GEO 100 Final

Describe the stages in the formation and evolution of Earth's atmosphere. Where does the ozone in the atmosphere come from, and why is it important?

- - it comes from O3 - it absorbs deadly UV radiation

How and why do seismic waves bend as they pass through the mantle?

- (see refraction)

How much of the Earth's surface is covered by oceans? What proportion of the world's population lives near a coast?

- 70.8% of the planet - 80% of people live within 62 miles of a coast, 40% live within 37 miles of a coastline (60% of people)

Describe the composition of air (considering both its gases and its aerosols). Why are trace gases important?

- 78% N2 and 21% O2. The remaining 1%, the trace gases of air, includes argon (Ar), CO2, neon (Ne), CH4, He, H2, and O3. Some of these trace gases serve an important function in the Earth System, as we've seen—CO2 and CH4 are greenhouse gases that regulate Earth's atmospheric temperature, and O3 protects the surface from ultraviolet radiation. - Aerosols include sulfuric acid, sea salt, volcanic ash, clay flakes, mineral dust, soot, viruses and bacteria, specks of decayed organic material, and pollen

What are seismic-velocity discontinuities, and what do they tell us?

- A boundary in the Earth at which seismic velocity changes abruptly - define boundaries in the mantle

How does air pressure change with elevation? Does the density of the atmosphere also change with elevation? Explain why or why not.

- Air pressure decreases as elevation increases - Air density increases from high elevation down toward the surface of the Earth - In the Earth's gravity field, the weight of air at higher elevations presses down on and compresses air at lower elevations

How does an unconformity develop? Describe the three kinds of unconformities.

- An unconformity occurs when sedimentary layers are deposited on top of a surface of erosion or nondeposition. - Three kinds of unconformities are recognized: nonconformities, disconformities, and angular unconformities. - Nonconformities arise when sedimentary strata are deposited on top of crystalline (metamorphic or igneous) rock. - Disconformities occur when strata are deposited on top of an erosional surface that was horizontal at the time, so that layers above and below the unconformity are parallel. - Angular unconformities arise when the new strata are deposited on top of older layers that have been tilted out of horizontality, so that, regardless of future tilting, layers on opposite sides of the unconformity are not parallel.

What observation let to the discovery of the Moho?

- Andrija Mohorovičić noted that P-waves arriving at seismometers more than 200 km from the epicenter traveled at an average speed of 6 km per second, whereas P-waves arriving at seismometers more than 200 km from the epicenter traveled at an average speed of 8km per second. To explain this observation, he suggested that P-waves reaching nearby seismometers followed a shallow path through the crust

How does a craton differ from an orogenic belt?

- At a shield within a craton, very ancient (greater than 1 billion years old), highly metamorphosed rocks are exposed at the surface. These metamorphic rocks—formed in ancient orogenies and now eroded flat—were once deep within the core of a mountain range, but cycles of erosion and isostatic uplift have worn away all other evidence of ancient mountains (such as the faulted and folded sedimentary that once overlaid the metamorphosed basement rock).

Describe the pattern of global atmospheric circulation. Why don't convective cells extend from the equator to the pole?

- Atmospheric circulation is the large-scale movement of air by which heat is distributed on the surface of the Earth. The wind belts and the jet streams girdling the planet are steered by three convection cells: the Hadley cell, the Ferrel cell, and the Polar cell. While the Hadley, Ferrel, and Polar cells are major players in global heat transport, they do not act alone. Disparities in temperature also drive a set of longitudinal circulation cells, and the overall atmospheric motion is known as the zonal overturning circulation.

Discuss the relationship between foliation and deformation

- Foliation is a layering resulting from the alignment of mineral grains or the development of compositional bands. Deformation involves folding and fracture of rock bodies. Both processes take place during orogenic events: foliation at microscopic scale and deformation at regional scale.

How do geologists obtain an isotopic date? What does the age of igneous rock mean? What does the age of metamorphic rock mean?

