of elements such as Uranium. Because the radioactive source of heat is deep within the mantle, the fluid asthenosphere circulates as convection currents underneath the solid lithosphere. This heated layer is the source of 1. lava we see in volcanos, 2. heat that drives hot springs and geysers, and 3. raw material which pushes up the mid-oceanic ridges and forms new ocean floor. Magma continuously wells upwards at the mid-oceanic ridges producing currents of magma flowing in opposite directions and thus generating the forces that pull the sea floor apart at the mid-oceanic ridges. As the ocean floor is spread apart cracks appear in the middle of the ridges allowing molten magma to surface through the cracks to form the newest ocean floor. As the ocean floor moves away from the mid-oceanic ridge it will eventually come into contact with a continental plate and will be subducted underneath the continent. Finally, the lithosphere will be driven back into the asthenosphere where it returns to a heated state.

Historical Perspective (3)



• Geomagnetic Anomalies

Historical Perspective (3)



The Earth is composed of 3 distinct layers: crust, mantle, and core.

Each layer has its own unique properties and chemical composition.

Crust The crust is the thin, solid, outermost layer of the Earth. The crust is thinnest beneath the oceans, averaging only 5 km thick, and thickest beneath large mountain ranges.Continental crust (the crust that makes up the continents) is much more variable in thickness but averages about 30-35km. Beneath large mountain ranges, such as the Himalayas or the Sierra Nevada, the crust reaches a thickness of up to 100 km.

Mantle The layer below the crust is the mantle. The mantle has more iron and magnesium than the crust,

making it more dense. The uppermost part of the mantle is solid and, along with the crust, forms the

lithosphere. The rocky lithosphere is brittle and can fracture. This is the zone where earthquakes occur. It’s

the lithosphere that breaks into the thick, moving slabs of rock, tectonic plates. As we descend into the Earth

temperature rises and we reach part of the mantle that is partially molten, the asthenosphere. As rock heats

up, it becomes pliable or ‘plastic’. Rock here is hot enough to fold, stretch, compress, and flow very slowly

without fracturing (like plasticine/silly putty). The plates, made up of the relatively light, rigid rock of the

lithosphere ‘float’ on the more dense, flowing asthenosphere.

Core At the center of the Earth lies the super-dense core. With a diameter of 3486 kilometers, the core is

larger than the planet Mars. The core of the Earth is made up of two distinct layers: a liquid outer layer and a

solid inner core. Unlike the Earth’s outer layers with rocky compositions, the core is made up of metallic iron

nickel alloy. The core is about 5 times as dense as surface rock.



A Section Through the Earth

Plates

The Main Tectonic Plates

Tectonic Evolution Movies1. Pangea to today

Tanya Atwater, UCSB : http://emvc.geol.ucsb.edu/downloads.php

Western USA Tanya Atwater, UCSB : http://emvc.geol.ucsb.edu/downloads.php

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Subduction Zone Tanya Atwater, UCSB : http://emvc.geol.ucsb.edu/downloads.php

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S. Atlantic Spreading : SectionTanya Atwater, UCSB : http://emvc.geol.ucsb.edu/downloads.php

Another View

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Movie of above clip : 400MA to now

More Movies

Pangea Breakup http://www.scotese.com/sfsanim.htm

Caribbean Tectonic Evolution http://www.scotese.com/caribanim.htm

Future Tectonics??? http://www.scotese.com/futanima.htm

Plate Boundaries

There are four types of plate boundaries:1. • Divergent boundaries -- where new crust is generated as the plates

pull away from each other.2. • Convergent boundaries -- where crust is destroyed as one plate

dives under another.3. • Transform boundaries -- where crust is neither produced nor

destroyed as the plates slide horizontally past each other.4. • Plate boundary zones -- broad belts in which boundaries are not

well defined and the effects of plate interaction are unclear.

