Connie Mendoza
October 30, 2000
MAJOR TECTONIC PLATES OF THE WORLD
Strato-volcanoes
Many of the world's great volcanoes, such as Vesuvius, Fujiyama, Egmont, and many others, are strato-volcanoes, with both lava flows and pyroclastic deposits. Many of these have erupted over a long
period, and indeed the strato-volcano is the commonest form developed by long-lived central volcanoes. The cones become gullied by erosion and lava flows commonly follow such gullies. New gullies then
form on the edges of flows, and so on. Scoria cones are built around the top of the volcano and pyroclastic flow and fall deposits may have a wide distribution on the flanks. Ngauruhoe, New Zealand, is a
typical strato-volcano, almost perfectly conical and about 1000 m high. The slopes are about 30 degrees steep and the crater is about 400 m across. The highest point on the rim is to the east, possibly
because of the prevailing westerly wind. Young aa lava flows have reached the base on all sides except the east.
Some strato-volcanoes are isolated, but many occur in groups. In McMurdo Sound, Antarctica, for instance, the large young strato-volcanoes, Mts Erebus (altitude 3700 m), Bird, Terra Nova, and Terror
coalesce to form Ross Island.
Hekla, Iceland, is intermediate in many respects between a typical Icelandic shield volcano and a strato-volcano such as Vesuvius. It has been built by repeated eruptions from a fissure, often with several
craters active at the same time. Shield volcanoes ... are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat,
domical shape, with a profile much like that a a warrior's shield. They are built up slowly by the accretion of thousands of flows of highly fluid basaltic (from basalt, a hard, dense dark volcanic rock)
lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some
of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The Hawaiian
Islands are composed of linear chains of these volcanoes including Kilauea and Mauna Loa on the island of Hawaii -- two of the world's most active volcanoes. The floor of the ocean is more than
15,000 feet deep at the bases of the islands. As Mauna Loa, the largest of the shield volcanoes (and also the world's largest active volcano), projects 13,677 feet above sea level, its top is over
28,000 feet above the deep ocean floor.
In some shield-volcano eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad
plateaus. Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake River in Idaho, and the Columbia River in Washington and
Oregon, these lava flows are beautifully exposed and measure more than a mile in total thickness.
Shield volcanoes ... are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat,
domical shape, with a profile much like that a a warrior's shield. They are built up slowly by the accretion of thousands of flows of highly fluid basaltic (from basalt, a hard, dense dark volcanic rock)
lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some
of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The Hawaiian
Islands are composed of linear chains of these volcanoes including Kilauea and Mauna Loa on the island of Hawaii -- two of the world's most active volcanoes. The floor of the ocean is more than
15,000 feet deep at the bases of the islands. As Mauna Loa, the largest of the shield volcanoes (and also the world's largest active volcano), projects 13,677 feet above sea level, its top is over
28,000 feet above the deep ocean floor.
In some shield-volcano eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad
plateaus. Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake River in Idaho, and the Columbia River in Washington and
Oregon, these lava flows are beautifully exposed and measure more than a mile in total thickness.
The earth's surface is broken into seven large and many small moving plates. These plates, each about 50 miles thick, move relative to one another an average of a few inches a year. Three types of movement
are recognized at the boundaries between plates: convergent, divergent and transform-fault.
At convergent boundaries, plates move toward each other and collide. Where an oceanic plate collides with a continental plate, the oceanic plate tips down and slides beneath the continental plate forming a
deep ocean trench (long, narrow, deep basin.) An example of this type of movement, called subduction, occurs at the boundary between the oceanic Nazca Plate and the continental South American Plate.
Where continental plates collide, they form major mountain systems such as the Himalayas.
At divergent boundaries, plates move away from each other such as at the Mid-Atlantic Ridge. Where plates diverge, hot, molten rock rises and cools adding new material to the edges of the oceanic plates.
This process is known as sea-floor spreading.
At transform-fault boundaries, plates move horizontally past each other. The San Andreas Fault zone is an example of this type of boundary where the Pacific Plate on which Los Angeles sits is moving slowly
northwestward relative to the North American Plate on which San Francisco sits.
Plate tectonics, the branch of science that deals with the process by which rigid plates are moved across hot molten material, has helped to explain much in global-scale geology including the formation of
mountains, and the distribution of earthquakes and volcanoes.
