unit 3
3 types of earthquakes
-Shallow focus earthquakes have focal depths of less than 70 km from the surface; -Intermediate focus earthquakes occur between 70 and 300 km from the surface; and -Deep focus earthquakes occur at depths of more than 300 km.
a volcano can be
-a conical mountain formed around a conduit where lava, gases, and volcanic debris are erupted, -a fracture in the Earth's crust where lava erupts and flows in layers across the landscape, or -a fracture in oceanic crust where pillow basalts are erupted on the ocean floor
where earthquakes occur most
A definite relationship exists between earthquake foci and plate boundaries (see map below), as approximately 95% of all earthquakes occur at plate boundaries. Earthquakes generated along divergent or transform boundaries are almost always shallow focus quakes. Many shallow focus quakes, as well as intermediate and deep focus earthquakes occur along convergent boundaries. Scientists are able to learn much about the Earth's interior by studying earthquake foci patterns; earthquakes near island arcs and their adjacent oceanic trenches can help geologists and seismologists determine the angle of a descending plate in a subduction zone. - Where do the other 5% of earthquakes occur? Not at plate boundaries. These earthquakes are called intraplate earthquakes and can be quite dangerous and deadly, often because no one is expecting them.
normal fault
A type of fault where the hanging wall slides downward; caused by tension in the crust
if not in the 3 catagories
Approximately 90% of all earthquake foci are at depths of less than 100 km, whereas only about 3% of all earthquakes are deep focus quakes. Shallow focus earthquakes are, with few exceptions, the most destructive because the energy they release has little time to dissipate before reaching the surface.
ridge-push
As plates move over the asthenosphere, they separate at oceanic ridges; as magma rises at a spreading center, it also raises the oceanic ridge higher than the surrounding oceanic crust. Gravity pushes the oceanic lithosphere away from the higher ridges and eventually it sinks forming the abyssal floor of the ocean basin. - newly formed oceanic crust is pushed away from the oceanic ridge by new magma rising from below.
body waves
Body waves spread outwards from the focus and propagate through the interior of the earth. The two most significant body waves in the study of earthquakes are primary (P) waves and secondary (S) waves. P-waves are also called compressional or push-pull waves as they travel through the earth with a push-pull motion (see figure below). They are referred to as primary waves because they travel the fastest of all the seismic waves and travel between 5km/s and 7km/s in the earth's crust, with an average velocity of 6.1 km/s. P-waves compress and expand earth material parallel to their direction of travel and are the first seismic waves to arrive at seismic stations. They are able to travel through solids, liquids, and gases.
Calderas
Calderas form as a result of a large volcanic eruption, when large volumes of magma are explosively ejected from the magma chamber all at once. The eruption can cause the overlying rocks to become unstable, and implode into the partially empty chamber, forming a circular to oval depression at the Earth's surface. These depressions can be small and isolated, or cover up to 100's of square kilometers. Crater Lake in Oregon is actually a steep-rimmed caldera, over 1,200 meters deep and covering ~ 60 km2. Caldera forming eruptions can be the largest and most deadly types of volcanic eruptions.
cinder cone volcano
Cinder cone volcanoes are small, rarely more than 400 meters high, with steep slopes of volcanic debris referred to as cinders. Many have a large, bowl-shaped crater, and if lava flows occur from these volcanoes, they usually break through the base or lower flanks of the mountains. Cinder cones can be isolated, singular volcanoes, but many form on the flanks or within the calderas of larger volcanoes and may represent the final stages of volcanic activity. They can be very active, but tend to be short-lived.
fires
Fires can be a major hazard in areas after the passage of an earthquake. Broken electrical wires could result in shortages and start major fires. If gas lines are ruptured, escaped gases could fuel massive fires for several days, as seen in the picture below left of Los Angeles. Water mains could be destroyed, leaving no water pressure to put out fires. Approximately 90% of the damage caused by the 1906 great San Francisco earthquake was caused by fire (picture below right).
flash flood
Flash floods occurs very rapidly in a small geographic area; sometimes the area being flooded has not experienced any significant precipitation, but is receiving water from areas upstream that have. Flash floods happen in semi-arid mountainous regions where precipitation occurs as intense thunderstorms and stream gradients are high.
volcanoes are categorized by
Lava composition Size Shape Location
Divergent Boundaries
Locations where plates are separating and new oceanic crust is being created are divergent boundaries, or spreading centers. Divergent boundaries most commonly occur along the crests of oceanic ridges such as the Mid-Atlantic Ridge (see picture D below). Oceanic ridges are characterized by rugged topography resulting from the displacement of rocks along large fractures, shallow depth earthquakes, high heat flow, and basaltic lava that forms new oceanic crust. - Divergent boundaries are also present under continents during the early stages of continental breakup. When magma wells up beneath a continent, the crust is initially elevated, stretched, thinned, and fractured (picture A below). Eventually the crust thins and separates enough that rift valleys are formed (picture B below). - Continued volcanic eruptions cover the valley floor and the denser material sinks, creating a low lying area that will be eventually covered by water. Continued separation creates narrow seas such as the Red Sea (picture C below), and eventually, if the rift valley continues to deepen and lengthen, a larger ocean basin will develop with the formerly joined landmasses separated by an extensive ocean (picture D below). The continents will eventually develop wide passive continental margins, including continental shelves, slopes and rises.
folded rock layers
Most folding occurs deep in the crust where pressure and temperature are high and rocks are more ductile than they are at or near the surface. The configuration of folds and the intensity of folding varies, but there are only three types of elongate folds: monoclines, anticlines, and synclines. Monoclines are a simple bend or flexure in otherwise horizontal or uniformly dipping rock layers. They are usually very broad structures and are interpreted over large geographic areas (see map below). The monocline in Wyoming formed when the Bighorn Mountains rose vertically along a fracture. The fracture did not penetrate the surface, so as uplift of the mountains proceeded, the near-surface rocks were bent so that they now appear to be draped over the margin of the uplift block (picture below, left). The East Kaibab monocline extends from Bryce Canyon in southern Utah to the San Francisco Peaks near Flagstaff and the Grand Canyon in northern Arizona (picture below, right). In a manner of speaking, a monocline is simply one-half of an anticline or syncline.
