EARTH SCIENCES - Notable Research and Discoveries

EARTH SCIENCES - Notable Research and Discoveries

been developed But any use of magnetometers to measure Earth’s

In 1912 the German scientist Alfred Wegener (1880–1930) made his astonishing proposal that continents drift Skepti- cal geologists could not imagine how something as large and heavy as a continent could “set sail,” but in the 1950s researchers discovered long, ocean-girdling ridges A ridge

is a range of hills or hilly terrain, but the ocean ridges also contained a crack—a deep valley or rift In the 1960s the researchers Harry Hess (1906–69) at Princeton University and Robert Dietz (1914–95) at the Scripps Institution of Oceanography suggested that these rifts form when rigid crustal plates separate Hot magma from below rises up to create a new section of the seafl oor.

Seafl oor spreading was a critical factor in the ment of plate tectonic theory Plate boundaries form where plates meet In some cases one plate slides past or dips be- neath another plate, but in other cases two plates separate, or diverge Divergence is what occurs at mid-ocean ridges, which are plate boundaries in which two plates slowly move apart.

establish-For example, the Mid-Atlantic Ridge runs along the middle

of the Atlantic Ocean, created by the separation of the North American and South American Plates from the African and Eurasian Plates These plates separate by about 0.4 inches (1 cm) every year (Part of the valley cuts through Iceland and is aboveground!) This region is a geologically active site;

nearly all earthquakes in the Atlantic Ocean occur at or near the Mid-Atlantic Ridge Plenty of young crust is available for inspection as well.

As geologists obtained samples from the seafl oor from the mid-ocean ridges, they found the ages of the rocks cor- responded to the movement of the plates—the newest and youngest rocks are at the rift center, where the plates are presently diverging, with a progressively increasing age far- ther away from the rift, at sites that formed longer ago.

orientations During some periods of time in Earth’s history, the mag-The spreading seafloor contains a record of pole reversals—as the

sea-floor spreads, rocks having one or the other magnetic orientation form

bands or chrons The most recent chrons, from oldest to newest, are

Gilbert, Gauss, Matuyama, and Brunhes (all named for scientists who

made important contributions to the study of geomagnetism).

Magnetic pole reversals are also evident in the Sun and its enor-This regular cycle is associated with other phenomena such as

sun-spots—areas of lower temperature and greater magnetic activity—but

astronomers do not fully understand these processes

On Earth magnetic reversals are much less predictable The “tape recorder” written on rocks stretches back millions of years, and major

in the inner core But the important question of why these reversals oc-cur at all remains unanswered

cesses, models and computer simulations are important tools to investi-

magnetic field strength is probably one of the hallmarks of a pole re-versal, which leads many people to wonder if Earth may be in the midst

also be affected, given the roles that the magnetosphere plays in shield-ing the planet

But according to the geological record, as written in volcanic rocks and mid-ocean ridges, a reversal takes thousands of years Scientists are

ca ,000 The poles of Earth’s magnetic field reverse, settling

ca 1000 c e Chinese navigators develop the compass

100 The British scientist William Gilbert (1544–1603)

publishes De Magnete (On the magnet), a book

in which he proposes that Earth behaves as a magnet

10 The Danish physicist Hans Christian Oersted

erates a magnetic field

(1777–1851) discovers that an electric current gen-11 The British scientist Michael Faraday (1791–1867)

observes that a changing magnetic field induces an electric current in a conductor

62) reaches the north magnetic pole

1 The German scientist Carl Friedrich Gauss

(1777–1855) publishes the earliest description

of a magnetometer

10 The Norwegian explorer Roald Amundsen (1872–

1928) becomes the second person to find the northern magnetic pole, but he discovers it had moved about 30 miles (50 km) north of Ross’s location

10 The British seismologist Richard D Oldham

(1858–1936) analyzes seismic waves to show that part of Earth’s core—the outer core—is liquid

1910) discovers rocks that suggest the poles of Earth’s magnetic field have been reversed in the past

earth ScienceS



11 The Irish physicist Sir Joseph Larmor (1857–1942)

proposes a dynamo theory to explain some of the Sun’s magnetic properties

10s Harry Hess (1906–69) and Robert Dietz (1914–95)

propose the crust is made of rigid plates that are separating to form mid-ocean ridges Rocks sur-rounding these ridges have stored a magnetic re-cord of the history of Earth’s magnetic field

000s Models and computer simulations suggest that

Earth’s magnetic pole reversals are due to internal processes

00 Signs of weakening in the Earth’s magnetic field

may indicate another pole reversal could occur soon

FuRtHER RESouRCES

Print and internet

Aczel, Amir D The Riddle of the Compass: The Invention That Changed

Livingston, James D Driving Force: The Natural Magic of Magnets.

Cambridge, Mass.: Harvard University Press, 1996 Magnets and

magnetism have interested people ever since the ancient Greeks

Livingston relates the history of research on magnets and magnetic

fields and explains how magnetism works

Moskowitz, Clara “Earth’s Magnetic Field Expected to Flip Soon.”

