Surface Features

Earth's surface features are unique in the solar system because they are dominated by the movement and chemical action of water and the existence of life.Water and vegetation cover the majority of the planet, and even in deserts the landforms are shaped predominantly by the infrequent rainstorms and floods, and less by wind.Water and vegetation also act to erode rocks and change landforms: Sharp edges become dulled by weathering and wearing away the rock, soil slumps down steep slopes, and glaciers carve away the surface.The ability of ice to carve bedrock and carry sediment is shown in the image on page 79 of Viedma glacier in Argentina. This glacier flows into Lake Viedma, one of three major lakes on the Patagonian side of the southern Andean ice sheet, at 50 degrees south. The slender dark lines of rock material (moraine) are carried in the slowly flowing ice. Lighter

Determining Age from Radioactive Isotopes EEach element exists in the form of atoms with several different-sized nuclei, called isotopes. Consider the element carbon. All carbon atoms have six protons in their nuclei, but they can have different numbers of neutrons. Protons determine the kind of element the atom is because protons have a positive charge, and to balance their positive charge, the atom has negatively charged electrons orbiting around its nucleus. It is the structure and number of electrons and the size of the atom that determine how it interacts with other atoms, and thus makes all atoms with the same number of protons act alike. Neutrons, on the other hand, make an atom heavier, but do not change its chemical interactions very much. The atoms of an element that have different numbers of neutrons are called isotopes. Carbon has three isotopes, with atomic masses 12, 13, and 14. They are denoted 12C, 13C, and 14C.

Most atoms are stable. A 12C atom, for example, remains a 12C atom forever, and an 16O (oxygen) atom remains an 16O atom forever, but certain atoms eventually disintegrate into a totally new atom. These atoms are said to be unstable or radioactive. An unstable atom has excess internal energy, with the result that the nucleus can undergo a spontaneous change toward a more stable form. This is called radioactive decay.

Unstable isotopes (which are thus radioactive) are called radioisotopes. Some elements, such as uranium, have no stable isotopes. Other elements have no radioactive isotopes. The rate at which unstable elements decay is measured as a half-life, the time it takes for half of the unstable atoms to have decayed. After one half-life, half the unstable atoms remain; after two half-lives, one-quarter remain, and so forth. Half-lives vary from parts of a second to billions of years, depending on the atom being considered. The radioactive element is called the parent, and the product of decay is called the daughter.

When rocks form, their crystals contain some amount of radioactive isotopes. Different crystals have differently sized spaces in their lattices, so some minerals are more likely to incorporate certain isotopes than others. The mineral zircon, for example, usually contains a measurable amount of radioactive lead (atomic abbreviation Pb). When the crystal forms, it contains some ratio of parent and daughter atoms. As time passes, the parent atoms continue to decay according to the rate given by their half-life, and the population of daughter atoms in the crystal increases. By measuring the concentrations of parent and daughter atoms, the age of the rock can be determined.

To learn the math to calculate the age of materials based on radioactive decay, read this section. Otherwise, skip to the end of the math section to learn about the ages of objects in the solar system.

Consider the case of the radioactive decay system 87Rb (rubidium). It decays to 87Sr (strontium) with a half-life of 49 billion years. In a given crystal, the amount of 87Sr existing now is equal to the original 87Sr that was incorporated in the crystal when it formed, plus the amount of 87Rb that has decayed since the crystal formed. This can be written mathematically as:

now original x original now'

The amount of rubidium now is related to the original amount by its rate of decay. This can be expressed in a simple relationship that shows that the change in the number of parent atoms n is equal to the original number ng times one over the rate of decay, called

X (the equations will now be generalized for use with any isotope system): ~dn - X

where dn means the change in n, the original number of atoms, and dt means the change in time. To get the number of atoms present now, the expression needs to be rearranged so that the time terms are on one side, and the n terms on the other, and then integrated, and the final result is:

The number of daughter atoms formed by decay, D, is equal to the number of parent atoms that decayed:

Also, from the previous equation, ng — neXt. That expression can be substituted into the equation for D in order to remove the term ng:

(continues)

Determining Age from Radioactive Isotopes (continued) Then, finally, if the number of daughter atoms when the system began was Do, then the number of daughter atoms now is

This is the equation that allows geologists to determine the age of materials based on radiogenic systems. The material in question is ground up, dissolved in acid, and vaporized in an instrument called a mass spectrometer. The mass spectrometer measures the relative abundances of the isotopes in question, and then the time over which they have been decaying can be calculated.