7 major plates
North American, South American, Pacific, African, Eurasian, Indo-Australian, and Antartic
Slab-pull
Slab-pull is explained by the density differences between the subducting slab of lithosphere and the warmer asthenosphere. The denser slab of lithosphere is pulled down, causing the overlying plate to move faster than the convecting cell below.
how do sinkholes form
Soluble rock below the soil is dissolved by seeping water, and openings in the rock are enlarged and filled in by the overlying soil. As groundwater continues to dissolve the rock, the soil is eventually removed, leaving shallow depressions with gently sloping sides. - Sinkholes also form when a cave's roof collapses, usually producing a steep-sided crater.
stress
Stress in rock material results from force applied to a given area, usually expressed in kg/cm2.
surface waves
Surface waves spread outward from the epicenter of the earthquake and propagate along the surface of the earth. Unlike the sharp jolting and shaking caused by body waves, surface waves generally produce a rolling or swaying motion. The two most important surface waves are Love (L) waves and Rayleigh (R) waves. Surface waves are slower than P-waves and S-waves, however, their surficial shearing motion are usually the principal cause of structural damages.
liquification
The degree of saturation also plays a role in ground shaking. Water saturated soils and sediments sometime lose their cohesive strength when shaken and behave like a liquid in a process called liquefaction. Buildings sometimes remain generally intact as they tip or topple into soft sediment, as shown in the picture below. These buildings are in Japan, where earthquakes are common and building codes are strictly enforced due to the likely event of geologic disturbance. The buildings remained mostly intact, but seismic waves traveling through the soft sediment caused the sediment grains to lose their cohesion and fail.
what causes plates to move
The lithosphere overlies the hotter and weaker asthenosphere. It is thought that movement resulting from some type of heat-transfer system within the asthenosphere causes the overlying plates to move
The Role of Gravity and Slope in Landslides
The most important factor that determines the stability of loose grains on a sloping ground surface is the force of gravity. Gravity tries to pull all objects toward the center of Earth. Its pull is constant, as shown by the fact that you cannot stand upright for very long. After some time passes, your body grows weary of fighting the pull of gravity, and you sit or lie down. If a sand or gravel particle is resting on a flat surface, gravity will hold it in place. If the particle is lying on a sloping surface, a part of the gravitational force is oriented parallel to the ground slope. So long as the part of the force oriented perpendicular to the surface is greater than the part parallel to the surface, the particle will not move. Friction between the particle and the ground surface holds the particle in place. When the slope is steeper, the frictional force will be overcome, and the particle will move down the slope. Normally, a large number of particles rests on the solid ground surface, rather than only one. On a gently sloping surface, the friction of particle against particle may be sufficient to keep them from moving. If the slope increases because of erosion or because of an excavation at its base, the friction will be overcome and the mass will slide down slope. If water is present, grains may slide on even very gentle slopes because water decreases the frictional attraction between grains and between the sloping surface.
plate tectonics + volcanoes
The tectonic setting of a volcano determines, to a large extent, the characteristics of magma and lava and the type of the volcano that forms. Generally speaking, relatively peaceful eruptions are characteristic of low viscosity, basaltic magmas generated at spreading zones; violent eruptions are characteristic of intermediate and felsic magmas generated at subduction zones. Eruptions that form in hotspots will be controlled by the location within the tectonic plate, the rocks melting to form the magma, and water content.
Composite Volcanoes
These volcanoes are what might be considered the "classic" volcano, with steeper sides and the conical shaped volcano profile. Composite volcanoes are built over time from alternating layers of lava and ash, and are generally associated with intermediate to felsic magma. They are sometimes referred to as stratovolcanoes, referencing the strata or layers found on the flanks of the volcanic mountain. The lava generated from these volcanoes is more viscous than mafic lava flows, so the eruptions are more volatile and explosive, with a significant ash and gas output during the eruption. Volcanoes with these attributes often form at convergent boundary subduction zones, either as part of a volcanic island arc complex as a result of oceanic-oceanic crust interactions, or as a volcanic arc system as a result of oceanic-continental crust interactions. Below, Mt. Fuji in Japan (picture A, below left) is part of a volcanic island arc formed along the Pacific plate subduction zone. Mt. Kilamanjaro (picture B, below right) is a composite volcano found in Tanzania, Africa.
seismic waves
When movement along a fault takes place, energy is released in the form of two kinds of seismic waves that radiate outward in all directions from an earthquake's focus
end of tsunami
When they enter shallow water, the waves slow down and water piles up to heights anywhere from two to many meters high. The 1946 tsunami that struck Hilo, Hawaii was 16.5 m high. The tremendous energy possessed by a tsunami is concentrated on the shoreline when it hits either as a large breaking wave or, in some cases, what appears to be a very rapidly rising tide. A common misconception is that a tsunami contains a single wave that crashes onto the shoreline. Tsunamis contain a series of waves that pour onshore for as long as 30 minutes followed by an equal time during which the water rushes back to sea. After the first wave hits, more waves follow at 20 to 60 minute intervals. Following the tragic 1946 tsunami that hit Hilo, Hawaii, the U.S. Coast and Geodetic Survey established a Pacific Tsunami Early Warning System (see picture below). This system utilizes seismographs and instruments that detect earthquake-generated sea waves. Whenever a strong earthquake takes place anywhere within the Pacific basin, its location is determined, and instruments are checked to see whether a tsunami has been generated. If it has, a warning is sent to evacuate people from low-lying areas that may be affected.
sinkhole
a depression or hole in the ground created by erosion and the drainage of water. They can be just a few feet across or large enough to swallow whole buildings. Most are caused by karst processes, the chemical dissolution of soluble rock such as limestone, gypsum, and rock salt. Although they're often the result of natural processes they can also be triggered by human activity. - Sinkholes are formed when the land surface above collapses or sinks into cavities in the subsurface; they can form relatively quickly or slowly over time. In areas underlain by soluble rock (most often limestone) the continual dissolution of rock by groundwater eventually leads to collapse of the surface. The collapsed area may contain only soil, or soil and a thin layer of bedrock beneath it.