Fox News, September 26, 2008 Available online URL: http://www

National Aeronautics and Space Administration “What Is the Magne-tosphere?” Available online URL: http://science.nasa.gov/ssl/pad/

sppb/edu/magnetosphere/ Accessed May 4, 2009 This accessible

series of Web pages explains the magnetosphere and includes nu-merous illustrations

National Geophysical Data Center “Geomagnetic Field Frequently

Asked Questions.” Available online URL: http://www.ngdc.noaa

gov/geomag/faqgeom.shtml Accessed May 4, 2009 The National

Geophysical Data Center manages data accumulated by various

geological, oceanic, and atmospheric observatories The frequently

asked questions (FAQ) listed on the Web page cover the basic prin-ciples of Earth’s magnetic field, along with such topics as compasses

and magnetic pole reversals

Public Broadcasting Service “Magnetic Storm.” Available online

URL: http://www.pbs.org/wgbh/nova/magnetic/ Accessed May 4,

2009 Nova, a science documentary series broadcast on PBS, aired

a program in 2003 on the possibility that Earth’s magnetic poles

will reverse in the near future This Web resource is an

and subsequent developments in the study of magnetism

Takahashi, Futoshi, Masaki Matsushima, and Yoshimori Honkura

“Simulations of a Quasi-Taylor State Geomagnetic Field

IntRoduCtIon

In 79 c.e Pompeii was a bustling city located near the Bay of Naples in a region of western Italy known as Campania Founded by an Italian peo-ple called the Oscans centuries earlier, the Romans had conquered and annexed the city by about 290 b.c.e Th e soil in Campania was rich in nutrients and extremely fertile Many wealthy Romans lived in or around the Bay of Naples or spent long vacations there in luxurious villas, and the population of Pompeii was about 10,000 to 20,000 people But on Au-gust 24, 79 c.e., a previously inactive volcano on nearby Mount Vesuvius erupted, covering the region in lava and ashes—and burying Pompeii

About 2,000 people in Pompeii died, blanketed in more than 30 feet (9.1 m) of ash, rock, and debris Many people outside of the city also per-ished, including Gaius Plinius Secundus (ca 23–79 c.e.)—better known

dition to his military duties, Pliny the Elder was a prolifi c author who was fascinated with history and nature; when Vesuvius began erupting, he sailed across the bay to investigate But the scientifi c expedition quickly evolved into a rescue operation as the extent of the disaster became clear, and Pliny the Elder died while helping the evacuation Pliny’s nephew, Pliny the Younger (ca 62–113 c.e.), had stayed behind at the naval base and later wrote about the tragedy Th is writing has survived to the present

as Pliny the Elder—the commander of a nearby Roman naval base In ad-day, collected in Th e Letters of Pliny the Younger.

Excavators digging in the area rediscovered the buried city in the middle of the 18th century Th e ash and debris preserved many of the structures from the ravages of time, and the site is one of the most popular tourist attractions in Italy, off ering an unsurpassed glimpse of what life was like in an ancient Roman city Th e tragedy at Pompeii also serves as a warning, as do other eruptions that have caused much destruction, such as the 1980 eruption of Mount St Helens in Washing-ton, which killed 57 people, and the 1985 eruption of Nevado del Ruiz

in Colombia, which claimed about 25,000 lives and covered hundreds

of square miles of land

vius was one of the earliest recorded observations of volcanic activity

Pliny the Younger’s description of the eruption of Mount Vesu-tablished around 1847 Th is was the only volcano observatory in the world for many years, until a catastrophe occurred on Martinique, an

Vesuvius is also the site of the earliest modern volcano observatory, es-3

Other types of volcanoes emit pyroclastic flows—these larly dangerous emissions include hot ash, gases, and dust that form a

tions can release huge quantities of material The mid-ocean ridges, dis-cussed in chapter 2, are the sites of many fissure eruptions

Magma is hot, which makes it less dense than surrounding rock, and so it rises, as does air heated over a stove Earth’s interior is clear-

tremen-1883, creating deadly tsunamis and a noise that people reported hearing all the way in Australia, 2,170 miles (3,500 km) away!

volve outpouring lava or pyroclas-tic flows Water is usually present

Volcanic activity need not in-in the ground, seeping through the soil after a heavy rain, and if such groundwater meets magma, it will

Geologists have determined that Earth’s lithosphere (crust and

up-permost mantle) is composed of slabs of rock called tectonic plates

Volcanoes congregate along the boundary of the Pacific plate and other

tectonic plates Mount St Helens, Krakatoa, and many other volcanoes

form a ring of fire encircling the Pacific Ocean

Most of Earth’s volcanoes—about 95 percent—occur at the borders between tectonic plates Magma surges through the cracks or seams,

providing the material for volcanic eruptions At some boundaries,

earth ScienceS



along, the volcanic activity will appear to move in the opposite direc-tion, as shown in the figure on page 77 This is because the hot spot is

(opposite page) Numerous volcanoes, shown as dots, line up at the plate

boundaries around the Pacific Ocean.