Earthquakes and Earth's Interior

shadow zone

Shortly after the discovery of the Moho, German seismologist Beno Gutenberg discovered the *second major boundary of Earth's interior, the one between the mantle and the core.* This boundary was not detected by an abrupt change in the arrival times of seismic waves but instead was revealed by the almost complete failure of the waves to arrive. *For any earthquake that is of sufficient magnitude to be detected at recording stations many thousands of kilometers from the epicenter, there is always a belt encircling the globe in which neither P waves nor S waves are detected*. This belt is known as the _____ ________, the edge of which is located about 11,500 kilometers (7000 miles) or 103 degrees around the globe from the epicenter of an earthquake

mantle; 83; seismic wave; mineral behavior

The _________ is the largest of Earth's internal subdivisions and comprises __ percent of Earth's volume. The _________ gets its name because it surrounds, or " _________s," the core. _________ samples are rarely brought to the surface, so most of our knowledge about its composition is based on variation in _________ _________ velocities and experimental studies of _________ _________ at very high pressures and temperatures. *_________ is generally assumed to be largely solid, chemically similar throughout, and made up of ultramafic rocks full of iron-and magnesium-rich silicate minerals.*

low-velocity zone

The _________-_________ _______ occurs within the upper mantle between the depths of 100 and 250 kilometers (60 to 150 miles) and is associated with a marked decrease in seismic wave velocities. The effects are more pronounced for S waves than for P waves and are best explained by the presence of a small amount of molten material in the zone. The presence of such melt is likely at this depth because it is in this region that the mantle comes closest to its melting point Because we know that S waves penetrate the _________-_________ _______ , the amount of melt must be small. In fact as little as one per- cent liquid would produce the observed effects. Although detected beneath both the oceans and continents, the _________-_________ _______ is not entirely continuous.

5.52

The average density of Earth is ________ g/cm^3. These calculations show that this difference in density cannot be due only to increasing pressure with depth, implying that the interior of Earth must be vastly different from its exterior.

2.7; Water

The average density of continental rocks is about _____ g/cm^3, that is, an average continental rock weighs about _____ times as much as the same volume of _________.

Moho (Mohorovičić Discontinuity)

The boundary, which separates rocks of the continental crust and oceanic crust from those of the underlying mantle, is formally named the _____________ _________, in honor of its discoverer, but it is *better known as simply the _____*

changing; less; lowermost; inward; inward (again)

The core must act on P waves in much the same way that a lens focuses light by ________ the path of light rays. Because the liquid core is ______ rigid than the __________ mantle, the P waves are bent _______ as they enter the core and bent ________ as they exit the core

outer core

The liquid portion of the core is the _____ _____. The presence of the _____ _____ entirely prevents S waves from penetrating the core-mantle boundary, thereby accounting for the absence of S waves within the shadow zone and beyond it

compositional convection

The mechanism for the circulation in the outer core, which powers the dynamo that generates Earth's magnetic field, is _________ _____________. Relies, like thermal convection, on light material rising while heavy material sinks. In _________ _____________, however, *the contrast in density is due to differences in composition rather than differences in temperature.* In the outer core, circulation is thought to be driven by the buoyancy of the less dense material left when iron solidifies and sinks to the surface of the inner core. This *links convection in the outer core to the growth of the inner core.*

surface waves

The seismic waves that travel along Earth's surface are ______ ________. These cause most of the damage associated with earthquakes, *producing low-frequency vibrations that roll the ground like ocean waves or whip it sideways. Compared with other seismic waves, surface waves travel slowly and take longer to diminish.* ________ _________are of limited use in probing Earth because they only travel along the surface and do not pene- trate the interior.

seismology

The study of earthquake waves. Because the speed with which seismic waves travel depends upon the properties of the material through which they pass, the pattern created by seismic waves as they travel through Earth's interior provides insight into the nature of the materials they encounter at depth. *Therefore the study of this helps us understand earth's interior*.