Deformation
a term which describes changes in the shape or volume of rock. Dynamic forces within the Earth continue to change rock through: Seismic activity earthquakes Volcanism Plate movements - They occur mostly as a result of plate tectonics and correlate well with specific tectonic environments
shear stress
forces act parallel to one another but in opposite directions; displacement occurs along closely spaced planes
landslide
also called mass wasting, occur on slopes when a mass of earth materials move downslope as a result of gravity. Some of history's worst disasters have been caused by these movements, and they have been increasing in more recent times. Expanding urbanization and construction in sloping terrain have created more opportunities for landslides that would have been avoided in earlier times. Through the advent of modern machinery and indirect influence, people move about 40 billion tons of soil and rock each year, a volume that equals or exceeds material transported by any other single agent, such as water, wind, or ice.
faults
are similar to joints in that they are fractures in rock, but in this case, movement has taken place along the fracture plane - also referred to as a fault plane. Rocks on either side of the fault plane move in opposite directions, either vertically or horizontally. If the displacement is vertical, geologists refer to these faults as dip-slip faults. If the movement is horizontal, geologists refer to these faults as strike-slip faults.
Convection currents
beneath the plates move the plates in different directions. The main source of heat driving the convection currents is radioactivity deep in the Earth's mantle.
what affects the worlds climate
complex interaction between wind and ocean currents
Plate movement is controlled by
convecting cells in the asthenosphere; the direction of the cells controls the direction in which the overlying plates will move.
earth layers
crust, mantle, core - lithosphere, asthenosphere, mesosphere, outer core, and inner core
types of plate boundaries
divergent, convergent, transform - Along these boundaries, new crust is formed, older crust is consumed, or crust slides past each other in opposite direction
joints
fractures along which no movement has taken place parallel with the fracture surface. Joints can be focal points for water and weathering processes, so they do expand perpendicular to the joint surface. Much of the spectacular scenery in the southwestern U.S. is formed as a result of jointing and fracturing at the Earth's surface. Most all near-surface rocks have joints that form in response to compression, tension, and shear stresses. They can vary from tiny fractures to those that extend for many kilometers and are often arranged in two or perhaps three prominent sets. Regional mapping reveals that joints and sets of joints are usually related to other geologic structures such as faults and folds.
effects of earthquakes
ground shaking, liquefaction, landslides, fires and tsunamis.
who discovered Mid Ocean Ridges
harry hess
ring of fire
includes the volcanoes in the Andes in South America; the volcanoes of Central America, Mexico, and the Cascade Range of North America; and the Alaskan volcanoes as well as those in Japan, the Philippines, Indonesia, and New Zealand. Also in the circum-Pacific belt are the southernmost active volcanoes at Mount Erebus that erupted in Antarctica during 2011 and a large caldera at Deception Island that erupted most recently during 1970. Another 20% of active volcanism occurs in the Mediterranean belt, including famous volcanoes such as Mounts Etna and Vesuvius, and the Greek volcano Santorini. Nearly all other remaining active volcanoes are associated with mid-ocean ridges or hot spots.
subduction zones
locations where oceanic plates converge and subduct under either oceanic or continental plates, creating a linear geographic cluster of active volcanoes. Volcanoes formed in this environment tend to take on the familiar classic conical shape; they are usually part of spectacular landscapes such as the Andes Mountains in South America and tend to be the most hazardous.
slow rising flood
low-rise flooding occurs of over a period of time, with extended periods of rain over weeks or months.
subduction zones
places where plates are pushed down into the upper mantle - As older oceanic crust is pushed away from the oceanic ridge, it eventually collides with other crust and older oceanic crust is subducted back into the mantle at trenches associated with subduction zones.
Compressional stress
rocks are compacted by forced directed towards one another along the same line; depending on the depth, rocks tend to fold or fracture to accommodate the stress
secondary waves
somewhat slower than primary waves and can only travel through solids. These waves are shear waves because they move material perpendicular to the direction of travel, producing shear stresses in the material they move through. Because liquids and gases are not rigid, they have no shear strength and s-waves cannot be transmitted through them.
Plate tectonic theory
the Earth's crust and upper mantle, known as the lithosphere, is broken into several variable sized pieces, called plates. These plates are sometimes referred to as lithospheric plates or tectonic plates, all plates contain both oceanic and continental crust. Plates can vary in both size and thickness, depending on their tectonic environment.
urbanization
the paving of land, building construction, stripping vegetation and soil - both increases the highest discharge that local streams attain and decreases the amount of infiltration that occurs (see hydrograph at right). Smaller floods are more affected by urbanization than are larger, less frequent floods. A larger flood is less impacted by urbanization because such extreme floods overwhelm the storage capacity of soil cover, no matter how extensive.
reverse fault
a type of fault where the hanging wall slides upward; caused by compression in the crust - Generally speaking, normal faults are produced by tensional stresses and reverse faults are produced by compressional stresses.
the jet stream
a west-to-east flow of air that normally flows at high altitudes over Canada during the summer months, stalled over Iowa and the Midwest. It formed a barrier to moisture-laden air that was moving northward from the Gulf of Mexico. That air bumped into the jet stream over Iowa and dumped its moisture almost without letup.
