4. Third rock from the Sun - restless Earth


Crust, mantle and core

Crust, mantle and core

. A relatively thin, rocky crust covers a thick silicate mantle. They overlie a liquid outer core, composed mainly of molten iron, and an inner core of solid iron. These nested layers have been inferred from seismic waves that travel through the Earth, changing velocity and direction at the layer boundaries (Figs. 4.2, 4.3).


Measuring earthquakes

Measuring earthquakes

. When an earthquake occurs beneath the surface of the Earth, it becomes the focus of seismic waves that travel through the Earth. Seismometers on the surface of the Earth record the arrival of the waves, and locate the position of the boundaries between internal layers of different composition, density, pressure and temperature (Fig. 4.3). Seismic waves known as S, or shear and shake, waves cannot pass though a fluid, and are reflected by it. The reflected waves shown here mark the boundary of the Earth's liquid outer core.


Layered structure of the Earth

Layered structure of the Earth

. The Earth's internal structure is determined by the varying velocity of earthquake waves. There are two types of waves that travel through the Earth. They are known as the compression P, or push and pull, waves and the shear S, or shake, waves. The P waves move almost twice as fast as the S waves, and the P waves pass through the fluid outer core which the S waves cannot do. The boundary between the mantle and core is marked be a precipitous drop in the velocity of the P waves at a depth of about 2.9 million meters. The S waves do not propagate beyond this boundary. The liquid outer core is separated from the solid inner core at a radius of 1.22 thousand meters where the P waves increase in velocity.


The Earth's double core

The Earth

. The mantle and part of the crust have been cut away here to show the relative sizes of the Earth's fluid and solid cores. The outer fluid core is about 55 percent of the radius of the Earth, and the inner solid core is slightly smaller than the Moon. The Earth's magnetic field is thought to be generated and sustained by moving currents in the planet's electrically-conducting, fluid outer core, which is composed of molten iron. Geophysicists have discovered that the route of the rapid, polar (north-south) waves through the Earth's interior is gradually shifting eastward because the inner core is rotating slightly faster than the rest of the planet. The fast rotation of the inner solid core may help explain how Earth's magnetic field reverses polarity (Courtesy of Paul Richards, Lamont-Doherty Earth Observatory.)


Planet Earth from space

Planet Earth from space

. As illustrated in this image of Africa, the Arabian peninsula, and the Indian Ocean, the Earth's surface consists of continents and oceans. The Antarctica ice cap is also shown, gleaming white at the bottom. Continents cover a little more than one-quarter of the Earth's surface, while ocean water covers almost three-quarters of the surface. This image was taken by Apollo 17 astronauts in December 1972 as they left the Earth en route to the Moon. (Courtesy of NASA.)


Continental fit

Continental fit

. The continents fit together like the pieces of a puzzle. Here the fit has been made along the continental slope at the depth of 910 meters, or 500 fathoms (gray areas). Within the present continents are ancient terrains between 1.7 and 3.8 billion years old (black areas). The close fit of the shorelines of the continents suggests that they once formed a single land mass known as Pangaea shown in Figure 4.7.


Continental drift

Continental drift

. Two hundred million years ago all of the continents were grouped into a signal supercontinent called Pangaea and the world contained only one ocean (top). The continents then drifted away from Pangaea, riding on the back of plates to the positions they now occupy (middle). The bottom diagram depicts the world geography 50 million years from now.


Bottom of the ocean

Bottom of the ocean

. Map of the world's oceans floor as acquired by the Seasat satellite. The mid-Atlantic ridge runs down the middle of the ocean floor separating Africa from North and South America. As shown here, a succession of great ridges runs through all of the world's ocean floors, although not always in the middle. (Courtesy of William F. Haxby, Lamont-Doherty Geophysical Observatory, Columbia University.)


Volcanic islands

Volcanic islands

. Lava erupting from the volcanic island Surtsey on 19 August 1966, almost three years after it rose out of the sea near the coast of Iceland. The volcanic island of Jolnir is in the background. It disappeared back into the sea about one month after this picture was taken, but Surtsey is still visited for research purposes today. All of these volcanic islands, including Iceland, mark points where a mid-ocean ridge has risen out of the ocean. (Courtesy of Hjalmar R. Bardarson, Reykjavik, from his book Ice and Fire.)