focus

The vibrations of an earthquake begin at the _______—the point where a locked fault suddenly lets go. *The ______ of an earthquake generally lies at some depth below Earth's surface.* From the _______, seismic waves spread outward in all directions in much the same way that ripples radiate when a pebble is thrown into a pond. The waves soon ripple through Earth's interior, literally filling Earth in the same way that sound waves fill a ringing bell.

seismic rays

To simplify matters, seismic waves are often described as _______ ______ rather than waves, in the same way that the illumination from a light source is described as rays of light.

phase change; increase; chemical composition

Under the intense pressures of Earth's interior, minerals are forced to adopt new crystal structures in which their component atoms are more tightly packed, a process known as _______ _______. These new structures __________ the density of the rocks concerned (and therefore the velocity of seismic waves passing through them) but do not affect their overall ________ __________.

logarithmic; tenfold; 10; 10,000

Unlike most of the scales with which we are familiar, the Richter scale is ____________, which means that each whole number increase measures a ________ increase in ground motion. This means that ground motion during an earthquake of magnitude 8 is __ times greater than an earthquake of a magnitude 7 and ________ times greater than one of magnitude 4.

ray path

We refer to the line of travel of an earthquake wave as a _____ _____. An analogy to the distinction between seismic waves and their _____ _____ would be the movement of the surf (or waves) can be described by the direction (or path) the surfers are traveling in.

Read

When portrayed in map view (Fig. 11.21a), tomographic results show a strong relationship between the velocity of rays in the mantle and the present distribution of litho- spheric plates to a depth of 100 kilometers (60 miles). Thus hot areas where seismic-wave velocities are low (in red) generally coincide with spreading ridges, whereas cold areas of high seismic velocity (in blue) occur beneath the oldest parts of the continents. This implies that, at least in the upper 100 kilometers of the mantle, the pattern of con- vection is relatively straightforward. (put read)

seismic waves

When the strain stored in the rocks on either side of a fault reaches a threshold, the rocks abruptly rupture and jerk violently past each other. The energy released sends out shock waves, known as _________ __________, in all directions for periods that can last for several minutes. This is found in the stick-slip motion of faults.

32

With each whole number increase on the Richter scale the amount of energy released by an earthquake increases by a factor of __.

seismogram

a weighted pendulum attached to a pen that produces a wiggly line on a revolving paper-covered drum whenever movement of the ground shakes its frame.

stony meteorites

are made of rocky material that closely resembles the peridotite of Earth's mantle.

iron meteorites

are metallic and mainly composed of iron and nickel. These are thought to re- semble the material of Earth's core.

thermal convection

geologists believe circulation of Earth's mantle is driven by ___________ ____________. During ___________ ____________, warm, less dense material rises while cool, denser material sinks. This process is far more efficient than conduction as a means of transporting heat because the material itself moves. In this way, hot material is transferred from the interior toward the exterior of the planet, and the heat produced in the interior travels to the surface where it can be radi ated away into space. Because the process also encourages the segregation of material according to their densities, it has also played a major role in developing Earth's internal layering.

epicenter

is the point on Earth's surface directly above the focus.

seismometer

is typically mounted directly on bedrock, and is often buried to avoid the noise of normal day-to-day activities. The instrument contains a weighted pendulum attached to a pen that produces a wiggly line (known as a seismogram) on a revolving paper-covered drum whenever movement of the ground shakes its frame. Depending on the way the instrument is set up, __________ can be used to detect either horizontal or vertical motion

plate tectonics; deep interior

it appears that the core-mantle boundary may be the graveyard of subducted slabs, potentially establishing a direct link between ______ __________ and the ______ __________ of Earth.

oldest; coldest; densest; younger; buoyant

velocity patterns beneath some subduction zones suggest that not all descending plates penetrate the transition zone between the upper and lower mantle. Some plates appear to be deflected by it instead. Perhaps only the ________, ________, and therefore ________ oceanic plates are heavy enough to push their way through the transition zone, whereas ________, more ________ subducting plates are not, and so are deflected by it. . It is even possible that subducting slabs may collect above the transition zone until they become heavy enough to avalanche to the core-mantle boundary. (put read)

lower; highly

Based on the available evidence, the ______ crust is thought to be broadly dioritic in composition and ______ metamorphosed due to elevated temperatures and pressures.

crust; greatest

The _____ is not only the thinnest of Earth's many layers but it also shows the _______ variety in its composition. This variety results from the dynamic processes that occur at or near Earth's surface.

