Research shows that interactive media which combines computer text and graphics, video, still images, and audio in classroom presentations are exciting, motivating, and flexible. Further studies show that interactive learning increases retention, significantly decreases learning time, and virtually guarantees mastery learning. An IBM study showed a 30%-50% increase in learning gain scores and a 300% increase in students reaching mastery level. Just as interactive media increases mastery, image processing increases the rate that data can be perceived and interpreted by the human brain. Language is processed at one hundred bits per second while images are processed at two hundred million bits per second. In effect a picture is worth one million words. By visualizing information students gain the power to see data in new and unique ways opening the door to original scientific discovery. As students manipulate images they explore data in a vareity of interactive processes making image processing an ideal vehicle for exploration and open ended discovery.
The goals for the activities are: to increase awareness and recognition of the fact that the Earth is an interconnected, interacting system; to reach a high level of understanding of the fact that earthquakes, volcanoes, and plate tectonics are parts of a large, integrated system that forms a consistent pattern of earth activity; to become aware of the fact that what happens in one small part of our planet is part of a larger whole, and that by fitting regional events into the bigger picture, a deeper understanding is reached; and to understand that events occurring in the lithosphere have far reaching consequences in the atmosphere, hydrosphere, cryosphere, and biosphere.
Earthquakes, volcanoes, and plate boundaries are interrelated and form a pattern that is truly global in scope. Recognition of patterns of volcanism and earthquakes, and their association with plate tectonics, stands as the major achievement in earth sciences in this century. Earthquakes may appear to strike randomly across the earth, but we are all familiar with the fact that California, parts of China, and Japan appear to receive more than other areas. Explosive volcanoes are common in the South Pacific, the Mediterranean, the Northwest coast of the United States and the Aleutian Islands of Alaska; when was the last time you read of a volcanic eruption in North Dakota or Sweden? Earthquakes and volcanoes are not randomly occurring events, there is a pattern to these geological events, and this pattern is associated with plate tectonics.
The Plate Tectonic Theory has truly become the "unifying theory" of the earth sciences. The distributions of earthquakes, volcanoes, mountains, mineral deposits, oil and gas accumulations, present and former plant and animal life, continental land masses, and many other observed elements of earth sciences can now be explained. The theory is not yet perfected, and there are still portions of the overall theory that are not universally accepted by the scientific community. However, it is apparent that scientists are now polishing the details; the major elements of the overall theory have withstood the rigors of scientific scrutiny.
The theory of Plate Tectonics rests on the following concepts:
The Earth's crust is made up of seven major plates and numerous minor plates that are moving in relationship to each other. Plate movement has been measured using various measuring devices on space platforms. Movement can be described with reference to a fixed point or relative to an adjacent plate. Hot spots in the mantle have been used as fixed points, as has a reflector station left on the moon, and the continent of Africa, which appears to be static at the present time. Relative movement is apparent from the measurements that have been made from satellites.
Tectonic plates interact along their margins in three ways:
The Earth is a differentiated planet. The differentiation is based on density, with the innermost sphere, the solid inner core, being the densest, and the outermost sphere, the atmosphere, being the least dense. Early in the Earth's history, the planet underwent a period of partial melting, during which iron and nickel accumulated in the core, dense silicate minerals accumulated in the mantle, and less dense silicate minerals accumulated in the crust. The source of heat for the period of melting came from the decay of radioactive elements trapped within the forming earth, kinetic energy from small asteroids accreting onto the forming Earth, and frictional heat generated as gravity compressed the forming earth. Most of the Earth's present-day heat is thought to originate from radioactive decay.
The early Earth is thought to have had a far different atmosphere than the present, composed mostly of ammonia, methane, and hydrogen. This early atmosphere was boiled off as the earth heated up, and was replaced with one lacking these gasses. The early Earth also had oceans derived from hydrogen and oxygen gasses vented during volcanic eruption, a process that continues to the present. The proportions of gasses in the atmosphere have fluctuated over time. The early atmosphere had little free oxygen; its abundance began to increase approximately 3 billion years ago due to the development of oxygen producing life.