- Geologists obtain an isotopic date by first observing the ratio of unstable parent isotope to the stable daughter product of the reaction within a mineral extracted from the rock of interest and then comparing this ratio to the known half-life for the nuclear reaction. - igneous: when magma solidified - metamorphic: the date it changed??

How are growth rings and ice layering useful in determining the ages of geologic events?

- Growth rings: annual layers from trees or shells - Rhythmic layering: annual layers in sediments - they provide points that geologist can track to compare the dates on an object

Describe the stages in the development of a hurricane. Describe a hurricane's basic geometry.

- Hurricanes develop in summer and late fall. Cyclonic low-P "tropical disturbances" pull air inward. This air rises, cools, and condenses, releasing latent heat. This heat buoys air, creates lower P, and pulls in more air. Over time, the storm gains size and strength. - Spiral arms (rain bands) mark rotating air pulled inward. - Converging winds accelerate, rise up, and spill out. - A region of calm, the "eye," is situated at storm center.

What factors control the climate of a region? What special conditions cause monsoons and El Nino?

- Latitude: north or south position: Determines amount of insolation and Controls isotherms: Hotter near the equator, Colder near the poles, and Seasonally varies - Altitude: height above sea level: Elevation linked to T and For the same latitude: Lower elevations warmer and Higher elevations colder - El Nino: low P develops off Peru, changing atmospheric and oceanic circulation. - monsoons: A monsoon is a seasonal shift in the prevailing wind direction. It occurs when the temperature on land is significantly warmer or cooler than the temperature of the ocean

Describe two different methods of correlating rock units. How was the correlation used to develop the geologic column? What is a stratigraphic formation?

- Lithologic correlation is the use of physical and chemical characteristics of rocks to determine that spatially isolated strata were once continuous (through original lateral continuity). Fossil correlation uses fossils with known stratigraphic ranges to determine the approximate temporal equivalence of two bodies of rock (which may be of disparate lithologies). Because a single locality does not provide strata that span in age throughout Earth's history, fossil correlation was required to provide time equivalence for units that are geographically isolated. With successful correlation, rock sequences from all parts of the world could be brought together to form an accurate sequence of relative time: the geologic column. A stratigraphic formation is a recognizable layer of a specific (usually sedimentary) rock type or set of types that were deposited within a certain time interval and can be traced over a broad region.

What are P-waves and S-waves shadow zones, and what do they tell us?

- P-wave shadow zone: A band between 103 degrees and 143 degrees from an earthquake epicenter. as measured along the circumference of the Earth, inside which P-waves do not arrive at seismograph stations - S-wave shadow zone: A band between 103 degrees and 180 degrees from the epicenter of an earthquake inside of which S-waves do not arrive at seismograph stations (do not travel through liquids)

In what ways do people try to modify or "stabilize" coasts? How do the actions of people threaten the natural systems of coastal areas?

- People use Groins, Jetties, Breakwaters and Sea Walls. Groins, Jetties, and Breakwaters all trap sand and must be dredged, making them difficult to deal with. Seal Walls reflect wave power back at the beach, washing sand off and allowing wave power to undermine themselves.

What does the process of radioactive decay entail?

- Radioactive decay is the nuclear breakdown of unstable isotopes in mineral crystals to form new (daughter) isotopes over time.

Do seismic waves travel at the same velocities in all rock types?

- Seismic waves travel at different velocities in different rock types. For example, P-waves travel at 3.5 km per second in sandstone (a porous sedimentary rock) but at 8 km per second in peridotite (an ultramafic igneous rock).

Under what conditions do thunderstorms develop? What provides the energy that drives clouds to the top of the troposphere? How do meteorologists explain lightening?