Sea Floor Spreading
the process by which new oceanic lithosphere forms as magma rises toward the surface and solidifies
Body Waves
those that emanate through the body of the Earth - There are two types of body waves, primary waves (p-waves) and secondary waves (s-waves). Primary waves are the fastest traveling seismic wave, and can travel through any medium: solids, liquids, or gases. They are compressional push-pull waves and move material back and forth along a line in the same direction that the waves are moving. The material through which the primary waves are traveling is expanded and compressed as the wave moves through it and returns to its original size and shape after the wave passes by. Because p-wave velocity is greater than s-wave velocity in all materials, p-waves always arrive at seismic stations first.
volcanic gases
All volcanic eruptions contain a variety of gases, with water vapor being the most common gas dissolved in a magma body. Depending on the volcano, water vapor may make up between 37% - 97% of all gases. Volcanic eruptions are responsible for circulating deeply seated water and bringing it to the surface to participate in the hydrologic cycle. Not all volcanic gases are so benign however, as magmas and lavas also contain carbon dioxide, sulfur dioxide, hydrogen fluoride, and others in minor quantities. As magma rises toward the surface, the pressure is reduced and the contained gases begin to expand. In highly viscous, felsic magma, expansion is inhibited and gas pressure increases. Eventually the pressure may become great enough and produce a violent explosion of volcanic debris (picture A below). In contrast, low-viscosity mafic magma allows gases to expand and escape easily, so volcanic eruptions tend to be more quiet (picture B below).
predicitng earthquakes
Although predicting the timing and effects of earthquakes remains problematic at best, where earthquakes will occur is much easier to predict: almost 95% of all earthquakes occur in seismic belts located at or near plate boundaries (see picture below). The other 5% are called intraplate earthquakes and occur away from plate boundaries. Many times, they can be more devastating as they are unexpected. The majority of all earthquakes ~ 80%, occur in the circum-Pacific belt encircling the Pacific Ocean basin. More than 900,000 earthquakes are recorded annually by the worldwide network of seismograph stations. Many of these are too small to be felt, except by sensitive seismic equipment. As our global population expands, more people live and work in earthquakes prone areas - such as the western coast of California! Earthquakes can be very destructive and cause death and injury to people in the area; they also affect the local economies in terms of clean-up costs, lost jobs, and lost business revenues.
Mid-Ocean Ridge Volcanoes
Although we have already discussed Mid-Ocean Ridges in the previous module, they are worth mentioning again as a type of volcano. Much of the mafic magma that originates at spreading ridges is emplaced within the crust as dikes and plutons, but some rises to the surface and forms submarine lava flows and pillow lava, which makes up the upper part of oceanic crust. Mafic lava is very fluid, and the immense pressure of the overlying water column prevents any sort of explosive eruption on the ocean floor. Lava that makes it to the ocean floor forms into bulbous masses referred to as "pillows", distinctive accumulations of these pillows form pillow lavas. While most of these eruptions go undetected, researchers in submarine vehicles have seen the results of recent eruptions.
Anticlines
Anticlines are up-arched or convex upward folds with the oldest rock in the center of the fold structure. A syncline is a down-arched or concave downward fold in which the youngest rock layers are in its core. Anticlines and synclines have an axial plane connecting the points of maximum curvature of each folded layer and divides folds into halves, with each half being a limb (see picture below). Because folds are most often found in a series of anticlines and synclines, an anticline and adjacent syncline share a limb. Folds can be exposed to view in areas of deep erosion, but it is important to remember that anticlines and synclines are simply folded rock layers and do not always correspond with areas of high and low topography at the surface. - Anticlines and synclines are elongate structures, meaning that their length greatly exceeds their width. In contrast, folds that are nearly equidimensional - oval to circular - are domes and basins. In a dome, all of the folded strata dip outward from a central point, as opposed to outward from a central line in an anticline, and the oldest rocks are exposed in the center of the fold (picture A below). In a basin, all strata dips inward toward a central point and the youngest rocks are exposed at the center of the fold (picture B below). We can characterize a dome as the circular equivalent of an anticline, and a basin as the circular equivalent of a syncline. Many domes and basins are so large that they can be visualized only on geologic maps or aerial photographs.
Convergent Boundaries
At these boundaries, two plates collide and the leading edge of one plate is subducted beneath the margin of the other plate and eventually incorporated into the asthenosphere. Convergent boundaries are characterized by deformation, volcanism, mountain building, metamorphism, earthquake activity, and deposits of valuable minerals. There are two types of crust, and three ways in which they can interact: oceanic-oceanic, oceanic-continental, and continental-continental. - Oceanic-oceanic convergent boundaries occur when two plates bearing oceanic crust converge, and one plate descends under the other forming an oceanic trench. As the subducting plate descends into the asthenosphere, it is heated and partially melted, generating magma of an intermediate (andesitic) composition. The magma will be less dense than the rock around it, and rises to the surface of the overriding plate to form a curved chain of volcanic islands called a volcanic island arc as shown in the picture below. Most present day active volcanic island arcs are located in the Pacific Ocean (Aleutian Islands, Japanese Islands, Philippine Islands) and the Atlantic Ocean (Scotia, Antillean).
proof of continental drift
Continental Fit: the continents fit together very well, particularly when you include the continental margins - Similarity of Rock Sequences: Marine, non-marine and glacial rock sequences in diff parts of world match, e trends of several major mountain ranges on separate continents match when the continents are repositioned. - Glacial Evidence: Multiple bodies of evidence, including glacial till (glacial sediments) and glacial striations (extensive scratch marks in the rock face made by moving glaciers) can be found in southern hemisphere continents, most of which are located in tropical to sub-tropical climates today. - Fossil Evidence: Wegener reasoned that these landmasses must once have been joined so that these widely separated continents were all in the same latitudinal belt, allowing those organisms to evolve and flourish, and eventually be preserved in the fossil record.