Mantle Plumes

Are believed to arise from a thermally unstable layer at the core-mantle bound- ary and rise upward through the entire mantle until they reach the base of the lithosphere. Here they spread out sideways, carrying the lithospheric plates away from the sites of upwelling as they do so. The more diffuse return flow is thought, once again, to be focused at the sites of subduction, where cold, dense oceanic lithosphere descends into the mantle.

body waves

Are not confined to Earth's surface, but instead travel through Earth's interior. Much of our knowledge of earth's interior is derived by these. Consists of primary waves, and secondary waves.

S waves; liquid; (liquid core)

Because _ _____ fail to reappear, at least a portion of Earth's core is interpreted to be _______ (Recall these cannot transfer through gases or liquids). All strong earthquakes, no matter where their epicenters are located, produce a similar shadow zone, which means that the ________ ________ is encountered everywhere at the same depth (29,000 km)

inner core

Because the shadow is not totally dark, geologists suspected this ______ _______ to be solid, to explain the previously undetected presence of faint P waves within the shadow zone that could only have been reflected from a discontinuity within the core. *The faster speed of P waves within the ______ ______ (consistent with its solid nature) causes their ray paths to be bent into the shadow zone because this time they accelerate on entry and therefore bend outward. The diameter of this ______ ______ is now known to be 2540 kilometers (1580 miles), but its exact size was not determined*

increases; increases; (increase); (faster); refracted ray paths

In reality, the speed of earthquake waves ________ with depth because the density of the materials they encounter ___________ as the pressure rises in Earth's deep interior. Because of this density _______, seismic waves move ______ at depth than they do near the surface. This causes seismic waves to adopt curved, or _________ ______ _______ bending up toward the surface as they travel downward into regions of faster velocity

geophysical methods

In the absence of direct observations, Earth scientists use ____________ _________, or the application of physical laws, to probe Earth's interior. These methods give us a good first approximation of the composition of Earth from core to surface.

crust; core; mantle

In this way, it was revealed that Earth is a layered planet with a very thin outer layer or _______, a massive metallic center or ______, and a thick intervening rocky layer or ________.

Density

Is a measure of how closely compacted a material is, and it is *one of the chief factors controlling the speed with which earthquake waves travel through Earth's interior.*

Seismic tomography

Is a relatively new technique that promises to improve our knowledge of the structure of the mantle in a dramatic fashion. Creates three-dimensional images of Earth's interior by combining seismic data from large numbers of earthquakes. As earthquakes occur in many places on Earth's surface, information can be synthesized from many sources, mimicking the CAT scan effect of the rotating X-ray beam. Because of its relatively uniform composition and structure, variations of seismic velocity in the mantle are probably due to temperature differences that are, in turn, related to mantle convection.

Richter magnitude (Scale)

Is calculated from the height or amplitude of the largest seismic wave recorded on the seismogram, and the distance of the seismic station from the epicenter as given by the time interval between the arrival of the first P and S waves.

seismic discontinuities

Known as ________ ___________, these abrupt velocity changes indicated the presence of distinct layers of material of differing composition or physical properties.

40 kilometers (25 miles)

Laboratory experiments tell us that most of the minerals present in *surface rocks* would not be stable at depths any greater than about _________ ___________ below the surface.

creep

Like when a large ball of silly putty slowly flattens itself into a disk. Movement is slow and occurs while the material is still solid. This kind of movement is known as solid-state flow or _______. *The ability of materials to ______ is greatly increased at elevated temperatures, especially when temperatures near the melting point of the material involved*. Near metling points make rocks soft and yielding, thus capable of flow.

dioritic; basaltic; ultramafic

Moho delineates an abrupt boundary separating lower crustal rocks of ______ to ______ composition from the ________ rocks of the uppermost mantle below.