Magnetic reversals and sea-floor spreading

Magnetic reversals and sea-floor spreading

. Radioactive dating of volcanic rocks on land have been used to determine the time scale of magnetic reversals on the Earth (bottom) They indicate that the Earth's magnetic field has flipped, or changed direction several times during the past five million years. The data describe normal epochs (white) when compasses would have pointed toward the geographic north, as they do now, and reversed epoch (gray) when compasses would have pointed south. The pattern of magnetic reversals on both sides of the volcanic mid-ocean ridge (top) is the same, indicating that sea-floor spreading has carried the solidified lava away from the central ridge. The sea floor is consumed at the other end, when it slides into a deep ocean trench at a subduction zone.


Moving plates

Moving plates

. The Earth's lithosphere is broken into numerous plates. They move in the directions shown by the arrows at rates of a few tenths of a meter per year. The lithosphere dives into the underlying asthenosphere at zones of subduction. They are denoted by the thick line with triangles, forming the famous ring of fire around the edge of the Pacific and Nazca Plates. Most of the Earth's earthquake and continental volcanic activity is concentrated along the subduction zones.


Los Angeles is moving

Los Angeles is moving

. The Pacific Plate is moving northwestward at the rate of 0.048 meters per year, or about 5 meters every century, slowly carrying Los Angeles towards San Francisco. The North American and Pacific Plates strike and rub against each other like immense grindstones, producing earthquakes along their boundary known as the San Andreas fault. The dated circles denote places where very major earthquakes have occurred with magnitudes of 8 and over on the Richter scale.


Convection inside the Earth

Convection inside the Earth

. Elongated convection cells in the asthenosphere may be aligned in long cylinders that drive the overriding lithosphere plates along like a conveyor belt. A larger-scale circulation transports heat from the volcanic mid-ocean ridge to a deep-ocean subduction trench. Similar large-scale convection currents may lift and lower entire continents. Heat-driven convection in the fluid outer core probably generates and maintains the Earth's magnetic field.


Converging plates

Converging plates

. Magma, volcanoes and earthquakes are generated at a subduction zone (top) where a dense oceanic plate is pushed under a lighter continental plate. When continents on two moving plates meet head on, new mountains are generated (center). In some situations, the advancing plate may become disrupted and the plate motion may stop. The two continents then become welded together forming a larger one, and a new subduction zone can be formed elsewhere (bottom).


Mount Kailash

Mount Kailash

. This mountain, sacred to both Hindus and Buddhists, is called Kang Rinpoche (The Precious Snow Mountain) by the Tibetans. It was formed by the collision of two former continents, now welded together in a seam known as the Himalayan mountain range. Buddhists consider it to be the palace of the diety Chakasamvara (Wheel of Supreme Bliss) and Hindus consider it to be the dwelling place of Shiva. There are also many nearby caves where famous hermits like Milarepa meditated for years. Pilgrims have been visiting and circumambulating Mount Kailash, in western Tibet, for thousands of years. (Courtesy of Matthieu Ricard, Shechen Monastery, Katmandu, Nepal.)


Hot spot forms the Hawaiian islands

Hot spot forms the Hawaiian islands

. A hot spot that is anchored deep within the Earth has recently fed molten lava through a long pipe to Mauna Loa on the big island of Hawaii (left). The moving Pacific Plate has carried three other volcanic islands away from the hot spot; these extinct volcanoes are shown in the accompanying radar image obtained from the Space Shuttle (right). As the plate moves on, wind and water erodes the peaks, reducing the older ones to sunken islands known as seamounts. An underwater volcano, named Loihi, is now forming over the hot spot; it should rise above the ocean to become another Hawaiian island in about 50 thousand years.


Hot spot forms the Hawaiian islands

Hot spot forms the Hawaiian islands

. A hot spot that is anchored deep within the Earth has recently fed molten lava through a long pipe to Mauna Loa on the big island of Hawaii (left). The moving Pacific Plate has carried three other volcanic islands away from the hot spot; these extinct volcanoes are shown in the accompanying radar image obtained from the Space Shuttle (right). As the plate moves on, wind and water erodes the peaks, reducing the older ones to sunken islands known as seamounts. An underwater volcano, named Loihi, is now forming over the hot spot; it should rise above the ocean to become another Hawaiian island in about 50 thousand years.


The rifting of a continent

The rifting of a continent

. A continental rift begins when molten lava rises up from deep in the Earth's interior and splits a continent open. As the fissure grows and widens, a future ocean floor spreads away from the ridge. Water should eventually flow into the cavity, making a new ocean.