Earthquakes and Plate Tectonics
The world's earthquakes are not randomly distributed over the Earth's surface. They tend to be concentrated in narrow zones. Why is this? And why are volcanoes and mountain ranges also found in these
zones, too?
An explanation is to be found in plate tectonics, a concept which has revolutionized thinking in the Earth's sciences in the last 10 years. The theory of plate tectonics combines many of the ideas about
continental drift (originally proposed in 1912 by Alfred Wegener in Germany) and sea-floor spreading (suggested originally by Harry Hess of Princeton University).
Plate tectonics tells us that the Earth's rigid outer shell (lithosphere) is broken into a mosaic of oceanic and continental plates which can slide over the plastic aesthenosphere, which is the uppermost layer of the
mantle. The plates are in constant motion. Where they interact, along their margins, important geological processes take place, such as the formation of mountain belts, earthquakes, and volcanoes.
The lithosphere covers the whole Earth. Therefore, ocean plates are also involved, more particularly in the process of sea-floor spreading. This involves the midocean ridges which are a system of narrow
submarine cracks that can be traced down the center of the major oceans. The ocean floor is being continuously pulled apart along these midocean ridges. Hot volcanic material rises from the Earth's mantle to
fill the gap and continuously forms new oceanic crust. The midocean ridges themselves are broken by offsets know as transform faults.
One of the keys to plate tectonics was the discovery that the Earth's magnetic field has reversed its polarity 170 times in the last 80 million years. As new basaltic material is squeezed up into the midocean
cracks and solidifies, it is magnetized according to the polarity of the Earth's magnetic field. If the field reverses its polarity, the strip of new material is magnetized in an opposite sense. As the oceanic floor
continues to spread, the new strips of rock are carried away on either side like a conveyer belt.
Using these magnetic strips as evidence of movement, it became obvious that the Earth's surface consisted of a mosaic of crustal plates that were continually jostling one another. If the Earth was not to be
blown up like a balloon by the continual influx of new volcanic material at the ocean ridges, then old crust must be destroyed at the same rate where plates collide. The required balanced occurs when plates
collide, and one plate is forced under the other to be consumed deep in the mantle.
We now know that there are seven major crustal plates, subdivided into a number of smaller plates. They are about 80 kilometers thick, all in constant motion relative to one another, at rates varying from 10
to 130 millimeters per year. Their pattern is neither symmetrical nor simple. As we learn more and more about the major plates, we find that many complicated and intricate maneuvers are taking place. We
learn, too, that most of the geological action - mountains, rift valleys, volcanoes, earthquakes, faulting - is due to different types of interaction at plate boundaries.
How are earthquakes connected with plate tectonics? In 1969, Muawia Barazangi and James Dorman published the locations of all earthquakes which occurred from 1961 to 1967. Most of the earthquakes
are confined to narrow belts and these belts define the boundaries of the plates. The interiors of the plates themselves are largely free of large earthquakes, that is, they are aseismic. There are notable
exceptions to this. An obvious one is the 1811-1812 earthquakes at New Madrid, Missouri, and another is the 1886 earthquake at Charleston, South Carolina. As yet there is no satisfactory plate tectonic
explanation for these isolated events; consequently, we will have to find alternative mechanisms.
Plate tectonics confirms that there are four types of seismic zones. The first follows the line of midocean ridges. Activity is low, and it occurs at very shallow depths. The point is that the lithosphere is very thin
and weak at these boundaries, so the strain cannot build up enough to cause large earthquakes. Associated with this type of seismicity is the volcanic activity along the axis of the ridges (for example, Iceland,
Azores, Tristan da Cunha).
The second type of earthquake associated with plate tectonics is the shallow-focus event unaccompanied by volcanic activity. The San Andreas fault is a good example of this, so is the Anatolian fault in
Northern Turkey. In these faults, two mature plates are scraping by one another. The friction between the plates can be so great that very large strains can build up before they are periodically relieved by
large earthquakes. Nevertheless, activity does not always occur along the entire length of the fault during any one earthquake. For instance, the 1906 San Francisco event was caused by breakage only along
the northern end of the San Andreas fault.