- Storms develop along steep pressure gradients. Centered by low pressure and fueled by warm, moist air - Thunderstorms: Short, local pulses of intense rain, wind, and lightning. Form when warm, moist, unstable air is carried upward, as a cold front moves in: Local conditions - Rising air forms cumulus clouds. Latent heat released by condensing water warms the air. - Lightning reflects electrical charge separation in clouds: lightening bolt is a giant spark or pulse of current that jumps across a gap of charge separation

Is seismic velocity constant at a given depth? Explain how seismologists learned the answer to this question and what the answer tells us about the mantle.

- Subsequent studies indicate that seismic velocities in the lower continental crust are faster than those in the upper continental crust. To interpret these observations, geologists have measured velocities in rock samples of various compositions under laboratory conditions. Such experiments suggest that the upper crust has, on average, felsic to intermediate composition, whereas the lower continental crust has, on average, mafic composition

What is the principle of isostasy, and why is the lithosphere able to be in isostatic compensation, in general?

- The condition that exists when the buoyancy force pushing lithosphere up equals the gravitational force pulling lithosphere down - lithosphere floats on the asthenosphere and thus can undergo isostatic adjustment

Explain the relation between the wind and variations in air pressure.

- warm air expands, becomes less dense, and rises (caused by wind) - This air is replaced by sinking, cooler, denser air.

What is a Wadati-Benioff zone, and why was it important in understanding plate tectonics?

- Wadati-Benioff: a sloping band of seismicity defined by intermediate-focus and deep-focus earthquakes that occur in the downgrading slab of a convergent plate boundary - it shows where one plate moves underneath the other

How do the shelf and slope of an active continental margin differ from those of a passive margin?

- active margin: narrow shelf and slope corresponds to the face of the accretionary prism(narrow shelf forms where apron of sediment spreads out over the top of an accretionary prism-the pile of material scraped off the downing subducting plate - passive: bigger shelf

What can we learn from seismic-reflection surveys?

- allows geologists to identify layering and structures in the sedimentary strata of the upper crust

Describe the differences among an anticline, a syncline, and a monocline.

- anticline: a fold with an arch-like shape in which the limbs dip away from the hinge - syncline: A trough-shaped fold whose limbs dip toward the hinge - monocline: A fold in the land surface whose shape resembles that of a carpet draped over a stair step

Describe the components of a beach profile.

- beach profile: cross section drawn perpendicular to the shore, illustrates the shape of a beach - beginning from the sea and moving landward, a beach consists of a foreshore zone, or intertidal zone, across which the tide rises and falls. then the beach face, steeper, concave-up par of foreshore zone, forms where smash of waves scours the sand. the back shore zone extends from a small step to the front of the dunes or cliffs that lie farther inshore. the back shore zone includes one or more berms, horizontal to landward-sloping terraces that received sediment during a storm

What changes do rocks undergo during formation of an orogenic belt?

- bending, breaking, shortening, stretching, and shearing

What is the difference between brittle and plastic deformation?

- brittle: The cracking and frfacturing of a material subjected to stress - plastic: The deformational process in which mineral grains behave like plastic and, when compressed or sheared, become flattened or elongate without cracking or breaking

Explain how liquefaction occurs in an earthquake and how it can cause damage.

- causes soil to lose strength - causes loose, wet sand to move

Why do clouds form? (Include a discussions of lifting mechanisms.) What are the basic categories of clouds?

- cloud-forming saturation occurs when air is lifted - Convective lifting—warmed air is buoyed upward. - Frontal lifting—air is carried upward along fronts. - Convergence lifting—converging winds force air upward. - Orographic lifting—air must rise to pass over mountains. - shape: Cirrus—wispy, thin, feathery; Cumulus—puffy, cottony; Stratus—stable, layered Prefixes narrow cloud types: Cirro—high-altitude; Alto—mid-altitude; Nimbo—rain-producing

How does a cold front differ from a warm front and from an occluded front?

- cold front: Steep Temp and Pressure gradient; cold, dense air flows beneath warmer air; rapid uplift of warm, humid air creates large storms - warm front: moves more slowly than cold fronts; warm air flows up a gentle incline over the colder air: pushes the cold air as a wedge and the incline reflects less steep temp and pressure gradients; warm air rising up the front produces broad cloud cover - occluded front: a combination front; a fast-moving cold front overtakes a slower warm front; the cold front lifts the warm front off the ground; generates strong winds and heavy precipitation

How do you recognize faults in the field?