The values of D and n are measured for a number of minerals in the same rock, or a number of rocks from the same outcrop, and the data is plotted on a graph of D vs. n (often D and n are measured as ratios of some stable isotope, simply because it is easier for the mass spectrometer to measure ratios accurately than it is to measure absolute abundances). The slope of the line the data forms is eXt - 1. This relation can be solved for t, the time since the rocks formed. This technique also neatly gets around the problem of knowing D the initial concentration of daughter isotopes: D ends up being the y-intercept of the graph.

Radiodating, as the technique is sometimes called, is tremendously powerful in determining how fast and when processes happened on the Earth and in the early solar system. Samples of most geological material has been dated: the lunar crustal rocks and basalts returned by the Apollo and Luna missions, all the kinds of meteorites including those from Mars, and tens of thousands of samples from all over the Earth. While the surface of the Moon has been shown to be between 3.5 and 4.6 billion years old for the most part, the Earth's surface is largely younger than 250 Ma (million years old). The oldest rock found on Earth is the Acasta gneiss, from northwestern Canada, which is 3.96 billion years old.

If the oldest rock on Earth is 3.96 billion years old, does that mean that the Earth is 3.96 billion years old? No, because older rocks probably have simply been destroyed by the processes of erosion and plate tectonics, and there is reason to believe that the Earth and Moon formed at nearly the same time. Many meteorites, especially the primitive chondritic meteorites, have ages of 4.56 billion years. Scientists believe that this is the age of the solar system. How, a critical reader should ask, is it known that this is when the solar system formed, and not some later formation event?

The answer is found by using another set of radioactive elements. These have half-lives so short that virtually all the original parent atoms have decayed into daughters long since. They are called extinct nuclides (nuclide is a synonym for isotope). An important example is 129I (an isotope of iodine), which decays into 129Xe (xenon) with a half-life of only 16 million years. All the 129I that the solar system would ever have was formed when the original solar nebula was formed, just before the planets began to form. If a rock found today contains excess 129Xe, above the solar system average, then it formed very early in solar system time, when 129I was still live. The meteorites that date to 4.56 Ga (billion years) have excess 129Xe, so 4.56 Ga is the age of the beginning of the solar system.

lines at right angles are patterns of crevasses.The glacier diverges into two lakes where calved icebergs can be seen floating in the lakes. The glacier here is about 1.1 miles (1.8 km) wide.

Because of this continuous action and the destructive work of plate tectonics, very little of early Earth history is left on the surface. Though other planets and moons are covered with impact craters from meteorite bombardment, the Earth has relatively few recognizable

The Viedma glacier flows down the Andes in Argentina.

(Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center, eol.jsc.nasa.gov, image number ISS008-E-12390)

craters.They have been eroded away over time. Because there are so few to be seen on the Earth, it took centuries for scientists to agree that giant meteorites have in fact struck the Earth.The reigning paradigm for the previous three centuries had been gradualism and uni-formitarianism, the ideas that Earth processes happened gradually and incrementally over vast amounts of time, leading in the end to the dramatic formations seen today. Now scientists think there are several good examples of the converse, catastrophism. Shown in the lower color insert on page C-1, sudden catastrophic processes that alter landforms include meteorite impacts, volcanic explosions, giant landslides, earthquakes, and storms. On Earth erosion generally acts against the ability to recognize craters.There are about 170 terrestrial craters known, ranging in age from a few thousand years old to 2 billion years old, and in size up to several hundred kilometers in diameter. Newly verified impact sites are presented at scientific conferences every year.The largest known craters are in the accompanying table. Not all can be seen at the surface of the Earth; Chicxulub, for example, is best detected by anomalies in the gravity field.