The third type of earthquake is related to the collision of oceanic and continental plates. One plate is thrust or subducted under the other plate so that a deep ocean trench is produced. In the Philippines,
ocean trenches are associated with curved volcanic island arcs on the landward plate, for example the Java trench. Along the Peru - Chile trench, the Pacific plate is being subducted under the South American
plate which responds by crumpling to form the Andes. This type of earthquake can be shallow, intermediate, or deep, according to its location on the downgoing lithospheric slab. Such inclined planes of
earthquakes are know as Benioff zones.
The fourth type of seismic zone occurs along the boundaries of continental plates. Typical of this is the broad swath of seismicity from Burma to the Mediterranean, crossing the Himalayas, Iran, Turkey, to
Gilbraltar. Within this zone, shallow earthquakes are associated with high mountain ranges where intense compression is taking place. Intermediate- and deep-focus earthquakes also occur and are known in
the Himalayas and in the Caucasus. The interiors of continental plates are very complex, much more so than island arcs. For instance, we do not yet know the full relationship of the Alps or the East African rift
system to the broad picture of plate tectonics.
How can plate tectonics help in earthquake prediction? We have seen that earthquakes occur at the following three kinds of plate boundary: ocean ridges where the plates are pulled apart, margins where the
plates scrape past one another, and margins where one plate is thrust under the other. Thus, we can predict the general regions on the Earth's surface where we can expect large earthquakes in the future. We
know that each year about 140 earthquakes of magnitude 6 or greater will occur within this area which is 10 percent of the Earth's surface.
But on a worldwide basis we cannot say with much accuracy when these events will occur. The reason is that the processes in plate tetonics have been going on for millions of years. Averaged over this
interval, plate motions amount to a several millimeters per year. But at any instant in geologic time, for example, the year 1977, we do not know exactly where we are in the worldwide cycle of strain buildup
and strain release. Only by monitoring the stress and strain in small areas, for instance, the San Andreas fault, in great detail can we hope to predict when renewed activity in that part of the place tectonics
arena is likely to take place.
In summary, plate tectonics is a blunt, but, nevertheless, strong tool in earthquake prediction. It tells us where 90 percent of the Earth's major earthquakes are likely to occur. It cannot tell us much about
exactly when they will occur. For that, we must study in detail the plate boundaries themselves. Perhaps the most important role of plate tectonics is that it is a guide to the use of finer techniques for
earthquake prediction.
II. Study the affects on the land in a geologic time scale
Geologic time scale- CHART http://www.geology.com/time.htr
A. Pangea and the movements of landmasses since it existed
The geologic time scale is divided into Cenozoic, Mesozoic, Paleozoic, and Precambrian, within these categories you will find different time periods. During the middle Mesozoic, Pangaea (all the land of Earth was joined together into a “super continent”) began to rotate huge masses of land at different rates and directions causing rifts to form. The first rift was a southwestward tear on the mouth. This produced an opening at the southern part of the North Atlantic Ocean and continued westward into the Gulf of Mexico. The Cordilleran arc firmly established along the Pacific margin of North and South America, they arc built on western N.A. and the Nevadan orogeny begins. Also the Cimmeria begins its collision with Laurasia to form the Cimmerian orogeny. During the Early Cretaceous period, the Atlantic continues to expand as the Pangaea breaks up. The cimmerian orogeny continues, Arcs, and micro continents slam into North America and Sevier orogeny begins. During the Late Cretaceous, the Atlantic lengthens and widens, the Sevier orogeny continues and the Caribbean arc is formed. During the Eocene, the plate positions continue to adjust to the opening of the Atlantic, the Rocky Mountains grow; the Alps and Pyrenees are formed. During this time the modern patterns of planet Earth begin to appear. The Miocene period caused southwestern North America and the East Pacific Rise to intercept a great extensional event. This event was the start of the Basin and Range orogeny. Orogeny continues in the Mediterranean region and India nears its junctions with southern Asia. From the Late Cretaceous to the Present rifts separated Africa from South America and then India, Australia, and Antarctica. North America rifted away from Europe. Then the rifted masses of old Gondwana, Africa, India, and Australia, moved northward towards Eurasia closing the Tethys Ocean and forming the great Alpine-Himalayan mountains. As the Pangea process continues, a new super continent appears to be in the near geologic future, centered about the North Pole.