- continuous features are displaced across a fault - faults may juxtapose different kinds of rock - friction may bend rocks near the fault into drag folds - brittle faulting results in shattered and crushed rock - fault breccia consists of rock fragments along a fault - fault gouge is made of pulverized, powdered rock - slickensides and linear grooves are slip lineations - scarps are visible when faults intersect the surface - fault zones with breccia and gouge preferentially erode - fault zones may be mineralized by fluid flow - ductile faults create plastically deformed rocks

Discuss the processes by which mountain belts form in convergent margins, in continental collisions, and in continental rifts.

- convergent margins: subduction-related volcanic arcs grow on overriding plate, accretionary prisms grow upward, compression shortens and uplifts overriding plate, and a fold-thrust belt develop landward of the orogen - continental collisions: Creates a welt of crustal thickening due to thrust faulting and flow folding. Fold-thrust belts extend outward on either side, but the high mountains may eventually collapse - continental rifts: Normal faulting creates fault-block mountains and basins, decompressional melting adds volcanic mountains, and increased heat flow expands and uplifts rocks

What basic layers of the Earth were recognized before the use of seismology?

- crust, mantle, and core

Explain the nature of cyclones and anticyclones, and note their relationship to high-pressure and low-pressure air masses. What is an extratropical (mid-latitude) cyclone?

- cyclones: cyclonic flow: A circulation of air around a high-pressure region in the atmosphere; it rotates clockwise in the northern hemisphere - anticyclones: anticyclonic flow: A circulation of air around a high-pressure region in the atmosphere; it rotates clockwise in the northern hemisphere. - extratropical cyclone: A large, rotating storm system, in mid-latitudes, associated with a regional-scale low-pressure zone.

Describe elastic-rebound theory and the concept of stick-slip behavior.

- elastic-rebound theory: the concept that earthquakes happen because stress builds up, causing rock adjacent to a fault to bed elastically until breaking and slip on a fault occurs; the slip relaxes the elastic bending and decreases stress - stick-slip behavior: stop-start movement along a fault plane caused by friction, which prevents movement until stress builds sufficiently

What is an estuary? What is the difference between an estuary and a fjord?

- estuary: seawater and river water meet they are delicate because oceanic and fluvial water combine to make nutrient rick brackish water with salinity between oceans and rivers. they inhabit unique species of shrimp, clams, oysters, worms and fish that can tolerate large changes in salinity - fjord: flooded glacial valleys

How is a fault different from a joint?

- fault: A fracture on which one body of rock slides past another - joint: Naturally formed cracks in rocks

Why might the Earth's magnetic field exist? What causes magnetic anomalies?

- flow of metal within outer core can produce an electric current - caused by non-dipolar field and magnetization of rocks in the crust

Explain what the geoid is and why it is so bumpy. What is gravity anomaly?

- geoid: hypothetical shape of the earth, coinciding with mean sea level and its imagined extension under or over land area - the large and small variations to the size, shape, and mass distribution of the earth cause slight variations in the acceleration of gravity - gravity anomalies: the difference between the observed acceleration of gravity on a planet's surface and the corresponding value predicted from a model of the planet's gravity field - positive anomaly: stronger gravitational pull - negative anomaly: weaker gravitational pull

Describe the types of damage caused by earthquakes.

- ground shaking and displacement: buildings and bridges sway, twist back and forth, or lurch up and down depending on the wave motion. Aftershocks worsen the problem because they may topple already weakened buildings. Seiche (water's rhythmic movement) capsize small boats - landslides: the tumbling and flow of soil and rock downslope - sediment liquefaction: liquefaction in beds of wet sand or silt happens because ground shaking causes the sediment grains to try and settle together, but because the spaces between grains are filled with water, water pressure in the pores increases and pushes the grains apart, and the wet silt or sand becomes a slurry (quicksand) - fires: the shaking during earthquake can tip over lamps, stoves, or candles with open flames, and it may break wires or topple power lines, generating sparks

Discuss the different types of coastal wetlands. Describe the different kinds of reefs and how a reef surrounding an oceanic island changes with time.