One of the most famous and most studied craters on Earth is Meteor crater. Meteor crater, also called Barringer crater, after the man who bought the rights to the land around it, is a clean, round crater three-quarters of a mile (1.2 km) in diameter, 560 feet (170 m) deep, and has a rim of rock blocks 150 feet (45 m) high around its edge. Meteor crater lies in the middle of a great flat plain in Arizona. When first found by Europeans, the crater had about 30 tons of pieces of iron meteorite scattered it over a diameter of about six miles (10 km).The view on page 82 of Meteor crater was taken by an astronaut on the International Space Station on February 11, 2004. The crater looks much as a lunar crater might appear through a telescope. The prominent gully meandering across the scene is known as Canyon Diablo, which drains northward toward the Little Colorado River and eventually to the Grand Canyon. The Interstate 40 highway crosses and nearly parallels the northern edge of the scene.

A good example of how hard it can be to confirm a circular feature as an impact crater on Earth is the five-mile (8-km) feature known as the Iturralde Structure. It is possibly the Earth's most recent large impact event, recording an impact that might have occurred between 11,000 and 30,000 years ago. Iturralde is in an isolated part of the Bolivian Amazon. Although the structure was identified on satellite

SELECTED TERRESTRIAL IMPACT CRATERS

Diameter

Approximate age

Crater name

State/Province

Country

(miles [km])

(millions of years)

Vredefort

South Africa

190 (300)

2020

Sudbury

Ontario

Canada

150 (250)

1850

Chicxulub

Yucatán

Mexico

110 (180)

65

Manicouagan

Quebec

Canada

60 (100)

214

Popigai

Russia

60 (100)

35

Acraman

South Australia

Australia

55 (90)

450

Chesapeake Bay

Virginia

U.S.

53 (85)

35

Puchezh-Katunki

Russia

50 (80)

175

Morokweng

South Africa

44 (70)

145

Kara

Russia

40 (65)

73

Beaverhead

Montana

U.S.

38 (60)

625

Tookoonooka

Queensland

Australia

34 (55)

130

Charlevoix

Quebec

Canada

34 (54)

360

Kara-Kul

Tajikistan

33 (52)

within 5 Myr

of present

Siljan

Sweden

33 (52)

368

Montagnais

Nova Scotia

Canada

28 (45)

50

photographs in the mid-1980s, its location is so remote that it has only been visited by scientific investigators twice, most recently by a team from NASA's Goddard Space Flight Center in September 2002.The feature is a closed depression only about 66 feet (20 m) in depth. Its rim cuts into the heavily vegetated soft sediments of this part of Bolivia.Thick vegetation makes its identification doubly difficult, since there are few hard rocks to preserve shock features.

Gosses Bluff, an impact crater sandwiched between the Macdonnell Range to the north and the James Range to the south in Australia's Northern Territory, is about 100 miles (160 km) west of Alice Springs. Australia makes a great natural laboratory for impact structure

Canyon Diablo and Meteor crater, one of the youngest and freshest craters on Earth, were photographed from the International Space Station. (Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center, eol.jsc.nasa.gov, image number ISS008-E-15268)

study, since it is so dry and old that many craters have been preserved on its surface. Gosses Bluff is one of the most studied (see image on page 83).The impactor, probably about 0.6 miles (1 km) in diameter, created the crater about 142 million years ago. The isolated circular feature within the crater consists of a central ring of hills about 2.8 miles (4.5 km) in diameter.The grayish feature surrounding the inner ring probably marks the original boundary of the outer rim.

Not all perfectly round features with concentric terraces are impact craters.This prominent circular feature, known as the Richat Structure, is in the Sahara of Mauritania. Richat has a diameter of 30 miles (50 km) and was, understandably, initially mistaken for a possible impact crater. It is now known to be an eroded circular dome of layered sedimentary rocks. Shown in the upper color insert on page C-4, this view, generated from a Landsat satellite image, exaggerates vertical expression by a factor of six to make the structure more apparent. The height of the mesa ridge in the center of the view is about 935 feet (285 m).