Divergent boundariesDivergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle. (like two giant conveyor belts, facing each other but slowly moving in opposite directions as they transport newly formed oceanic crust away from the ridge crest).Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the Earth. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. (This rate, similar to fingernail growth, may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers). Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today. The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, is a natural laboratory for studying on land the processes also occurring along the submerged parts of a spreading ridge. Iceland is splitting along the spreading center between the North American and Eurasian Plates, as North America moves westward relative to Eurasia. The consequences of plate movement are easy to see around Krafla Volcano, where existing ground cracks have widened and new ones appear every few months. From 1975 to 1984, numerous episodes of rifting (surface cracking) took place along the fissure zone. Some of these rifting events were accompanied by volcanic activity; the ground would gradually rise 1-2 m before abruptly dropping, signalling an impending eruption. Between 1975 and 1984, the displacements caused by rifting totalled about 7 m. In East Africa, spreading processes have already torn Saudi Arabia away from the rest of the African continent, forming the Red Sea. The actively splitting African Plate and the Arabian Plate meet in what geologists call a triple junction, where the Red Sea meets the Gulf of Aden. A new spreading center may be developing under Africa along the East African Rift Zone. When the continental crust stretches beyond its limits, tension cracks begin to appear on the Earth's surface. Magma rises and squeezes through the widening cracks, sometimes to erupt and form volcanoes. Whether or not it erupts, it puts more pressure on the crust, producing additional fractures and, ultimately, the rift zone. East Africa may be the site of the Earth's next major ocean. Plate interactions in the region provide scientists an opportunity to study first hand how the Atlantic may have begun to form about 200 million years ago. If spreading continues, the 3 plates at the triple junction will separate completely, allowing the Indian Ocean to flood the area and making the easternmost Africa a large island.

Convergent boundariesThe size of the Earth has not changed significantly during the past 600 million years, and very likely not since shortly after its formation 4.6 billion years ago - this implies that the crust must be destroyed at about the same rate as it is being created.Such destruction (recycling) of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks (is subducted) under another. The type of convergence -- called by some a very slow "collision" -- that takes place between plates depends on the kind of lithosphere involved : 1.oceanic and a largely continental plate, or 2. between two largely oceanic plates, or 3. between two largely continental plates.Oceanic-continental convergence: the surface of the Pacific Ocean, beneath the sea, contains a number of long narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor. Trenches are the deepest parts of the ocean floor and are created by subduction. Off the coast of South America along the Peru-Chile trench, the oceanic Nazca Plate is pushing into and being subducted under the continental part of the South American Plate. In turn, the overriding South American Plate is being lifted up, creating the towering Andes mountains, the backbone of the continent. Strong, destructive earthquakes and the rapid uplift of mountain ranges are common in this region. Even though the Nazca Plate as a whole is sinking smoothly and continuously into the trench, the deepest part of the subducting plate breaks into smaller pieces that become locked in place for long periods of time before suddenly moving to generate large earthquakes. Such earthquakes are often accompanied by uplift of the land by as much as a few meters. Can produce massive, deep events such as 9 June 1994, Mw8.3 event struck about 320 km northeast of La Paz, Bolivia, within the subduction zone between the Nazca Plate and the South American Plate depth of 636 km. One of deepest and largest subduction earthquakes recorded in South America. Oceanic-continental convergence also sustains many of the Earth's active volcanoes, such as those in the Andes and the Cascade Range in the Pacific Northwest. The eruptive activity is clearly associated with subduction, but scientists vigorously debate the possible sources of magma: partial melting of the subducted oceanic slab, or the overlying continental lithosphere, or both?

Convergent boundariesOceanic-oceanic convergence As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed. The Marianas Trench (paralleling the Mariana Islands) marks where the fast-moving Pacific Plate converges against the slower moving Philippine Plate. The Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior (nearly 11,000 m) than Mount Everest, the world's tallest mountain, rises above sea level (about 8,854 m). Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas, the Outer Antilles and the Aleutian Islands have formed and why they experience numerous strong earthquakes. Magmas that form island arcs are produced by the partial melting of the descending plate and/or the overlying oceanic lithosphere. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.

Continental-continental convergence The Himalayan mountain range dramatically demonstrates one of the most visible and spectacular consequences of plate tectonics. When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights. Most of this growth occurred during the past 10 million years. The Himalayas, towering as high as 8,854 m above sea level, form the highest continental mountains in the world. Moreover, the neighboring Tibetan Plateau, at an average elevation of about 4,600 m, is higher than all the peaks in the Alps except for Mont Blanc and Monte Rosa, and is well above the summits of most mountains in the United States.