There are two ways of looking at the Earth's interior. On the basis of composition, the core of the earth extends from the center at 6,370 km to 2,900 km and is composed of a mixture of mostly iron and nickel, with smaller amounts of silicon and sulfur. The mantle of the earth extends from 2,900 km to about 10 km under ocean crust, and approximately 20-70 km under continental crust. The mantle is composed of relatively dense silicate minerals, rich in iron, magnesium, and calcium. The crust of the earth extends from about 10 km to the rock surface in ocean basins, and from 20-70 km to the surface for continents. The crust is composed of less dense silicate minerals, less rich in iron, magnesium, and calcium compared to the mantle, and relatively richer in silicate, aluminum, potassium, and sodium.
On the basis of the physical states of the Earth's interior, additional subdivisions can be made. The compositional core is divided into a solid inner core and a liquid outer core, both thought to be approximately the same composition. There is a complex interplay between pressure, temperature, depth, and melting point of core material. Temperature dominates in the outer core, keeping it liquid, and pressure dominates in the inner core, keeping it solid. Similarly, the mantle can be divided into an upper portion, the asthenosphere (literally "weak sphere") from about 100 to 350 kilometers deep which is approximately 10% molten, and a lower portion, which is 100% solid. The increase in temperature and pressure with depth and the melting temperature of the mantle interact to produce the asthenosphere. The uppermost mantle, from about 100 kilometers deep to the base of the crust, is 100% solid, as is the crust. Taken together, the solid, brittle upper mantle and the solid, brittle crust, make up the Earth's lithosphere (literally "rock sphere.")
The asthenosphere is also called the low velocity zone because earthquake energy slows down as it passes through, due to the presence of the molten material. The 10% liquid component of the asthenosphere is the source material for new ocean crust at spreading centers. Both the asthenosphere and the lower mantle respond over the long term to stresses as a plastic-like material; in other words, the mantle convects in response to the temperature difference between its base and the upper portion, albeit slowly. The asthenosphere is more plastic-like in its response to long term stresses because of the presence of the molten fraction.
Hot Spots, Mantle Plumes, Convection, and Volcanoes
Hot Spots are seemingly fixed areas in the upper mantle that are hotter than average. They produce a plume of hot mantle that convects towards the surface. Hot spots are assumed to be more or less fixed in the mantle, although they may be slowly moving. Hot mantle from hot spots is less dense than the surrounding mantle, causing it to rise towards the surface. Some geologists think that the hot material rises as a relatively narrow plume, while others believe that nearly the entire mantle is involved in general mantle convection. In both cases, hot mantle spreads out beneath the overlying lithosphere plate. Several hot spots are located along divergent boundaries, and they may have provided the convection necessary to rift the lithosphere.
Other hot spots are beneath plate interiors, such as the ones thought to lie beneath Yellowstone Park and the Hawaiian Islands. These intraplate hot spots produce a chain of volcanoes on the overlying plate. There is a line of volcanoes stretching from the west coast of North America almost due eastward, ending at Yellowstone Park. The volcanic activity is oldest in the west, and youngest at Yellowstone, and is thought to represent movement of the North American plate westward over the more or less fixed location of the Yellowstone hot spot. Another example is the Hawaiian Island chain of volcanoes, which stretches in a straight line northwestward, and continues as a chain of submerged and eroded volcanoes (seamounts) in the same direction. This chain makes an abrupt change in direction to nearly due north, and continues towards the Aleutian trench as the Emperor Seamount chain. The southeastern-most big island of Hawaii is the youngest and most volcanically active of all the Hawaiian Islands. They get progressively older and smaller towards the northwest until finally, past Kauai, the volcanic mountains are eroded off to below sea level.
Divergent Boundaries comprise one of the largest geological features on the planet, with a combined length of about 40,000 kilometers. Divergence is a good general term for the process, although sea floor spreading is a more precise description of the actual process and is used synonymously. Mid-oceanic ridge refers to the geographic ridge formed during the process of divergence, and rift generally refers to geographic fault-controlled valleys on continents or at the crest of mid-oceanic ridges. These terms (divergent boundaries, spreading centers, mid-oceanic ridges, rift valleys) refer to the same general process and are, in casual usage, used interchangeably. The newest oceanic crust is found within and adjacent to the spreading center; progressively older ocean crust is found with increasing distance from the divergent boundary. In addition, the load of unconsolidated sediments on the ocean crust, mainly shells and fragments of microscopic organisms and clay, is thinnest in the vicinity of the ridge, and progressively thicker away from the ridge.