- in temperate climates- swamps(wetlands dominated by trees), marshes(wetlands dominated by grasses) and bogs(wetlands dominated by moss and shrubs) tropical or semitropical-mangrove swamps reefs - fringing reef: forms directly along coast - barrier reef: develops offshore(separated from coast by lagoon) - atoll: makes circular ring surrounding a lagoon eventually reef itself sinks too far below sea level to remain alive and it becomes the cap of a guyot

Describe the principle of isostasy and how it affects the elevation of a mountain range.

- isostasy: the condition that exists when the buoyancy that force pushing lithosphere up equals the gravitational force pulling lithosphere down - In places where a mountain consists of particularly dense rock but is small enough to be held up by the strength of the lithosphere, there may be an excess of mass and therefore a positive gravity anomaly.

Where does the salt in the ocean come from? How does the salinity in the ocean vary? How does the temperature of the oceans vary?

- most cations in sea salt come from general chemical weathering of rocks and the anions come from volcanic gases. rivers deliver over 2.5 billion tons of salt to the sea every year. salinity varies with location. it reflects the balance between the addition of freshwater by rivers or rain and the removal of freshwater by evaporation. it may also depend on water T bc warmer water can hold more salt general correlation of average T with latitude exists bc the intensity of solar radiation varies with latitude

Compare normal, reverse, and strike-slip faults.

- normal fault: two blocks of rock are pulled apart by tension - reverse fault: two blocks that push into each other - strike-slip fault: one block of crust slides literally past each other (most common cause)

Compare the motion of normal, reverse, and strike-slip faults.

- normal: a fault in which the hanging-wall block moves down the slope of the fault - reverse: a steeply dipping fault on which the hanging-wall block slides up - strike-slip: a fault in which the block on the opposite fault plane from a fixed spot moves to the right of that spot

Contrast numerical age with relative age

- numerical age: "absolute age" The age of a geologic feature given in years - relative age: The age of one geologic feature with respect to another

What is the age of the oldest rocks on Earth? What is the current estimate of the numerical age of the Earth? Why is there a difference?

- oldest rocks on Earth's surface: 4.03 GA - the age of the Earth is about 4.54 Ga - there is a difference because of the rock cycle and of meteorites and moon rocks

Why do prevailing winds develop at the Earth's surface? Why do the jet streams form?

- prevailing winds: surface winds that generally flow in the same direction for long time periods - A region's prevailing and dominant winds are often affected by global patterns of movement in the Earth's atmosphere. - jet streams: a fast-moving current of air that flows at high elevations - In two special places, over the polar front and over the horse latitudes, air masses of very different temperatures come in contact. This temperature difference causes a step at the top of the troposphere. Along these steps, high-altitude westerlies flow particularly fast—at speeds of between 200 and 400 km per hour—producing the jet streams

What is the difference between refraction and reflection of waves?

- refraction: the bending of a ray as it passes through a boundary between two different materials : the angle depends on the contrast in wave velocity between the two materials in contact at the boundary and on the angle at which a wave hits the interface - reflection: ray bounces off the material : the angle at which a reflected wave bounces off a boundary is always that same as the angle at which the incoming, or incident, wave strikes the surface

What is the relative humidity of the atmosphere? What is the latent heat of condensation, and what is its relevance to the evolution of a thunderstorm or hurricane?

- relative humidity: the ratio between the measured water content of air and the maximum possible amount of water the air can hold at a given condition - latent heat of condensation: the energy released when water vapor condenses into water droplets - The process is most readily observed in atmospheric clouds in thunderstorms. A storm maintains its intensity as thunderclouds release this latent heat

Why do changes in atmospheric temperature depend on latitude and the seasons?

