EARTH SCIENCES - Notable Research and Discoveries Part 2 potx

Shear or transverse waves consist of up-and-down motions perpen-dicular to the wave’s propagation... For in-stance, the United States Geological Survey USGS, an agency devoted which ar

Despite seemingly permanent features, such as Mount Rushmore in

South Dakota, Earth is constantly, albeit slowly, changing (William Walsh/

iStockphoto)

earth ScienceS



Disturbances can propagate in several different ways A transverse wave propagates in a direction perpendicular (at a 90 degree angle) to

(opposite page) Compression waves consist of contractions and

expan-sions in the same direction (longitudinally) as the propagation of the

wave Shear or transverse waves consist of up-and-down motions

perpen-dicular to the wave’s propagation.

Seismic recording equipment, part of the Earthquake Arrival Recording

Seismic System (EARSS) in New Zealand (New Zealand © GNS Science/SSPL/

The Image)

disturbance’s origin, which is called the earthquake’s focus For

in-stance, the United States Geological Survey (USGS), an agency devoted

which arrive at the sensor stations first and are called P

waves or pri-mary waves P waves travel through rock at an average speed of about

13,000 MPH (20,800 km/hr) and through water and air at about the

same speed as sound Secondary waves or S waves are shear waves that

propagate at a little more than half the speed of P waves Because S

waves are shear waves, they cannot propagate through liquids Other

types of waves are involved in earthquakes but are less important for

studying Earth’s interior

In 1935 the California Institute of Technology researcher Charles Richter (1900–85) established a scale to measure the intensity of earth-

quakes The Richter scale, which is still sometimes used, calculates the

magnitude of an earthquake based on seismic wave amplitude—the

eploring earth’s Depths

Land surveys to delineate boundaries and establish maps have

always been an important function of governments After the

United States won its independence in the Revolutionary War,

the government established a Surveyor General in 1796 and

tasked this offi ce with surveying western territories Much of

this land was sold or granted to the public, but the

disposi-tion of mineral lands—areas rich in natural

resources—gener-ated a lot of debate as to who got what and where The

sci-ence of geology was in its infancy at the time, so people had

trouble determining where the natural resources were buried

But as the science grew and developed, geologists became

more effective at locating resources, and on March 3, 1879,

President Rutherford Hayes signed a bill establishing a new

agency, the United States Geological Survey (USGS) The job

of this agency was to classify lands according to their

geologi-cal properties and mineral resources.

USGS’s responsibilities have grown tremendously since its establishment Although fi nding minerals and natural resources

liquid, so the existence of a liquid center inside the planet would explain

why seismometers fail to record shear waves on the other side of the

remains a valuable service, geologists have expanded their knowledge and expertise into all aspects of Earth science, environmental issues, and biological phenomena USGS em- ploys 10,000 researchers and support staff to study and un- derstand the planet and its resources, to reduce the danger and negative effects of natural disasters such as earthquakes and landslides, and to manage natural and environmental resources.

Among the agency’s many projects are Priority systems Science, which supports the management of eco- systems that are of concern and value to society and is currently studying Florida’s Everglades, San Francisco Bay, the Mojave Desert, the Platte River, and the Chesapeake Bay USGS also maintains the Earthquake Hazards Program and the Advanced National Seismic System, which monitors about 20,000 earthquakes occurring in the United States each year (Most are too small to be felt, but are important

Eco-indicators of stress and strain at various locations.) Other

programs involve energy resources, coastal and marine ogy, habitats, water resources, fisheries, volcano hazards, and remote sensing with satellites.

geol-eploring earth’s Depths

ing from the continental surface to an average depth of about

(opposite page) (1) Boundaries between the layers of Earth’s interior

bends or refracts P waves, causing shifts in speed and altered paths that

leave “shadows”—areas that receive few or no waves.

(2) S waves fail to penetrate the liquid outer core, leaving a large shadow

on the other side of the earthquake’s origin.

eploring earth’s Depths

which results in slower progress and higher costs Temperature also

Project Mohole—an ambitious

attempt to Reach Earth’s Mantle

Project Mohole was an attempt to drill a hole to the mantle

and retrieve a sample from this great frontier—a frontier

separated by vast quantities of hard rock Suggested in

1957 by Walter Munk, a member of the U.S National

Acad-emy of Sciences, the project got funds for preliminary work

in 1958 from the National Science Foundation (NSF), one of

the main government agencies that supports basic scientifi c

research A sample from the mantle would provide a large

amount of information on the exact composition of this layer,

its age, and internal dynamics The question of mantle

dy-namics was particularly important during this time period,

as continental drift was being hotly debated.

The thickness of Earth’s crust varies widely, and the nest section is beneath the ocean In some areas of the sea-

thin-fl oor, the crust is only about three miles (4.8 km) thick,

al-though the average is considerably more The plan of Project

Mohole consisted of three phases, the fi rst of which was an

experimental program to develop techniques to drill through

deep water and into the crust Drilling for oil in the

rela-tively shallow areas of the sea is common, but Mohole

scien-tists needed to drill in deeper parts of the oceans, in places

where the crust is thinner In the fi rst phase of the project,

beginning in early 1961, researchers drilled in 11,700 feet

(3,570 m) of water off Guadalupe, Mexico The platform was

depth at which rocks begin to change phase, or state Rocks change

a ship named CUSS I, a converted naval barge (The ship’s

name came from the initial letters of the names of oil panies that had outfitted the ship—Continental, Union, Shell, and Superior.) Researchers drilled a series of holes, one of which extended into the ocean crust to a depth of 557 feet (170 m) Although this does not seem very far, the project became the first to drill successfully in deep water.

com-Phase two never got started Cost estimates ballooned from $5 million to nearly $70 million Although Phase one had succeeded, the project called for drilling through even deeper water and farther into the crust below, but no one was able to think of a cost-effective means of doing this

Project Mohole lost its funding in 1966 amid arguments about how the project should proceed and whether it was worth the money (Another budget problem faced by Project Mohole was the existence of an even bigger and more ex- pensive project that was competing for funds at the same time—the Apollo Moon landings.)

The project’s failure was an embarrassment to the NSF, since the promising beginning had crumbled so quickly A journalist Daniel S Greenberg wrote a series of articles

on the project in 1964 for Science magazine, and, as he

watched the plan disintegrate, he wrote, “The Mohole ness is a very sorry episode .” Yet Project Mohole was not a complete failure, and geologists were able to identify

busi-a second sublbusi-ayer of crust, consisting of rock cbusi-alled bbusi-asbusi-alt, from the samples obtained at 557 feet (170 m) in the ocean crust.

eploring earth’s Depths

surement, geologists can only estimate the core’s temperature based on

ies, and the depth of the plates includes the crust plus a little bit of the

upper part of the mantle The crust and uppermost mantle composes

eploring earth’s Depths

motions—the study of which may help explain the underlying

haves in many ways as a gigantic magnet, with the north pole of the

magnet somewhat close to the North Pole (which is located along the

planet’s rotational axis), and similarly for the south pole This field

eploring earth’s Depths

waves traveling, say, perpendicular to it But iron tends to become

isotropic—without orientation—at high temperature and pressure

gested that iron in the core adopts a certain crystal pattern called