It is important to consider that rifting, once begun, produces oceanic type crust, even if the rifting involves continental crust. As the continental crust separates, partial melting of the upper mantle produces basaltic magma that wells up into the rift, solidifying as oceanic type crust. As an example, the triple junction of the Great African Rift Valley and Gulf of Aden-Red Sea system in east Africa represents continental divergence. If this process continues in this area, the distance separating the opposite sides of the rift will increase, and basins floored with oceanic crust will form. The African Rift Valley system is partially occupied by a series of lakes, and the Gulf of Aden-Red Sea is occupied by an arm of the Indian Ocean. The African Rift Valley system probably represents the inactive, failed arm of the triple junction, and the gulf of Aden-Red Sea arms may well continue to widen.
An interesting question to ponder at this point is why are almost all divergent zones hidden beneath the ocean; the answer, of course, is that divergence, once begun and continued for a geologically significant length of time, always produces oceanic crust. There are not many places where continents are being actively rifted; East Africa, mentioned above, is one of the main continental rifts; another would be the western United States. Much of the Great Basin area of Nevada, Utah, and portions of surrounding states is slowly being pulled apart. There is little, if any, ocean type crust yet formed in this area, but the crust is greatly thinned by the stretching, and high heat flow, volcanic activity and earthquakes are common. Another interesting question to speculate on is: what will the future geology and geography of the western United States look like if this divergence continues for a geologically significant length of time?
Earthquakes in divergent zones are shallow and due to tensional forces and normal faulting. They are shallow because the brittle lithosphere is thinned by stretching, and because convection brings hot, plastic-like mantle material close to the surface. Under stress, the plastic-like mantle will deform by flowing, while the overlying brittle lithosphere will deform by fracturing.
Volcanic activity is generally quiet, non-explosive eruptions of basaltic lava, forming a wide (up to 1,500 km), high (up to 3.5 km from the sea floor) complex of shield volcanoes, fissure volcanoes and flood basalt. The topographic relief of mid-oceanic ridges is partly due to the buildup of basalt, but some is thought to be due to heat inflation. The presence of hot, expanded mantle a few kilometers immediately beneath the ridge increases its elevation.
Paleomagnetism, literally "old magnetism," is a powerful tool used to support the plate tectonic theory and to reconstruct past configurations of continents and oceans. The earth has a self generating magnetic field related to movements within the liquid outer core. Convection in the liquid outer core, as in the mantle, is not uniform; it speeds up and slows down, depending upon the transfer of heat at the inner core-outer core boundary. The Earth's magnetic poles are only approximately coincident with the geographic poles. The present day north magnetic pole is, for example, in Baffin Bay. It will wander around at the top of the globe, but will remain within a few degrees of the geographic pole.
The magnetic field of the Earth occasionally weakens and disappears, only to build back up with the opposite polarity. The north magnetic pole does not wander to the south while the south magnetic pole is wandering to the north, the field merely dies down and builds back up with the opposite polarity. Our present period of polarity is called "normal polarity" and it began approximately 800,000 years ago. Prior to that time, the Earth was in a period of "reversed polarity," that is, the north end of a magnetic compass needle would have pointed towards the south pole. The length of each period of polarity is quite variable, but they average approximately 500,000 years long. There have been numerous reversals of the Earth's polarity throughout geological time.
Rocks can record the Earth's magnetic field in several ways. Cooling lava, such as the basalt produced at spreading centers, will take on the alignment of the Earth's current magnetic field. Once the rock is cooled and crystallized, the magnetic alignment is locked in and can be measured. In addition, very fine-grained sedimentary rocks can lock in the alignment of the magnetic field as tiny mineral grains settle into place. Since igneous rocks such as basalt can be dated using radiometric methods and the alignment of the magnetic field measured, there exists a very good record and correlation between magnetic reversals and rock ages.
Among the best lines of evidence pointing to sea floor spreading are the symmetrically arranged patterns of magnetic reversals and rock ages associated with spreading centers. The youngest ocean crust adjacent to the spreading center is normally polarized in two strips parallel to and symmetrical about the rift. The width of the normally polarized zone is a function of the rate of spreading and the length of the current normally polarized epoch. Older ocean crust is found with increased distance from the spreading center. The patterns are reversed and normally polarized rocks on either side of the spreading centers are as distinctive as tree ring growth patterns, and, once dated, can be used to determine the area and rate of spreading of the ocean floor.