Convergent boundariesOceanic-oceanic convergence As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed. The Marianas Trench (paralleling the Mariana Islands) marks where the fast-moving Pacific Plate converges against the slower moving Philippine Plate. The Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior (nearly 11,000 m) than Mount Everest, the world's tallest mountain, rises above sea level (about 8,854 m). Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas, the Outer Antilles and the Aleutian Islands have formed and why they experience numerous strong earthquakes. Magmas that form island arcs are produced by the partial melting of the descending plate and/or the overlying oceanic lithosphere. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.

Continental-continental convergence The Himalayan mountain range dramatically demonstrates one of the most visible and spectacular consequences of plate tectonics. When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights. Most of this growth occurred during the past 10 million years. The Himalayas, towering as high as 8,854 m above sea level, form the highest continental mountains in the world. Moreover, the neighboring Tibetan Plateau, at an average elevation of about 4,600 m, is higher than all the peaks in the Alps except for Mont Blanc and Monte Rosa, and is well above the summits of most mountains in the United States.


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Convergent boundaries - Summary

Transform boundariesThe zone between two plates sliding horizontally past one another is called a transform-fault boundary, or simply a transform boundary. The concept of transform faults originated with Canadian geophysicist J. Tuzo Wilson, who proposed that these large faults or fracture zones connect two spreading centers (divergent plate boundaries) or, less commonly, trenches (convergent plate boundaries). Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes. However, a few occur on land, for example the San Andreas fault zone in California. This transform fault connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge, another divergent boundary to the north. The San Andreas fault zone, which is about 1,300 km long and in places tens of kilometers wide, slices through two thirds of the length of California. Along it, the Pacific Plate has been grinding horizontally past the North American Plate for 10 million years, at an average rate of about 5 cm/yr. Land on the west side of the fault zone (on the Pacific Plate) is moving in a northwesterly direction relative to the land on the east side of the fault zone (on the North American Plate). Oceanic fracture zones are ocean-floor valleys that horizontally offset spreading ridges; some of these zones are hundreds to thousands of kilometers long and as much as 8 km deep. Examples of these large scars include the Clarion, Molokai, and Pioneer fracture zones in the Northeast Pacific off the coast of California and Mexico. These zones are presently inactive, but the offsets of the patterns of magnetic striping provide evidence of their previous transform-fault activity.



Plate-boundary zonesNot all plate boundaries are as simple as the main types discussed above. In some regions, the boundaries are not well defined because the plate-movement deformation occurring there extends over a broad belt (plate-boundary zone). An example is the Mediterranean-Alpine region between the Eurasian and African Plates, within which several smaller fragments of plates (microplates) have been recognized. Because plate-boundary zones involve at least two large plates and one or more microplates caught up between them, they tend to have complicated geological structures and earthquake patterns.Puerto Rico and Hispaniola are often described as lying on microplates between the Caribbean and North American Plates

Rates of Motion of PlatesHow do you determine the rates of plate movement, both now and over geologic time?

• The oceans hold one of the key pieces to the puzzle. Because the ocean-floor magnetic striping records the flip-flops in the Earth's magnetic field, scientists, knowing the approximate duration of the reversal, can calculate the average rate of plate movement during a given time span. These average rates of plate separations can range widely. The Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr). • Evidence of past rates of plate movement also can be obtained from geologic mapping studies. If a rock formation of known age -- with distinctive composition, structure, or fossils -- mapped on one side of a plate boundary can be matched with the same formation on the other side of the boundary, then measuring the distance that the formation has been offset can give an estimate of the average rate of plate motion. This simple but effective technique has been used to determine the rates of plate motion at divergent boundaries, for example the Mid-Atlantic Ridge, and transform boundaries, such as the San Andreas Fault. • Current plate movement can be tracked directly by means of ground-based or space-based geodetic measurements; geodesy is the science of the size and shape of the Earth. Ground-based measurements are taken with conventional but very precise ground-surveying techniques, using laser-electronic instruments. However, because plate motions are global in scale, they are best measured by satellite-based methods. The late 1970s witnessed the rapid growth of space geodesy, a term applied to space-based techniques for taking precise, repeated measurements of carefully chosen points on the Earth's surface separated by hundreds to thousands of kilometers. 3 most common space-geodetic techniques -- very long baseline interferometry (VLBI), satellite laser ranging (SLR), and the Global Positioning System (GPS) -- based on technologies developed for military and aerospace research (radio astronomy and satellite tracking). To date GPS has been the most useful for studying the Earth's crustal movements. GPS : 21 satellites are currently in orbit 20,000 km above the Earth, continuously transmiting radio signals back to Earth. To determine its precise position on Earth (longitude, latitude, elevation), each GPS ground site must simultaneously receive signals from at least four satellites, recording the exact time and location of each satellite when its signal was received. By repeatedly measuring distances between specific points, geologists can determine if there has been active movement along faults or between plates. By monitoring the interaction between the plates, scientists hope to learn more about the events building up to earthquakes and volcanic eruptions. Space-geodetic data have already confirmed that the rates and direction of plate movement, averaged over several years, compare well with rates and direction of plate movement averaged over millions of years.