The Role of Earthquakes in Landslides
Earthquakes can trigger landslides in much the same way that water can. In an earthquake, the grains in a rock or sediment are separated by shaking. The earthquake jostles the grains so that they lose cohesion and separate for a moment, which is sufficient time to cause a slope failure. Earthquakes can also cause landsliding for another reason. As seismic waves travel through the solid rock, it can cause rocks to fracture and break; the fracturing of the rock, combined with the jostling caused by the quake, allows gravity to have an instantaneous effect and catastrophic downslope movement occurs.
tsunamis
Earthquakes sometime generate seismic sea waves called tsunamis which may rise to be several meters high as they come ashore. They are generated when large amount of energy from submarine earthquakes are rapidly released in to a body of water. Tsunamis are able to travel great distances across open seas at speeds of several hundred meters per hour and inflict massive damage thousands of kilometers away from the epicenter. The picture below left is from the island of Hawaii in 1946, when a tsunami generated by an earthquake in the Aleutian Islands engulfed a large segment of the island.
earthquakes
Earthquakes take place because rocks are capable of storing energy, but their strength is limited, so if enough force is present, they rupture and release their stored energy. Most earthquakes occur when movement is initiated along faults, most of which are related, at least indirectly, to plate movement. Once rupturing begins, it moves along the fault at several kilometers per second for as long as conditions for failure exist. The longer the fault along which movement occurs, the more time it takes for the stored energy to be released, and therefore the longer the ground will shake. - When an earthquake occurs, the released energy radiates out from the point of rupture in the form of seismic waves.These waves move out from this point in all directions and emanate through the Earth. The seismic waves are recorded by instruments called seismographs, instruments that record and measure the vibrations produced by an earthquake. These seismographs measure both types of movement: horizontal (below left) and vertical (below right). Modern seismographs have electronic sensors and record movements precisely using computers, rather than relying on the drum strip charts as shown below. The record of the waves made by the instrument is called a seismogram; seismograms are interpreted by seismologists, scientists who study earthquakes. - The location within the Earth's crust where rupturing begins and the point where energy is first released is called the focus. The place on the Earth's surface directly above the focus is called the epicenter. When we first learn of an earthquake from the news or otherwise, most often the report gives the location of the epicenter. The focal depth of an earthquake requires more extensive data evaluation and will not be available in the initial report.
plate tectonics + earthquakes
Earthquakes, along with volcanic eruptions, are part of the Earth's release of energy. Because our Earth is tectonically active, the interaction of the lithospheric plates creates a dynamic environment to propagate earthquakes. Earthquakes occur at all types of plate boundaries; convergent, divergent, and transform boundaries. Movement along these boundaries (and associated faults) releases energy in the form of seismic waves. When energy is released in one area, it causes stress to increase elsewhere. As a result, the constant shifting of the plates and the earthquakes that happen when the plates move, are part of the dynamic interaction between the Earth's crust and the mantle and core. Although predicting the timing and effects of earthquakes remains problematic at best, where earthquakes will occur is much easier to predict: almost 95% of all earthquakes occur in seismic belts located at or near plate boundaries. The other 5% are called intraplate earthquakes and occur away from plate boundaries. Many times, they can be more devastating as they are unexpected. The majority of all earthquakes ~ 80%, occur in the circum-Pacific belt encircling the Pacific Ocean basin. More than 1,000,000 earthquakes are recorded annually by the worldwide network of seismograph stations. Many of these are too small to be felt, except by sensitive seismic equipment.
ground shaking
Ground shaking is usually the most obvious indicator of an earthquake. The amount of shaking is dependent upon several factors such as epicentral distance, magnitude, and the geology of the underlying rocks. Ground shaking effects such as collapsed buildings, flying glass from broken windows and walls, toppled objects and potential injuries caused from falling objects may result in more loss of life than any of the other earthquake hazards. Structures built on loosely consolidated surfaces are likely to experience a greater degree of ground shaking as the amplitude and duration of seismic waves generally increase as they transition from consolidated bedrock to unconsolidated materials.
earthquake magnitude
If earthquakes are to be compared quantitatively, we must use a scale that measures the amount of energy released and is independent of intensity as earthquakes can show enormous variations in strength and produce amplitudes that are significantly different in size. The Richter scale, named after its inventor Charles Richter, is used to measure the size of an earthquake. To accommodate this wide variation in size, the Richter scale is a logarithmic scale where a tenfold increase in wave amplitude corresponds to an increase of 1 on the magnitude scale. Therefore, the amount of ground shaking associated with a magnitude 6 is 10 times greater than that produced by a magnitude 5; 100 times greater than a magnitude 4. Additionally, the amount of energy produced by a magnitude 6 is approximately 30 times greater than a magnitude 5 and 900 (30x30) greater than that of a magnitude 4. When an earthquake occurs, initial reports will classify the earthquake with a certain magnitude. Often, after data is processed and initial reports are revised, the magnitude of an earthquake will be adjusted. The December 26, 2004 Sumatra-Andaman earthquake that triggered a series of tsunamis throughout the Indian Ocean was originally classified as a magnitude 9.0 earthquake, and was later revised to a magnitude 9.2.
Pyroclastic Materials
In addition to lava flows, erupting volcanoes eject pyroclastic materials such as volcanic ash. If the ash is ejected into the atmosphere (picture A below) and eventually settles to the surface (pictures B below), it is referred to as an ash fall. If the ash flows downslope in a cloud of ash and gas (picture C below), and solidifies (picture D below) it is referred to as an ash flow. Ash flows can move rapidly, sometimes up to 100 km/hour. There are two types of pyroclastic ash flows: nuee`ardent and lahar. Nuee`ardent flows are sometimes called a "glowing cloud" characterized by a dense, hot mass of ash, gas, and volcanic material traveling downslope at high velocities (picture C below). - Lahars are similar, only water is added to the mix from precipitation or melting snow. Due to the water content, the ash will have a consistency of wet cement, creating a highly viscous fluid that can endanger everything in its path (pictures A and B below).
