Further Reading

Anderson, Don L. Theory of the Earth. Oxford: Blackwell

Scientific Publications, 1989. Kious, Jacquelyne, and Robert I. Tilling. "U.S. Geological Survey. This Dynamic Earth: The Story of Plate Tectonics." Available online. URL: http://pubs.usgs. gov/gip/dynamic/dynamic.html. Last modified March 27, 2007.

Moores, Eldridge M., and Robert Twiss. Tectonics. New York: W. H. Freeman, 1995.

Kauai

60 miles

Oahu

Oahu

Molokai

Lanai Kahoolawe Moiokini r\Maui

Lanai Kahoolawe Moiokini

Hawaii

PACIFIC OCEAN

Hawaii

Hot spot

Map of Hawaii-Emperor chain formed by a hot spot track as the Pacific plate moved over a mantle plume. The youngest volcanoes are found on the island of Hawaii (Mauna Loa), and they get progressively older toward the northwest.

Ringwood, A. E. Composition and Petrology of the Earth's

Mantle. New York: McGraw-Hill, 1975. Skinner, Brian, and B. J. Porter. The Dynamic Earth: An Introduction to Physical Geology. 5th ed. New York: John Wiley & Sons, 2004.

Mars The fourth planet from the Sun, Mars is only 11 percent of the mass of Earth and has an average density of 3.9 grams per cubic centimeter and a diameter of 4,222 miles (6,794 km). Mars orbits the Sun every 687 days at a distance of 142 million miles (228 million km) and has a period of rotation about its axis of 24 hours, 37 minutes, and 23 seconds.

When viewed from Earth, Mars shows several striking surface features, including bright polar caps that consist mostly of frozen carbon dioxide (dry ice) that change in size with the seasons, almost disappearing in the Martian summer. Some spectacular canyons are also visible, including the 2,485-mile (4,000-km) long Valles Marineris. This canyon probably formed as a giant crack or fracture on the surface of the expanding bulge in the Tharsis region. Running water may have later modified its surface. Mars is prone to strong surface winds that kick up a lot of dust and generate dust storms that occasionally obscure the surface for long periods of time, an observation that led early observers to suggest that the planet may host vegetation and other life-forms.

Northern main channel of Kasei Valles on Mars (ESA/ DLR/FU Berlin, G. Neukum)

Mars shows evidence for widespread volcanism in its past—its surface is covered with basaltic volcanic rocks, flows, and cones and hosts several large shield volcanoes in the Tharsis and Elysium regions. The Tharsis region is a huge, North America-sized bulge on the planet that rises on average 6.2 miles (10 km) above the elevation of surrounding regions. These volcanoes are huge compared to shield volcanoes on Earth. The largest volcano, Olympus Mons, is 435 miles (700 km) across, and several others are 220 to 250 miles (350-400 km) across and rise 12.5 miles (20 km) over the surrounding terrain. The northern hemisphere of Mars is made of rolling volcanic plains, similar to lunar maria (the dark flat areas on the Moon, shown to be lava flows), but formed by much larger volcanic flows than on Earth or the Moon. In contrast, the southern hemisphere consists of heavily cratered highlands, with a mean elevation several kilometers higher than the volcanic plains to the north. Estimates put the average age of the highland plains at 4 billion years, whereas the age of most of the volcanic plains in the north may be 3 billion years, with some volcanoes as young as 1 billion years old.

Recent high-resolution images of the surface of Mars have strengthened earlier views that water once ran across the surface of the planet. outflow channels and runoff channels are common. Runoff channels form extensive systems in the southern hemisphere and resemble dried-up river systems. outflow channels, which are most common in equatorial regions, may have formed during a catastrophic flooding epi sode about 3 billion years ago in the early history of the planet. Flow rates are estimated to have been at least one hundred times that of the Amazon River. The current absence of any water or visible water ice on the surface suggests that much of this water is frozen beneath the surface in a permafrost layer, reflecting a severe global cooling since 4 billion years ago.

The atmospheric pressure on Mars is only 1/150th that of Earth, and carbon dioxide makes up most of the gas in the atmosphere, with a few percent nitrogen, argon, oxygen, and carbon monoxide and less than one-tenth of a percent water vapor. The temperature rises from about -244°F at 62 miles (-153°C or 120 K at 100 km) above the surface, to about -10°F (-23°C or 250 K) at the surface, with surface temperatures on average about -82°F (-63°C or 50 K) cooler than on Earth.

Many early speculations centered on the possibility of life on Mars and several spectacular claims of evidence for life have been later found to be invalid. To date, no evidence for life, either present or ancient, has been found on Mars. The National Aeronautic and Space Administration (NASA) launched twin robot geologists to Mars in June and July of 2003, and these Rovers have been exploring the Martian surface since 2004. They have mapped a wide variety of rocks and surface structures on the planet, and returned numerous analyses and photographs. Some of the most exciting show evidence for past running water on the Martian surface and evidence that some of the landforms were carved by running water.

See also Earth; Jupiter; Mercury; Neptune; Saturn; solar system; Uranus; Venus.

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