http://www.vishnu.glg.nau.edu/rcb
B. The formation of landmasses and oceans
Around 200 million years ago all the land was joined together into a “super continent.” However Pangea (super continent) began to split apart. Scientists believe there are four types of plate movements. First divergent boundaries, where new crust is generated as the plates pull away from each other. It spreads centers where plates are moving apart and new crust is created by magma pushing up from the mantle. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year. This rate may seem slow but this process has been going on for millions of years, resulting the Atlantic Ocean to grow from a “lake” into the ocean that exists today. Second Convergent boundaries, is where the crust is destroyed as one plate dives under another. The plates move toward each other and one plate sinks under another. This occurs between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates. Thirdly transform boundaries, where the crust is neither produced nor destroyed as the plates slide horizontally past each other. Most are found on the ocean floor. They usually offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquake. Fourth plate boundary zones, broad belts in which boundaries are not well defined and the effects of plate interaction are unclear. The plate-boundary zones involve at least two large plates and one or more micro plates caught up between them, they tend to have complicated geological structures and earthquakes patterns. Today current plate movements can be tracked directly by means ground based or space-based measurements. These measurements are taken with conventional ground serving techniques, using laser-electronic instruments. Because plate motions are global in scale, they are best measured by satellite-based methods. These satellites transmit radio signals back to earth measuring distances between specific points, geologists can determine if there has been active movement along faults or between plates.
http://pubs.usgs.gov/publications/text/understanding.htn
Plate Tectonics
There are several specific examples that show the effects of plate tectonics on our planet. A first example would be the creation of mountain ranges such as the Himalayan mountain range. The Himalayas were created by the collision of the continents of Asia and India over 50 million years ago, which caused the Eurasian Plate to override the Indian Plate. Buckling and upward motions occurred in the crust ( the three layers of the continental crust, lithosphere, and asthenosphere) due to the collision. The two plates continued to converge and eventually led to the Himalayas being pushed up to heights of 8,854 m above sea level. (USGS Website – The Dynamic Earth – Understanding plate motions)
Ridges and valleys also have been effected by plate tectonics. One example is the Mid-Ocean ridge, which winds around Earth like the seams on a baseball. It is more than 50,000 km long and 800 km across and rises close to 4,500 meters above the sea floor. Ridges also play a role in the formation of new crust – magma will rise and erupt from spreading ridges to form new crust. Another notable ridge is the Mid-Atlantic Ridge, which tracks from the Arctic Ocean to the southern tip of Africa. It spreads approxiamately 2.5 cm per year and has caused plate movement in the thousands of kilometers. It also separates the North American Plate from the Eurasian Plate. Iceland happens to be in the middle of the ridge and the two plates and as a consequence is literally splitting due to the pull of the plates, which also causes quite an amount of volcanic activity in the region.
Another effect of plate tectonics is the creation of volcanoes. Volcanoes are formed by the process of subduction as oceanic-oceanic plates converge. Over time (millions of years) “erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above seal level to form an island volcano.” The best example of the relationship between volcanoes and plate tectonics is Hawaii. It is believed that the creation of Hawaii occurred because the Pacific Plate moved over a stationary “hot spot” in the mantle. The heat from the “hot spot” is a consistent source of magma partly because it melts the overriding Pacific Plate. Over time as the magma builds up in the ocean and eventually forms and island volcano. As the plate moves though, the island volcano is carried away from the “hot spot”, or magma source, and beomes inactive. It is a continuous cycle – as one island volcano ceases, another will develop over the “hot spot”. This explains why there is a long trail of volcanic islands across the Pacific Ocean floor. The oldest and most eroded volcanic islands are in the northwestern region of Hawaii and the youngest (still over the “hot spot”) are in the southeast region. (USGS website – This Dynamic Earth: The Story of Plate Tectonics – “Hotspots”: Mantle Thermal Plumes)
The upper part of the mantle and crust form the lithosphere (rigid layer). It is thinnest under the oceans and in volcanically active regions. The lithosphere is broken into the plates that contain continents and oceans. The lithosphere floats on the asthenosphere, which is a mobile zone composed of semi-solid and hot matter. This matter can “flow after being subjected to high temperature and pressure over geologic time”. (USGS website – This Dynamic Earth - Inside the Earth)
The earth does not get larger due to sea floor spreading – old crust is consumed in trenches, magma rises from spreading ridges to form new crust, cycle of creation of new crust and destruction of old lithosphere.