other
In addition to volcanic ash, volcanoes also erupt lapilli and volcanic blocks and bombs. Bombs have a twisted, streamlined shape which indicates they were erupted as globs of magma that cooled and solidified during their flight through the air. Blocks are angular pieces of rock ripped from the volcanic conduit or pieces of a solidified lava flow. Lapilli, blocks and bombs are usually confined to the immediate area of the volcano because they are too large to travel far suspended in the air column.
landslides
In areas of high relief ground failure triggered by earthquakes can be particularly destructive or fatal. If slopes are unstable, the shaking caused by earthquakes could cause ground failure. Over 2,000 residents lost their lives when most of the town of Port Royal, Jamaica sunk below sea level as a result of an earthquake and tsunami on June, 7 1692 (Figure 6.7). About 3,000 people died in the days following the earthquakes due to injuries and disease.
earthquake intensity
Intensity is a subjective or qualitative measure of the kind of damage done by an earthquake as well as people's reaction to it. Since the mid-19th century, geologists have used intensity as a rough approximation of the size and strength of an earthquake. The Modified Mercalli Intensity Scale, named after its inventor Guiseppe Mercalli, has 12 levels (I - XII) of earthquake intensity with XII being the most intense. It is based on the P-S time interval along with the amplitude (height) of the largest seismic wave and takes into account the fact that seismic waves weaken with increasing distance away from the focus. Even though earthquake intensity is subjective, it does qualify the kind of damage done by an earthquake and insurance companies still classify earthquakes on the basis on intensity.
largest volcano
Mauna Loa in Hawaii (picture A, below left) is the largest volcano. In our solar system, Olympus Mons on Mars (picture B, below right) is the largest volcano. Olympus Mons stands 25 km high, about three times the size of Mt. Everest, and measures more than 500 km across its base. The base of the volcano covers an area about the size of the state of Ohio.
Mafic lava flows
Most mafic lava flows do not flow very rapidly, and because they are fluid, they follow low areas. Once a flow begins moving, determining the path it will take is fairly easy, and anyone in areas likely to be affected can be evacuated. From April 1990 to January 1991, lava flows (picture B, below right) covered Kalapana, Hawaii and in the process destroyed 180 homes, highways (picture A, below left), cultural landmarks, and points of archaeological interest. Civil Defense authorities working with geologists at the Hawaiian Volcano Observatory made important decisions regarding evacuations and road closures, and as a result, there were no injuries or fatalities. - Mafic lava flows can be characterized as pahoehoe or aa, both initially used to describe Hawaiian eruptions but this terminology is now used elsewhere. Pahoehoe lava (picture A, below left) has a smooth, ropy texture, and the flows are hotter and thinner than aa flows. Aa flows (picture B, below right) consist of jagged, angular blocks and fragments, and can be viscous enough to break into blocks and move forward as a wall of rubble. If a pahoehoe flow cools enough and its viscosity increases, it can change into an aa flow; however, an aa flow cannot change into a pahoehoe flow.
lava dome
Most volcanoes are classified as one of the above types, a less common type of volcano is referred to as a lava dome volcano (also called a volcanic dome or resurgent dome). Lava domes are steep-sided bulbous mountains that form when highly viscous felsic magma, and sometimes intermediate magma, is forced toward the surface by immense pressure from below. These eruptions can be some of the most violent and destructive, as the rising pressure eventually blasts hot gases and volcanic debris downslope at great speeds.
s-wave (secondary wave)
S-waves are also called shear or side to side waves as they travel through the earth with a side-to- side motion (Figure 6.5). They are referred to as secondary waves and are recorded at seismic stations later than P-waves. S-waves travel between 3km/s and 4.5 km/s, with an average velocity of 4.1 km/s at the surface and displace earth material perpendicular to the direction of travel. S-waves cannot travel through fluids (liquids and gases) and are therefore unable to penetrate the outer core of the earth. - The velocities of primary and secondary waves are determined by the density and elasticity of the materials through which they travel. Seismic waves travel more slowly through rocks of greater density, but more rapidly through rocks with greater elasticity. Elasticity is a property of solid rock, and means that once they have been deformed by an applied force, they return to their original shape when the force is no longer present. Because primary wave velocity is greater than secondary wave velocity in all materials, primary waves always arrive at seismic stations first.
strain
Strain is deformation caused by stress - Elastic - non-permanent deformation, rocks rebound to their original shape and location - Brittle - rocks fracture and break, often occurs if the force is applied rapidly to rocks located at shallow depths - Ductile - rocks bend, stretch, and fold over a longer period of time, particularly if the rocks are located at depth
strain (deformation)
Strain is deformation in rocks caused by stress, and is characterized as elastic, brittle, or ductile. As stress is applied, rocks respond first by elastic strain, but when strained beyond their elastic limit, they undergo ductile strain and deform by folding, or behave like brittle solids and deform by fracturing. In either folding or fracturing, the strain is permanent; that is, the rocks do not recover their original shape or volume even in the stress is removed. Whether the strain is elastic, brittle or ductile depends on the kind of stress applied, pressure and temperature, rock type, and the length of time the rocks are subjected to stress. A small stress applied over a long period of time, such as heavy books on a bookshelf, will cause the rocks to sag and experience ductile deformation. If the bookshelf were struck with a hammer, the shelf would fracture and experience brittle deformation. Rock type is also important because all rocks do not have the same internal strength and respond to stress differently. Rocks that experience brittle deformation show little to no plastic strain before they fracture, but rocks that experience ductile deformation can bend and stretch a great deal before they finally fracture. Many rocks show the effects of deformation that must have taken place deep within the crust. At or near the surface, rocks will behave like brittle solids and fracture, but at depth, they more often exhibit ductile deformation and become more ductile with increasing pressure and temperature.
Rock layers may be crumpled into folds or fractured as a result of stress, which results from force applied to a given area of rock. The rock's internal strength resists stress, but if the stress is great enough the rock undergoes strain, which is simply deformation caused by stress.