Transform faults form plate boundaries where the motion between plates is neither towards nor away, only slipping horizontally past each other. Lithosphere is not created nor is it destroyed at transform boundaries. Transform faults are a prominent feature of the ocean floor, where they offset the mid-oceanic ridges and the associated paleomagnetic and age patterns. (see the map of Sea Floor Ages, and the Sea Floor Topography map)
If the lithosphere opened smoothly at a spreading center, it would have to open considerably wider in the central portion than towards the ends because of the spherical shape of the earth. Spreading centers open about a pole of spreading (not in any way connected to the pole of rotation). Even though the angular rate of spreading may be constant, the linear rate at the surface of the earth is going to vary with the distance from the pole of spreading. A portion of the spreading center not far from the pole of spreading will have a small linear velocity at the surface of the earth, while a portion near the center of the spreading center would have a large linear velocity. The differences in these linear velocities along the spreading center set up stresses perpendicular to the ridge that are accommodated by transform faulting. Across the offset portions of the ridge, along the transform fault, ocean crust is moving in opposite directions only between the ridge segments.
Shallow earthquakes and a lack of volcanic activity characterize ocean floor transform boundaries. However, occasionally transform movement affects continental crust, as in the San Andreas Fault of California. Here spreading motion of the East Pacific Rise in the Gulf of California is transformed along the San Andreas-Mendoceno to the Gorda Ridge off the coast of northern California. Earthquakes along the San Andreas Fault have the potential to be particularly dangerous because this fault is a major plate boundary that cuts continental crust in a densely populated area.
Convergent boundaries are where moving plates come together. If both of the plates are oceanic lithosphere, one of them is overridden by the other and is consumed or subducted back into the mantle. Convergent zones involving oceanic crust are marked by an arcuate, linear trench where the subducting plate bends downward, and an arcuate, linear chain of volcanoes behind the trench, on the overriding plate. If one of the two plates is composed of continental crust, it is the overriding plate and the ocean crust is subducted beneath it. The linear chain of volcanoes is then called a continental arc, with the best example being the Andes Mountains. If both plates are continental crust, the process of subduction stops and the movement is taken up elsewhere. Continental crust is too thick and too low in density to be subducted and recycled back into the mantle. The Himalayan Mountains and plateau are a good example of this. When India struck Asia, that subduction zone ceased to exist. The double thickness of continental crust where Asia tried to override India has produced the world's largest, highest altitude area.
Although not precisely synonymous, the terms trench, island arc, convergent zone, and subduction zone are often used interchangeably.
There are numerous good examples of trench-island arc systems in the Pacific Ocean, including the Aleutian Islands of Alaska. The Pacific Plate is subducting into the Aleutian Trench. An island arc (Aleutian Islands) and a continental arc (the Alaskan Peninsula and mainland) are forming on the overriding plate. The University of Alaska is a good source for information on volcanoes in this area.
The Cascade Mountain range in the northwestern United States is also a good example of a continental arc, formed as the small Juan de Fuca plate is being subducted beneath the North American plate.
Earthquakes associated with subduction exhibit a wide variety of behaviors that fall into a pattern consistent with plate tectonic theory. Earthquakes nearest the subduction zone are shallow (approximately 50 km or less), located in the overriding plate and at the top of the subducting plate, and are due to shear stresses. Earthquake foci increase in depth with increase distance in back of the trench. Earthquakes are, in general, due to tensional stresses in the intermediate zone (approximately 50-200 km) and compressive stresses in the deep zone (up to 700 km).
This pattern forms a seismic zone that dips away from the trench and under the overriding plate, named the Benioff Zone in honor of its discoverer. Earthquakes get deeper and deeper away from the trench, consistent with the concept of a subducting slab of brittle oceanic lithosphere. Shear stresses in the overriding plate and in the upper portion of the subducting plate are consistent with the convergence of two brittle plates, and tensional stresses in the intermediate zone are consistent with brittle material being pulled or sinking into the plastic-like mantle. Deep compression stresses are consistent with the increasing pressure experienced by the brittle subducting slab at those depths. Earthquakes are restricted to the Benioff Zone, which has the size, shape, and brittle characteristics one would expect of a subducting slab of oceanic lithosphere. The lack of any earthquakes deeper than about 700 km indicates that by the time the subducting slab has reached this depth, it has been heated to the point of plasticity and has been essentially reabsorbed back into the mantle.