- seasons result from the 23.5 degrees tilt of Earth's axis of rotation - The hemisphere pointing toward the Sun experiences summer - The hemisphere pointing away from Sun experiences winter

How does beach sand migrate as a result of longshore drift? Explain the sediment budget of a coast.

- sediment follows a sawtooth pattern of movement that results in gradual net transport of beach sediment parallel to the beach(longshore drift) - Sediment budget refers to the balance between sediment added to and removed from the coastal system

Why can't we date sedimentary rocks directly? How do we assign numerical ages to intervals on the geologic column, to produce a geologic time scale?

- sedimentary rocks are made up of different sediments that have varied dates - a detrital rock is composed of fragments from other rocks, and you don't get the date of sedimentation

What factors determine whether a rock will behave brittlely or plastically?

- stress causes deformation to take place

How are stress and strain different?

- stress: The push, pull, or shear that a material feels when subjected to a force; formally, the force applied per unit area over which the force acts - strain: The change in shape of an object in response to deformation (i.e., as a result of the application of a stress)

What factors control the direction of the surface currents in the ocean? What is the Coriolis effect, and how does it affect oceanic circulation? Explain thermohaline circulation.

- surface currents result from interaction between the sea surface and the wind and the rotation of the Earth. - Coriolis effect: causes surface currents in the northern hemisphere to veer toward the R and surface currents in the southern hemisphere to veer toward the left of the average wind direction. it's caused by earth's rotation thermohaline circulation: rising and sinking of water driven by density contrasts caused by differences in T and salinity

How do plate tectonics, sea-level changes, sediment supply, and climate change affect the shape of a coastline? Explain the difference between emergent and submergent coasts. What is the difference between an erosional and depositional coast?

- tectonic setting of a coast plays a role in determining whether the coast has steep sided mtn slopes or a broad plain that borders the sea(active margin creates mtns and passive margin leads to broad coastal plain) - emergent coast: land is rising and terraces form along some coasts - submergent: land sinks relative to sea level causes estuaries and fjords - erosional vs accretionary coasts - warmer climate means weathering happens less quickly

How are long-term and short-term earthquake predictions made? What is the basis for determining a recurrence interval, and what does a recurrence interval mean?

- they can be predicted in the long term, but cannot be predicted in the short term - long term: determination of seismic zones by: mapping historical epicenters and evidence of modern or ancient earthquakes - recurrence interval: historical records and geologic evidence - requires radiometric dating of events - sand volcanoes, offset strata, and drowned forests - short term: no reliable predictions are possible

Describe how waves affect a rocky coast and how such coasts evolve.

- they don't have a beach so a breaker can pick up boulders and smash them together until they shatter and it can squeeze air into cracks making enough force to widen them. the shattering, wedging, and abrading(wave erosion) gradually undercut a cliff face and the cliff retreats if it becomes unstable enough and breaks away

What causes the tides? Why do the range and reach of tides vary with location?

- tides are caused by the tide-generating force-due partly to the gravitational attraction of the Sun and Moon and partly to centrifugal force caused by the revolution of the Earth-Moon system around its center of mass. forces causing tides: gravitational pull, centrifugal force, earth-moon system, center of mass - range and tide reach varies bc of tilt of Earth's axis, the moon's orbit, the sun's gravity, and the focusing effect of bays, basin shape, and air pressure

What conditions lead to the formation of a tornado? Where do most tornadoes appear?

- tornado: A near-vertical, funnel-shaped cloud in which air rotates extremely rapidly around the axis of the funnel. - Tornadoes develop along steep P gradients. Strong west winds aloft carry polar air. Strong southeast surface winds carry warm moist air. Strong shear initiates horizontal rotation. Drafts tip the rotating cylinder upright. - Tornadoes are common in the midwestern United States. Favorable conditions exist from March to September.