Plates and Spreading Rateshttp://www2.umt.edu/Geology/faculty/sheriff/437-Seismology_Magnetics/Images/Tectonic_Map_World.jpg

HotspotsThe vast majority of earthquakes and volcanic eruptions occur near plate boundaries, but not all : eg Hawaiian Islands, which are entirely of volcanic origin, have formed in the middle of the Pacific Ocean more than 3,200 km from the nearest plate boundaryIn 1963, J. Tuzo Wilson :"hotspot" theory. He noted in various locations, volcanism has been active for very long periods of time. This could only happen, he reasoned, if relatively small, long-lasting, and exceptionally hot regions -- called hotspots -- existed below the plates that would provide localized sources of high heat energy (thermal plumes) to sustain volcanism. Specifically, Wilson hypothesized that the distinctive linear shape of the Hawaiian Island-Emperor Seamounts chain resulted from the Pacific Plate moving over a deep, stationary hotspot in the mantle, located beneath the present-day position of the Island of Hawaii. Heat from this hotspot produced a persistent source of magma by partly melting the overriding Pacific Plate. The magma, which is lighter than the surrounding solid rock, then rises through the mantle and crust to erupt onto the seafloor, forming an active seamount. Over time, countless eruptions cause the seamount to grow until it finally emerges above sea level to form an island volcano. Wilson suggested that continuing plate movement eventually carries the island beyond the hotspot, cutting it off from the magma source, and volcanism ceases. As one island volcano becomes extinct, another develops over the hotspot, and the cycle is repeated. This process of volcano growth and death, over many millions of years, has left a long trail of volcanic islands and seamounts across the Pacific Ocean floor.According to Wilson's hotspot theory, the volcanoes of the Hawaiian chain should get progressively older and become more eroded the farther they travel beyond the hotspot. The oldest volcanic rocks on Kauai, the northwesternmost inhabited Hawaiian island, are about 5.5 million years old and are deeply eroded. By comparison, on the "Big Island" of Hawaii -- southeasternmost in the chain and presumably still positioned over the hotspot -- the oldest exposed rocks are less than 0.7 million years old and new volcanic rock is continually being formed.

Global Hotspots

Largest Earthquakes in last 100 years



Caribbean Tectonics

Current Plate Boundaries

Earthquake record



Puerto Rican Tectonics and

Seismic Hazard

Puerto Rico is located on a microplate, sandwiched between the obliquely subducting North American and Caribbean plates (Figure 1), which accommodates approximately 30mm/yr of deformation. The main sources of seismic activity in the region are at the supposed boundaries of the microplate; the subduction zones to the North (the Puerto Rico Trench) and South (the Muertos Trough), the Anegada Trough to the East, and the Mona Canyon region to the West. All regions are capable of producing events greater than M7.0, and all have evidence of having done so in the recorded history of the island (Ascencio, 1980; Moya and McCann, 1992; Macari, 1994). Other shallow faults with less potential for large magnitude events are interspersed, mainly with E-W trends, across the island. On average Puerto Rico is strongly shaken with Modified Mercalli Intensity (MMI) >VII once every hundred years, and MMI > VI is experienced on the island once every 50years.