Volcanic activity associated with island arcs and continental arcs alternates between violent explosive eruptions of volcanic ash and fragments, and more quiescent, viscous andesitic lava flows. The volcanoes are generally steep sided, conical, composite volcanoes that are more widely spaced than volcanoes at spreading centers. The origin of the magma for arc volcanism is partial melting of the subducting oceanic lithosphere. The deeper it plunges, the hotter it gets, and the greater percentage that becomes molten. The partial melt is lower in density that the surrounding lithosphere and mantle, so it rises towards the surface. Some of the magma becomes lodged with the upper lithosphere-crust as igneous intrusions, and some makes it to the surface as eruptions of lava.
The oceanic lithosphere being subducted is, in general, old ocean crust and thus has had time to accumulate a load of water-saturated marine sediments. Much of this sediment is scraped off and added to the overriding plate, but some of it is subducted along with the oceanic lithosphere, and is eventually partially melted. The presence of water in these sediments influences the melting temperature of the oceanic lithosphere, lowering the melting temperature of the substances (such as silicate molecule) that will eventually form andesitic magma. Water itself becomes a gaseous constituent of the magma, as does carbon dioxide, nitrogen oxide, sulfur oxide, and chlorine from various marine sediments and organic material. The combination of these gasses (chiefly water), and silicate, which increases the viscosity of the magma, results in the explosive nature of arc volcanoes. Andesitic lava is very viscous due to its silicate content, while at the same time expanding gasses such as water create tremendous forces. The history of most composite volcanoes in island and continental arcs is that of alternating explosive eruptions of ash and other fragments, and quieter outpourings of viscous andesitic lava. The gasses contained in these eruptions escape into the atmosphere, where they have the potential to influence atmospheric processes. For example, carbon dioxide is a greenhouse gas and can be produced in great quantities in some volcanic regions, and chlorine, even in small quantities, may have an effect on stratospheric ozone concentrations.
The arcuate pattern of trenches is distinctive, and can be visualized as a dimple in a Ping-Pong ball. By pushing a finger into the ball, a dimple is produced with the same radius of curvature as the original ball. There has been no stretching or other deformation, just a reversal of direction of curvature. An arc of the dimple is a good model of a subducting slab; notice how most arcuate trenches are concave in the direction of movement of the subducting slab.
Earthquakes are sudden releases of energy stored in stressed rocks. Tensional stresses originate as plates move away from each other, compressive stresses originate as they move towards each other, and shear stresses originate as plates slide past each other. In the brittle lithosphere, stresses can at first be accommodated by elastic strain; in other words, the lithosphere will bend. When bent beyond its strength, the lithosphere will suddenly break, releasing the energy it stored during bending. Similar stresses in the plastic-like asthenosphere can be accommodated by plastic flow; no breaking occurs and no earthquake energy is released.
The point of first release of earthquake energy is called the earthquake's focus. The energy travels outward in all directions from that point as a spherical wave front. The earthquake's epicenter is where this wave front first encounters the surface of the earth, immediately above the focus. Earthquake energy travels through the earth from the focus as two types of body waves. The first of these is the P-wave, also known as the primary wave, which travels as the rocks are alternately compressed and expanded (push-pull). The second is the S-wave or shear wave, which travels through the rock by causing it to shear up and down. The rate of travel of P-waves is almost twice as fast as that of S-waves; both travel faster through denser material. Earthquake energy can also travel outward from the epicenter as a surface wave or L-wave. Body waves reaching the epicenter cause the surface of the earth to vibrate; these vibrations, in turn, cause surface waves that affect the upper few kilometers of the crust, moving with a side to side motion and an up and down rolling motion.
Arrival times of earthquake energy (both P-waves and S-waves) at seismograph stations can be used to triangulate back to the focus and epicenter of the earthquake. Earthquake energy is also used to obtain information on the structure and physical makeup of the Earth's interior. Since earthquake waves travel faster through denser material, slower through less dense or partially molten material, and reflect and refract at interfaces between dissimilar material. They can be used to determine the composition and physical state of material inside the Earth. As an example, S-waves cannot travel through a liquid, so the presence of the S-wave shadow zone on the side of the Earth opposite an earthquake pinpoints the location and nature of the liquid outer core.