An infant ocean

An infant ocean

. An ocean is being born where the Arabian peninsula and the African continent are moving apart, a process that began about 20 million years ago. In a few hundred million years, the Red Sea could be as wide as the Atlantic Ocean is now. (Satellite photograph courtesy of NASA.)


An infant ocean

An infant ocean

. An ocean is being born where the Arabian peninsula and the African continent are moving apart, a process that began about 20 million years ago. In a few hundred million years, the Red Sea could be as wide as the Atlantic Ocean is now. (Satellite photograph courtesy of NASA.)


Sun-layered atmosphere

Sun-layered atmosphere

. The pressure of our atmosphere (right scale) decreases with altitude (left scale). This is because fewer particles are able overcome the Earthís gravitational pull and reach higher altitudes. The temperature (bottom scale) also decreases steadily with height in the ground-hugging troposphere, but the temperature increases in two higher regions that are heated by the Sun. They are the stratosphere, with its critical ozone layer, and the ionosphere. The stratosphere is mainly heated by ultraviolet radiation from the Sun, and the ionosphere is created and modulated by the Sunís X-ray and extreme ultraviolet radiation.


Hole in the sky

Hole in the sky

. A satellite map showing an exceptionally low concentration of ozone, called the ozone hole, that forms above the South Pole in the local winter. In October 1990 it had an area larger than the Antarctic continent, shown in outline below the hole. Eventually spring warming breaks up the polar vortex and disperses the ozone-poor air over the rest of the planet. (Courtesy of NASA.)


Rise in Atospheric Carbon Dioxide

Rise in Atospheric Carbon Dioxide

. The average monthly concentration of atmospheric carbon dioxide, or CO2 for short in prats per million (106), abbreviated ppm, of dry air plotted against time in years observed continuosly since 1958 at the Mauna Loa Observatory, Hawaii. It shows that atmospheric amounts of the principal waste gas of industrial societies, carbon dioxide, have risen steadily for more than forty years. The up and down fluctuations, that are superimposed on the systematic increase, reflecta a seasonal rise and fall in the absorption of carbon dioxide by trees and other vegetation. Summertime lows are caused by the uptake of carbon dioxide by plants, and the winter hights occur when the plants' leaves fall and some of the gas is returned to the air. (Courtesy of Dave Keepling and Tim Whorf, Scripps Institution of Oceanography.)


The Sun in X-rays

The Sun in X-rays

. The bright glow seen in this X-ray image of the Sun is produced by ionized gases at a temperature of a few million degrees Kelvin. It shows magnetic coronal loops which thread the corona and hold the hot gases in place. The brightest features are called active regions and correspond to the sites of the most intense magnetic field strength. This image of the Sunís corona was recorded by the Soft X-ray Telescope (SXT) aboard the Japanese Yohkoh satellite on 1 February 1992, near a maximum in the 11-year cycle of solar magnetic activity. Subsequent SXT images, taken several years after activity maximum, show a remarkable dimming of the corona (Fig. 4.22), when the active regions associated with sunspots have almost disappeared. (Courtesy of Gregory L. Slater, Gary A, Linford, and Lawrence Shing, NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


X-ray view of the solar cycle

X-ray view of the solar cycle

. Dramatic changes in the solar corona are revealed in this four-year montage of images from the Soft X-ray Telescope (SXT) aboard Yokhoh. The 12 images are spaced at 120-day intervals from the time of the satelliteís launch in August 1991, at the maximum phase of the 11-year sunspot cycle (left), to late 1995 near the minimum phase (right). The bright glow of X-rays near activity maximum comes from very hot, million-degree coronal gases that are confined within powerful magnetic fields anchored in sunspots (Fig. 4.21). Near the cycle minimum, the active regions associated with sunspots have almost disappeared, and there is an overall decrease in X-ray brightness by 100 times. (Courtesy of Gregory L. Slater and Gary A, Linford, NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


Variations in the solar constant

Variations in the solar constant

. Observations with very stable and precise detectors on several Earth-orbiting satellites show that the Sunís total radiative input to the Earth, termed the solar constant or solar irradiance, is not a constant, but instead varies over time scales of days and years. Measurements from five independent space-based radiometers have been combined to produce the composite solar irradiance over two decades since 1978. They show that the Sunís output fluctuates during each 11-year sunspot cycle, changing by about 0.1 percent between maximums (1980 and 1990) and minimums (1987 and 1997) in magnetic activity. Temporary dips of up to 0.3 percent and a few daysí duration are due to the presence of large sunspots on the visible hemisphere. The larger number of sunspots near the peak in the 11-year cycle is accompanied by a rise in magnetic activity that creates an increase in luminous output that exceeds the cooling effects of sunspots. The solar constant, or irradiance, is given in units of watts per square meter, where one watt is equivalent to one joule per second. (Courtesy of Claus FrŲhlich.)