Stress Stress is the force applied to a given area of rock, usually expressed in kilograms per square centimeter (kg/cm2). Although stress is force per unit area, it comes in three varieties: compression, tension, and shear, depending on the direction of the applied forces. In compression, rocks or any other object are squeezed and compressed by forces directed toward one another along the same line - such as squeezing a rubber ball in your hand. Rock layers deformed by compression tend to be shortened in the direction of stress by either folding or fracturing. - Tension results from forces acting along the same line, but in opposite directions and tends to lengthen rocks or pull them apart. - Shear stresses act parallel to one another but in opposite directions, resulting in deformation by displacement along closely spaced planes.
The Role of Water in Landslides
The addition of water to material on a slope can make landslides more common. This is because water adds significant weight to the slope as it seeps into the ground, becoming groundwater, and adding to the gravitational force. Water lowers the strength of the material which can make it less able to withstand the force of gravity. Water also reduces friction between grains and between the grain and the sloping surface, making it easier to move material downhill. These processes help to explain why landslides are much more common during the rainy season, and especially common during or right after large storms. If the amount of water is increased in a sediment body that contains clay minerals, the water can cause these minerals to lose their static attraction to each other. The cohesion between the flat, flaky clay minerals is lost, and they will slide much like a tilted deck of cards. Water can also dissolve mineral cements that hold sand grains together. If the sandstone is bound by a calcite cement, this cement will easily dissolve in water and release the sand grains.
Strike-slip faults result from shear stresses and exhibit horizontal movement with blocks on either side of the fault plane sliding in opposite directions.
These faults are characterized as right-lateral or left-lateral, depending on the relative sense of movement. In our second picture to the right, this fault would be a left-lateral strike slip fault because the sense of movement is to the left. If I stand on the block of rock on the left hand side of the picture and look across the fault plane, the other block would appear to be moving left. If I stand on the block of rock on the right hand side of the picture and look across the fault plane, the other block would appear to be moving left. One of the most famous strike-slip faults is the San Andreas Fault on the western coast of California and is shown in our third picture to the right. This fault also functions as a transform plate boundary between the North American Plate and the Pacific Plate.
Fissure Volcanoes
These volcanoes lack a central vent or cone, instead the mafic lava flows out of long fractures or linear rifts in the crust. Because lava is very fluid with such low viscosity, it spreads easily covering vast geographic areas. One of the features associated with fissure volcanoes are basalt plateaus; the Columbia River basalts are found in eastern Washington and parts of Oregon and Idaho, and are huge accumulations of basalt that cover over 164,000 km2 with an average thickness of more than 1,000 meters. Currently, fissure eruptions only occur in Iceland, and the bulk of the island is composed of basalt lava erupted from fissure volcanoes.
Tsunami
Tsunamis are destructive sea waves generated when the seafloor undergoes sudden vertical movement. The word tsunami means "harbor wave" and it is in harbors and estuaries that the damage is magnified. When a tsunami strikes a relatively straight coastline, the energy is distributed along the entire shore. When the wave enters an embayment like a harbor or river mouth, the water is funneled into a narrow space and the wave can build to heights much greater than 10 m (almost 35 feet) and cause unimaginable damage. Sometimes incorrectly called "tidal waves", tsunamis are not related to tides and have nothing to do with the regular movement of water that occurs because of the Earth's relationship between the Sun and Moon. Many tsunamis result from submarine earthquakes, but volcanoes at sea and submarine landslides have also caused them. Once a tsunami is generated, it can travel across an entire ocean and cause devastation far from its source. In the open sea, tsunamis can travel at several hundred kilometers per hour and commonly go unnoticed as they pass beneath ships because they are usually less than 1 m high and the distance between wave crests is typically hundreds of kilometers. These waves pose no danger in mid-ocean, but can result in coastal disasters when they begin to "break" as they move into shallower water near land (see picture below).
Shield Volcanoes
Volcanoes that look much like the outer surface of a shield laying on the ground are called shield volcanoes. They are composed almost entirely of low-viscosity mafic lava flows, so the flows spread out and form thin layers with gentle slopes. These volcanic eruptions are sometimes referred to as Hawaiian-type eruptions, and are quiet when compared with eruptions of other types of volcanoes, particularly those that occur at convergent plate boundaries. Shield volcanoes do not occur at plate boundaries, they form in areas where the crust is thinning and magma from the upper mantle makes its way to the surface. These volcanoes occur most commonly in ocean basins, but some are present on the continents. The island of Hawaii is made up of five huge shield volcanoes; two of which, Kilauea and Mauna Loa (picture below), are still active much of the time.
deformation
a general term for all changes in the shape or volume of rocks. So the term "solid as a rock" should probably be followed by some sort of qualifier or disclaimer. - The fact that dynamic forces continue to operate within Earth is obvious from seismic activity, volcanism, plate movements, and the evolution of more recent mountain ranges in South America, Asia, and elsewhere. Earth is an active planet with many processes driven by internal heat, especially plate movements, and many of the earthquakes, volcanic eruptions, and mountain building takes place at convergent plate boundaries
earthquake
along with volcanic eruptions, are part of the Earth's release of energy. Because our Earth is tectonically active, the interaction of the lithospheric plates creates a dynamic environment to propagate earthquakes. Earthquakes occur at all types of plate boundaries; convergent, divergent, and transform boundaries. Movement along these boundaries (and associated faults) releases energy in the form of seismic waves. When energy is released in one area, it causes stress to increase elsewhere. As a result, the constant shifting of the plates and the earthquakes that happen when the plates move, are part of the dynamic interaction between the Earth's crust and the mantle and core. Geologists define an earthquake as the sudden vibration or shaking of the Earth caused by the rapid release of stored energy, usually caused by movement along faults and fractures in the Earth's crust. The released energy radiates from the source also known as the focus, the point inside the Earth where the movement actually occurs. Once the movement occurs, the released energy spreads in all directions in the form of seismic waves. The surface location directly above the focus is known as the epicenter (see picture below).