The most common scheme used to measure the intensity of earthquakes is the Richter scale, developed in 1935 by Charles Richter of Cal Tech. The Richter scale is a logarithmic scale measuring the amplitude of seismographic recordings. Each increase of one unit on the Richter scale corresponds to a 10-fold increase in the amplitude of the recording. The Richter scale is proportional to the amount of energy released during an earthquake. Each increase of 1 unit on the Richter scale corresponds to a 30-fold increase in energy release.
The Mercalli Intensity Scale is still occasionally used, even though it is subjective and gives no real information on the strength or location of earthquakes. It ranks earthquakes based on subjective evaluations of how noticeable the earthquake was to people, and the amount and type of damage it caused. How noticeable an earthquake is depends upon the person, how strong the earthquake is, and how far away it is. The degree of damage depends more upon local geology and construction techniques than it does on the strength of the earthquake.
Mountains and Accretion of Terranes
Most of the world's mountains occur in long, linear chains of mountain systems, at or near present or former convergent zones. As the crust of the earth is shortened during convergence, a similar amount of thickening must occur. This can be accomplished by adding mass to the edge of a continent by partial melting of a subducting slab of oceanic lithosphere and the resultant intrusive and extrusive volcanic activity, such as the Andes Mountains. It can also be accomplished by folding and faulting of sedimentary rocks and crust rocks caught between two colliding plates, such as the Himalayan Mountains caught between India and the Asian continent. Most commonly it is a combination of the two. Intensely deformed rocks are found in these orogenic ("mountain building") zones. The Appalachian Mountain chain is an example of a former zone of collision that took place many millions of years ago between North America and Africa-Europe, prior to the more recent opening up of the Atlantic Ocean.
As part of the shortening and thickening of the earth's crust, consider what would happen to shallow marine deposits, a volcanic island arc, and an island composed of a fragment of continental crust. As convergence progresses, each of these becomes shortened and thickened and added to the edge of the continent. Each of these fragments is then known as an "exotic terrane," exotic because it originated in a place other than where it eventually ended. Much of the western portion of North America and parts of Alaska were added (accreted) to the continent by just such a process. It is interesting to speculate on the future geology and geography of the western portion of the Pacific Ocean. This is the oldest oceanic crust in the world, and the area is a maze of trenches, island arcs, shallow sedimentary basins, and continental crust fragments. Western North America's past may be eastern Asia's future.
The average elevation of the Earth's crust is an important control on the average temperature of the Earth. In general, the higher the elevation, the cooler is the average temperature. Many of the Earth's major mountain systems have formed in the last 65 million years or so, coinciding with a length period of declining temperatures, culminating in the ice ages of the Pleistocene some 2-3 million years ago. There is evidence in the geological record to suggest that there were lengthy periods of time when orogenesis or mountain building was much slower than the current episode. During these periods it is likely that erosion reduced the average elevation of the Earth's surface to such a degree that average temperatures could remain much higher than at present.
Formative assessment is seen as a means to guide students and acknowledge that they are on the right path. Quick feedback sometimes stops important processing that allows knowledge to move from short term memory to long term memory. The formative assessment strategies suggested here are meant to mirror these ideas.
The Hot Spot Track stack was created by using images downloaded from the this site.
For calibrating NIH Image in the Hot Spot Track stack, the length of the Yellowstone caldera is approximately 64 km and the width is approximately 44 km.
Active volcano - one that is erupting or has erupted in recent past.
Aftershock -earthquakes that occur hours to weeks to months after the main quake. They originate at or near the focus of the larger earthquake .
Anticline - a structure of folded stratified rocks in which the rocks dip in two directions away from a crest. The crest is called the axis, the oldest rocks occur at the center.
Asthenosphere - combination of the lower mantle and the core.
Batholith - large body of intrusive igneous rock, with surface exposure greater than 100 sq km. Usually coarse grained rock, ie. granite.
Caldera - depression in crust resulting from a volcano emptying its lava chamber and then the empty chamber caving in due to enormous weight of earth above it, roughly a steep sided volcanic basin with a diameter at least three or four times its depth. (Examples of calderas)
Composite volcanic cone - composed of interbedded lava flows and pyroclastic material (any material thrown from a volcano) characterized by slopes of close to 30 degrees at the summit reducing to 5 degrees near the base.
Cinder cone volcano - small conical volcano built of tephra (pyroclastic material), usually without lava flows.
Composite volcano - formed from alternating layers of lava and volcanic ash. (Example)
Continental drift theory - the continents were once a solid piece of ground called Pangea and moved apart. They are still breaking up and moving and account for present arrangement of continents and ocean basins.