whole mantle convection

Often considered the simplest mantle convection model. In this model several large convection cells extend through the entire mantle from the core-mantle boundary to the base of the lithosphere, including both the upper and lower mantle layers. *To link this motion to plate movement, warm, buoyant mantle material rises beneath the mid-ocean ridges, then spreads laterally beneath the lithosphere, which it carries in a conveyor-belt fashion. Eventually, the material cools and, beneath sites of subduction, sinks back into the mantle and is reheated.*

Primary (P) waves

One of the two types of body waves. Are generated by sudden compression or extension of the ground at the site of an earthquake, like a push or pull on the end of a stretched spring. Like sound waves, these temporarily affect the volume of the material they pass through by alternately compressing and expanding this material in the direction in which they travel. Travel through solids, liquids, and gases because all three materials react to the wave by resisting a change in volume when they are compressed. *Travel most quickly through dense rigid rocks*. *Are elastic. That is, both require the material through which they are traveling to re- bound when the wave has passed through, just as a compressed spring rebounds when it is let go.*

Secondary (S) waves

One of the two types of body waves. Are generated by the shearing or sliding motion at an earthquake site. Are therefore shear waves and behave like oscillations in a rope. Cause the material in their path to heave vertically or sideways, perpendicular to the direction in which they are traveling. This motion affects the shape of the material through which the oscillation travels. *Only travels through solids as they have definite shapes. Blocked by liquids and gases*. *Are elastic. That is, both require the material through which they are traveling to rebound when the wave has passed through, just as a compressed spring rebounds when it is let go.*

transition zone; upper mantle; lower mantle; transition zone

Other discontinuities discovered within Earth's mantle are thought to record the transformation of upper-mantle minerals to mineral structures that are stable at higher pressures. Increases in seismic wave velocities indicate that such transformations occur from 400 to 700 kilometers (250 to 435 miles) in depth. This interval, known as the __________ ______, separates the __________ ______ from the __________ ______. Unlike the abrupt compositional boundary at the Moho, the discontinuities of the __________ ______ are not sharp. Instead, they range over a significant depth interval. *For this reason, the discontinuities are generally attributed to changes in the crystal structure of the minerals present rather than to changes in the chemistry of the mantle rocks.*

deeper; greater; higher; shallower

P waves that followed the _________ ray paths to more distant stations traveled at _______ speeds and so must have passed through rocks of _______ density than the rocks encountered along the ________ ray paths to nearby stations

7; first; slower; 4.5; after

P waves travel through crustal rocks at speeds up to __ kilometers (4 miles) per second and are the ______ to arrive at seismographic stations. S waves are _______ than P waves, traveling through the crustal rocks at speeds up to ____ kilometers (3 miles) per second. As a result, the characteristic signals of S waves are recorded by seismometers _____ those of P waves

layered mantle convection

Proposes that more vigorous circulation occurs in the upper mantle because the mantle rocks are closest to their melting points. Proponents of this model argue that the upper mantle is probably compositionally distinct from the lower mantle because it is the source of mid-ocean ridge basalts. The upper mantle has therefore had basalt melt extracted from it, unlike the lower mantle, which is probably closer to its original composition. *In this model, more vigorous convection in the upper mantle is thought to occur independently of the slower convection in the more rigid lower mantle so that the convective systems are decoupled across the transition zone. Convective circulation in the upper mantle is thought to have had a direct bearing on plate tectonics in that upwelling occurs beneath mid-ocean ridges. Downwelling, on the other hand, occurs where cold oceanic lithosphere descends in subduction zones.*

elastic rebound theory

Seismic Waves' motion and cause form an explanation for the origin of earthquakes known as the ________ _______ ________, which lies at the heart of ongoing research into earthquake prediction

mantle plumes

the descent of subduction slabs demands a compensating return flow, like the water that rushes up when a diver enters the water. The pattern of this flow, however, remains ambiguous. The main candidate is that of _______ ________, which many geophysicists believe to ascend from the core-mantle