Puerto Rican Tectonics and

Seismic Hazard

For Mayagüez, the most important earthquake sources are 1) the Puerto Rico trench, with damaging events occurring at moderate depths about 70km to the North, Mmax8.0; 2) the Mona Canyon, 35km distant, Mmax7.5; 3) the local Mayagüez and Cordillera Faults, about 10km distant, each with Mmax6.5 (Moya and McCann, 1992). The first 2 sources would produce longer period, longer duration motions across the entire city, whereas the local faults could generate very high intensities at short periods over a small region. The most recent large earthquake to hit the island is the M7.5 1918 event, located in the Mona Canyon to the west of Aguadilla. 114 people were killed from the strong shaking and tsunami triggered by the event (the tsunami was 2m in height at Mayagüez, a maximum height of 6m was reached at Aguadilla). Structural damage was widespread throughout Western Puerto Rico, with Mayagüez being severely affected. The USGS (USGS, 2001) suggest the seismic hazard at Mayagüez is similar to that at Seattle, Washington State. The overall seismic hazard of the island is similar to that found in the Basin and Range province of the Western United States (http://eqhazmaps.usgs.gov/html/prvi2003doc.html).The local geology at Mayagüez creates the potential for ground motion amplifications, which increases the seismic risk for the town. Quaternary alluvial deposits are widespread across the low-lying valley where population is most concentrated. This is especially true of the test-bed site, which is only tens of meters from the banks of the Yagüez River, the main river running through the city. These unconsolidated sediments are typically of NEHRP site class E, and are of unknown depth – boreholes in excess of 45m do not reach competent bedrock. These sediments can produce amplification of the seismic waves as they pass from the bedrock to the surface, and also increased duration of strong shaking as trapped waves can resonate through the soil column. Capacete et al (1972) note extensive structural damage occurred right at the test-bed site during the 1918 event, Moya and McCann (1992) suggest that these damages could be caused by the amplification of the unconsolidated sediments.

Plate Kinematics

Euler discovered relative motion of irregular plates on a spherical surface can be simplified as rotation around a pole, now called a Euler Pole.

Euler Poles - movement of a plate on a sphere can be compactly described as rotation about a pole. Euler poles (latitude, longitude, and rotation rate) have been compiled for all of the plates. The latitude and longitude of a point can be used to determine the distance between the point and the Euler pole. The rotation rate can be converted into a linear velocity using this distance from the Euler pole.

• This center of plate rotation (Euler pole) is like an axle, running through the plate, down to the centre of the earth.

• North and South Poles, around which the whole earth rotates, give an idea of how a Euler pole works .

• Close to the poles, rotation is slow, but as you move from the pole, rotation speeds up. The same as a bicycle wheel, where the rim rotates faster than the hub. As earth rotates on its axis, the equator moves faster than anywhere else, and is 90 degrees of latitude from the North and South Poles.

• Plate motion is also fastest at 90 degrees from the Euler Pole, then slows again as you go even further from the rotation axis.

Plate Kinematics cont…• all plates are moving about on a sphere• Euler's theorem: the relative motion

between two plates across the surface of the sphere is uniquely defined by a singular angular rotation about a pole of rotation (the Euler Pole)

• the Euler pole tends to remain fixed for long periods of time

• transform faults are geometrically linked to the Euler poles

– for the transform fault to act with true tangential motion, it must lie on a small circle, the centre of which is the Euler pole.

Euler Pole

vij=ij x rvij :linear velocity of plate i with respect to plate jij : angular velocity of plate i with repect to plate jr : distance from Euler pole to point on boudary

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| v |= vEW2

( ) + vNS2

( )

vNS = a |ω | cosϑ sin(μ −ϕ )

vEW = a |ω | [sinϑ cosλ − cosϑ sinλ cos(μ −ϕ )

EulerPole :ϑ = latitude;ϕ = longitude

Pointonboudarybetweenplatei, j : λ = latitude;μ = longitude

Homework due Wed 24 Aug 05

1. Briefly discuss the hazard and risk Natural Hazards pose to Puerto Rico, with focus on hazards from earthquakes.

2. Explain why the Australian continent has few earthquakes. However, note that there are very active earthquake zones near Australia.

3. Compare the earthquake activity, volcanic activity and topography of the west and east coasts of South America. Why are these continental margins so different?

4. Stein p367 Problems1,2,3,5 (handout in class)

Note : Seismic moment = M0 = AD (dyne cm) = rigidity of rock (assume 3x1011dyne/cm2)A = area of fault slipD = average displacement/slip over the area of fault slip

Moment Magnitude = Mw = (log M0 / 1.5) - 10.73

2 Plate Tectonics

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