Convection theory - theory in which hot magma rises and cold magma sinks within the asthenosphere and upper mantle. Causes plates to move.
Convergent plate movement - two lithospheric plates colliding.
Core - inner central mass of the earth composed largely of iron and nickel. Solid interior and liquid outer zone.
Crust - outside shell of the earth, 10 to 15 km thick.
Divergent plate movement - two plates spreading apart.
Domed mountain - formed when magma flows up between two layers of rock.
Dormant volcano - a volcano that has not erupted for a long time but shows signs of some activity.
Earthquake - the sudden movement of layers of rock caused from great pressures being built up over a period of time resulting in a new position that eases the pressures.
Epicenter - point on surface of earth directly above the earthquake focus.
Extinct volcano - have not erupted for a long time and show no signs of activity.
Fault - crack or break of a rock layer with movement of one layer with respect to the other.
Fault-block mountain - layers of rock on one side of a fault are pushed up higher than those on other side creating block shapes.
Focus - the place inside the earth where an earthquake begins.
Folded mountain - mountains that result from crustal compression.
Folds - wave-like bends in the layers of rocks caused by compression.
Foreshock - a faint or low magnitude quake that occurs in advance of a major earthquake.
Fumerole - a vent that emits volcanic gasses, (mostly steam) at high temperatures.
Geyser - jetlike emission of hot water and steam from a narrow vent at a geothermal locality.
Isoseismal - a line drawn on a map that connects points which register same intensity during an earthquake, usually the numbers come from the Mercalli Scale.
Lava - molten rock emerging on the earth's surface, exposed to water and air.
Lithosphere - combination of the crust and the upper portion of the mantle (rests on the core) from 50-125 km thick.
Magma - high temperature molten rock inside earth with dissolved gasses.
Mantle - rock layer inside the earth that surrounds the core (about 2900 km thick).
Mantle plume - hypothetical column of heated mantle rock thought to cause a hot spot in overlying lithospheric plate. ex. Hawaii, Yellowstone
Mercalli scale - scale of measurement of intensity of earthquakes which uses observations of earthquake's effects on structures and the earth's surface
Mountains - huge masses of rock pushed high into the air by forces inside the earth.
Mud volcano - a vent filled with thick, liquid mud formed by hot, very acidic water dissolving overlying rock. The escaping gasses cause bubbling and gurgling.
Obsidian - rock formed from lava that has cooled very fast.
Plate boundary - the area where the edges of two plates meet.
Primary wave- earthquake body waves that travel fastest and advance by a push-pull mechanism also known as longitudinal, compressional or p-waves.
Pumice - very light rock that hardened while gases were escaping from them.
Pyroclastic material (tephra) - Rock fragments blown out of a volcanic vent under pressure of rapidly expanding gasses.
Rift zone -a system of fractures in the earths crust. Often associated with the extrusion of lava.
Richter Scale - logarithmic scale of magnitude that gives the quantity of energy released by an earthquake. ex. 4.2 is two times more energy than 4.0, and 5.0 is ten times more energy than 4.0.
Seismic wave - waves that travel through earth's crust and begin at focus point of earthquake.
Seismograph - instrument used to detect and measure earthquakes.
Shield volcano - A volcano built up almost entirely of lava with slopes seldom as great as 10 degrees at the summit and 2 degrees at the base. Caused by nonviscous (low silica content) lava.
Surface waves- wave that travels along the earth's surface. Earthquake surface waves are sometimes represented by the symbol L.
Syncline - trough-like structure caused by folding.
Tectonic plate - huge chunks of the lithosphere that float around on the asthenosphere. The motion is thought to be caused by convection.
Transform (lateral) plate movement - two plates slide past each other.
Tsunamis - huge sea wave caused by earthquake with its epicenter in the ocean.
Tuff - volcanic ash that has become consolidated to form rock.
Volcanic ash - Finely divided pyroclastic material blown from a volcanic vent.
Volcanic cinders - coarse particles of lava.
Volcanic crater - Summit depression associated with the principal vent of the volcano.
Volcanic dust - smallest particles of lava.
Volcanic mountain - formed by lava and other materials that are extruded out of earths interior onto the earth's surface.
Volcano -Conical mountain that has been built by lava, ash, and rocks ejected from the earth's interior.