Transform Boundaries
also called transform faults, occur when plates slide laterally past each other; this type of movement most often occurs along fractures in the seafloor, perpendicular to the spreading ridge (picture A below). In this type of plate movement, crust is neither created nor destroyed, but the movement between plates results in a zone of intensely shattered rock and numerous shallow-depth earthquakes. Although the majority of transform faults are in oceanic crust, one of the best known transform faults is the San Andreas Fault in California (picture B below). It separates the Pacific plate from the North American plate, and connects spreading ridges in the Gulf of California with the Juan de Fuca and Pacific plates off the coast of the northern California. Many of the earthquakes that affect California are the result of movement along this fault. Finally, the Juan de Fuca plate and its relationship to the Pacific plate and the North American plate exhibits all three types of plate boundaries: ocean-continent convergent boundary along the coast of Oregon, Washington, and British Columbia, divergent boundary along the Juan de Fuca Ridge, and transform boundary between the spreading ridge and the convergent boundary (picture C below).
deformation definition
any change in the volume and shape of rocks, but this refers to rock layers rather than individual stones or rock samples you might find in a stream channel or along a hiking path. The type of rock is irrelevant - igneous, sedimentary, or metamorphic - but layered rocks such as sedimentary rocks or lava flows show the effects of deformation most clearly. We call the resulting changes in rocks associated with deformation geologic structures and define them based on the types of deformation with descriptive terminology such as folds and faults. Many/most of our interpretations of geologic structures occurs millions or even billions of years later than the actual deformation event, so interpretations of these structures help us understand what has happened to large packages of rocks, the timing of those events, and are an essential part of our understanding of the engineering capabilities of competent rocks layers when choosing sites of dams, bridges, and nuclear power plants. Studies of geologic structures are also important for many aspects of mining resource minerals and exploring for petroleum and natural gas.
spreading zones
areas where the Earth's crust is thinning or separating, erupting mafic lavas that spread across the surface, mostly on the ocean floor. On the continent, magma rises through rift zones and forms flood-basalt flows that spread over large geographic areas such as the Pacific Northwest.
hotspot volcanoes
far fewer in number but generally produce large volumes of magmas. These volcanoes form within a plate, rather than at or near the edge, and are driven by a magma source in the mantle. Although the sources of heat are derived from the mantle, if these hotspots are found within a continent they will erupt felsic lavas such as those found in Yellowstone National Park. If the hotspots are found within oceanic crust, the resulting eruptions will generate mafic lavas similar to those found in the Hawaiian volcanoes.
tensional stress
forces act along the same line in opposite directions, rocks tend to fracture to accommodate the stress
whats used to measure an earthquake
intensity and magnitude, to measure the size of an earthquake. Intensity assesses the degree of earthquake shaking at a given location based in the amount of damage to surficial structures including buildings and roads, as well as individual descriptions of the event. Intensity is a qualitative measure of the earthquake while the quantitative measure is called magnitude which does not rely on damage.
Oceanic-continental convergent boundaries
occur when oceanic crust and continental crust converge; the denser oceanic crust is subducted under the less dense continental crust. The magma generated by subduction rises beneath the continent and either crystallizes as large intrusive bodies (plutons) before reaching the surface, or erupts at the surface to produce a chain of intermediate (andesitic) volcanoes called a volcanic arc. An excellent modern example of this type of plate boundary is the Pacific Coast of South America where the oceanic Nazca plate is currently being subducted under South America. The Peru-Chile Trench marks the site of subduction, and the Andes Mountains are the resulting volcanic mountain chain on the western edge of the continent.
Continental-continental convergent boundaries
occur when two landmasses, previously separated by an ocean basin, collide as a result of complete subduction of the oceanic crust between them. Continental crust cannot be subducted underneath other continental crust; the density differences are not that great and the relative thickness of continental crust makes it a physical imposssibility. The two continents will be welded together along a zone marking the former site of subduction, forming an interior mountain belt consisting of deformed sediments and sedimentary rocks, igneous intrusions, metamorphic rocks, and fragments of oceanic crust. The Himalayas in central Asia resulted from the collision between India and Asia that occurred as India moved north to eventually be joined to the Eurasian landmass.
All movement on dip-slip faults takes place
parallel with the fault's dip; that is movement is vertical, either up or down the fault plane. The type of dip-slip fault is described by the apparent movement of the hanging wall, the block of rock that is above the fault plane. The block of rock below the fault plane is referred to as the footwall. This terminology came from miners; most underground mining for precious metals occurred along fault planes. As the miners walked into the mines, they would walk along the footwall. When they reached their destination in the underground mine, they would hang their lanterns from the hanging wall such as in the picture below.
Precipitation
rainfall - can lead to a rapid increase in a stream's water volume within the channel, referred to as stream discharge. When a stream's discharge becomes so great that it exceeds the capacity of its channel, it overflows its bank in a flood. Floods are the most commonly experienced natural hazards. In the United States, rainstorms and their resulting floods and debris flows accounted for over 70% of federally declared disasters during 1965-2005. Flooding is a normal, inevitable part of any stream's life over the years. Whether or not the flooding becomes an environmental problem depends on how close the stream is to population centers. It also depends on the elevation difference between a population center and the top of the stream bank. Most floods occur when rainfalls are so great that temporary storage in soil pores is insufficient to keep the stream from rising above bank level.
surface waves
travel along the surface of the ground or just below it Seismologists recognize several types of surface waves, the two most important being Rayleigh waves and Love waves, named after the scientists who discovered them. Surface waves are slower moving than body waves, and generally produce a rolling, pitching motion to the ground, much like the experience of being on a boat in turbulent water. Rayleigh waves are generally the slower of the two types of surface waves, and behave like water waves with the particles moving in an elliptical motion. Sometimes referred to as the "drunk" wave. - Love waves are sometimes referred to as the "snake" wave, with the particles moving back and forth in a horizontal plane perpendicular to the direction of wave travel.