Describe the atmosphere's structure from base to top. What characteristics define the boundaries between layers?

- troposphere: the mixing layer; all weather occurs here; thinner at poles; thicker at equator; Temp decreases to -55^o C - tropopause: marked by T stabilization - stratosphere: does not convect; T stays the same for 10km; then T warms to 0^o C - stratosphere - mesosphere: T decreases to -85^o C; meteors start to burn - mesopause: marked by T shift - Thermosphere: Outermost layer; sparse gas concentrations - ionosphere: most of the mesosphere; lower part of thermosphere; creates aurorae borealis and aurorae australis

Describe the principle that allows us to determine the relative ages of geologic events

- uniformitarianism: processes observed today were the same in the past

Explain how the vertical and horizontal components of an earthquake motion are detected on a seismometer.

- vertical: records up and down movement using a pen, weight, spring, and rotating drum - horizontal: records back and forth movement using a pen, weight, spring, and rotating drum

Describe the typical topography of a passive continental margin, from the shoreline to the abyssal plain. How does the lithosphere beneath a passive margin differ from that beneath an abyssal plain?

- when rifting stops and sea-floor spreading begins, the stretched lithosphere at the boundary gradually cools and sinks. sand and mud that washed of the continent, along with the shells of marine creatures that grow on the sea floor or settle bury the sinking crust, making a pile of sediment up to 20 km thick. the flat surface of the pile it the continental shelf.

Describe the motion of water molecules in a wave. How does wave refraction cause longshore currents?

- within a deep ocean wave, water molecules follow a circular path and the radius of the circle decreases with depth. beneath the wave base, water modules are not affected by the wave. - wave refraction occurs when waves approach the shore at an angle. if the wave reaches the beach at an angle, it causes a longshore current and beach drift of sand

What is a tsunami, and why does it form?

-A tsunami is a giant wave traveling at speeds of about 800 km/hr (500mph). They are formed by the rise of the sea floor shoving up overlying water. Then the water from above the upthrust sea floor moves outward from above the fault zone.

What processes are involved in sculpting the rugged topography of mountain belts? How can rocks from depth beneath a mountain range be exhumed?

-Erosion: The formation of the rugged topography (deep valleys, sharp peaks) characteristic of mountain belts is due to the sculpting of the landscape by rivers and glaciers.

What types of structure are most prone to collapse in an earthquake? What types are most resistant to collapse?

-Structures built on land underlain by weak mud that could liquefy are prone to collapse in an earthquake. Also structures built on top of, on, or at the base of steep escarpments could fail and produce landslides. Also, buildings downstream of dams are dangerous because they could crack and collapse causing a flood. Buildings that are resistant to collapse include ones that are wider at the base and that have cross beams added for strength. Wrapping a bridge's support columns in cable and bolting the span to the columns will prevent the bridge from collapsing so easily. Placing buildings on rollers or shock absorbers lessens the severity of the vibrations.

How does seismicity on mid-ocean ridges compare with seismicity at convergent or transform boundaries? Do all earthquakes occur at plate boundaries?

At a divergent plate boundary (mid-ocean ridges) two oceanic plates form and move apart. Divergent boundaries are segmented and spreading segments are linked by transform faults. Therefore, two kinds of faults develop at divergent boundaries. Along spreading segments , stretching generates normal faults, whereas along the transform fault that link spreading segments, strike slip faulting occurs. Seismicity along mid ocean ridges takes place at shallow depths.

What are the four types of seismic waves? Which are body waves, and which are surface waves?

Body Waves: - P-waves: compression and expanding wave; fastest wave; parallel to propagation : cause the ground to go up and down - S-waves: moving material back and forth; movement is perpendicular to direction of propagation; only travels through solids : cause the ground to go back and forth Surface Waves: - R-waves: P-waves that intersect the land surface; cause ground to ripple up and down like water : make the ground surface roll in wave-like motions - L-waves: S-waves that intersect land surface : undulate the ground laterally