Hunters in the snow

Hunters in the snow

. This section of a painting by Pieter Bruegel the Elder depicts a time when the average temperatures in Northern Europe were much colder than they are today. Severe cold occurred during the Maunder Minimum, from 1645 to 1715, when there was a conspicuous absence of sunspots and other signs of solar activity. This picture was painted in 1565, near the end of another dearth of sunspots, called the SpŲrer Minimum. (Courtesy of the Kunsthistorisches Museum, Vienna.)


Ice age temperatures and greenhouse gases

Ice age temperatures and greenhouse gases

. Ice core data indicate that changes in the atmospheric temperature over Antarctica closely parallel variations in the atmospheric concentrations of two greenhouse gases, carbon dioxide and methane, for the past 160,000 years. When the temperature rises, so does the amount of these two greenhouse gases, and vice versa. This strong correlation has been extended by a deeper Vostok ice core, to 3,623 meters in depth and the past 420,000 years. The carbon dioxide and methane increases could amplify orbital forcing of climate change. The ice core data does not include the past 200 years, shown as a dashed lines at the right. They show that the present-day levels of carbon dioxide and methane are unprecedented during the past four 100,000-year glacial-interglacial cycles.


Unusual Heat

Unusual Heat

. The temperature in the Northern Hemisphere (top curve, scale at left) has been warmer in the 20th century than in any other century of the last one thousand years, and the 1990s were the hottest decade during all that time. The sharp upward jump in temperature during the past 100 years was recorded by thermometers near the Earth's surface; earlier temperature fluctuations were inferred from tree rings, lake and ocean sediments, coral reefs, and ice cores. There has been an exponential rise in the amount of atmospheric carbon dioxide over the past two and a half centuries (bottom curve, scale at right). (Courtesy of Michael E. Mann, top curve, and Charles D. Keeling, bottom curve.)


The Sunís fate

The Sunís fate

. In about 8 billion years the Sun will become much brighter (top) and larger (bottom). Note the different time scales, expanded near the end of the Sunís life to show relatively rapid changes. (Courtesy of I-Juliana Sackmann and Arnold I. Boothroyd ).


Coronal mass ejection

Coronal mass ejection

. A huge coronal mass ejection, abbreviated CME, is seen in this image. The white circle denotes the edge of the photosphere, so this mass ejection is about twice as large as the visible Sun. The black area corresponds to the occulting disk of the space-borne telescope, called a coronagraph, that blocks intense sunlight and permits the tenuous, million-degree corona to be seen. This coronagraph image, taken on 27 February 2000 with the Large Angle Spectrometric COronagraph (LASCO) on the SOlar and Heliospheric Observatory. (SOHO). (Courtesy of the SOHO LASCO consortium. SOHO is a project of international cooperation between ESA and NASA.)


Spiral path of interplanetary electrons

Spiral path of interplanetary electrons

. The trajectory of flare electrons in interplanetary space as viewed from above the Sun's polar regions using the Ulysses spacecraft. As the high-speed electrons move out from the Sun, they excite radio radiation at successively lower frequencies; the numbers denote the observed frequency in kiloHertz, or kHz. Since the flaring electrons are forced to follow the interplanetary magnetic field, they do not move in a straight line from the Sun to the Earth, but instead move along the spiral pattern of the interplanetary magnetic field, shown by the solid curved lines. The squares and crosses mark Ulysses radio measurements of type III radio bursts on 25 October 1994 and 30 October 1994. The approximate locations of the orbits of Mercury, Venus and the Earth are shown as circles. (Courtesy of Michael J. Reiner. Ulysses is a project of international collaboration between ESA and NASA.)


The Sun getting ready to explode

The Sun getting ready to explode

. When the magnetic fields in the low solar atmosphere get twisted into an S, or sigmoid, shape, they become dangerous, like a coiled rattlesnake waiting to strike. Statistical studies indicate that the appearance of such a large S or inverted S shape in soft X-rays is likely to be followed by an explosion in just a few days. This image was taken on 8 June 1998 with the Soft X-ray Telescope (SXT) aboard the Yohkoh satellite (Courtesy of Richard C. Canfield, NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


Summary diagram

Summary diagram

. Summary diagram.