The runoff that erodes away the evidence of meteorite impacts on Earth eventually drains into the planet's oceans. Oceans cover nearly 70 percent of the planet, and the action of the water that flows down to them controls much of the shape of continents, the patterns of vegetation, and of human population. Seen in the upper color insert on page C-3, of the Earth's surface water, 97.2 percent is in the oceans, 2.15 percent in ice caps and glaciers, and only 0.65 percent in lakes, streams, ground water, and the atmosphere combined (this tiny percentage emphasizes the importance of preserving clean groundwater supplies for drinking).

Oceans are thought to have formed as early as 100 million years after the Earth formed, at 4.4 billion years before the present. They have not always existed in the places and shapes they do now, though; as shown in the upper color insert on page C-5, plate tectonics controls the shapes, ages, and positions of the ocean basins on Earth. At the moment on Earth, the Pacific Ocean is surrounded by subduction zones, while the Atlantic Ocean is surrounded for the most part by passive margins, where no relative movement is occurring between

The 142-million-year-old Gosses Bluff impact crater lies about 100 miles (160 km) west of Alice Springs in Australia.

(Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center, eol.jsc.nasa.gov, image number ISS07-E-5697)

The Nile River, Earth's longest river, meanders across its floodplain. Both Mars and Venus have longer channel systems, but those on Venus are thought to have been caused by flowing lava rather than by water. (Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center, eol.jsc.nasa.gov, ISS009_E_8271)

the oceanic plates and the continents.The Atlantic Ocean is growing in area at the rate at which new crust is produced at its mid-ocean ridges.The Pacific Ocean, on the other hand, is losing area all the time to the subduction zones around its perimeter. In this way the sizes of oceans wax and wane.

About 200 million years ago, the Atlantic Ocean was closed. The continents on each side had been driven toward each other because of tectonic forces on their opposite sides and because of mantle convention currents, and the ocean basin had gradually disappeared as the continents were driven to the point that they collided into one giant land mass. At 200 million years ago, for reasons that are not well understood, a great rift began to open more or less along the suture zone where the continents collided. Immense volumes of lava poured out, forming basaltic rocks that now line the shores of the continents along their lengths. A mid-ocean ridge began to form, creating new oceanic crust and pushing the continents apart again. The new ocean basin did not form exactly where the old one had been: Part of New England used to belong to Africa but remained on this side when the ocean basin opened.

The Mississippi River displays meander bends and oxbow lakes, which are characteristic of water flow. (NASA)

By reconstructing the original types and ages of rocks found on North and South American, Europe, and Africa, it is now known that the Atlantic had gone through this cycle previously as well. The opening and closing of ocean basins was first developed in a paper by J.Tuzo Wilson, a professor of geophysics at the University of Toronto, published in 1963. In it he began to describe the cycle:A rift forms in a continent, developing into a mid-ocean ridge and the growth of oceanic crust. The two continents move farther apart. Eventually, tectonic forces from their other sides begin to push them together again, and subduction zones form on one side or both of the ocean. Finally, the two continents collide. When continents collide, neither one subducts; they are too rigid and too buoyant. Instead, as seen in the lower color insert on page C-4, mountains are thrust up where the continents meet, as the Himalayas are forming today in the continued collision of Asia and India. The idea of the Wilson cycle was critical in the development of plate tectonics.

Plate tectonics form and remove oceans and also take an important role in the water cycle of the planet. Sediments that sink to the bottom of the ocean carry water bound into their crystal structures. When oceanic plates subduct under continental plates at volcanic arcs, the plates carry some percentage of their

This large distributary fan on Mars is evidence for long-term water flow. (NASA/JPL/ Malin Space Science Systems)

wet sediments into the Earth. Some of this water is released from the sediments by pressure and heat and helps the overlying mantle melt, eventually being erupted onto the surface in volcanic flows. Some other percentage of the water is carried down into the mantle with the subducting plate and mantle. Some scientists estimate that there is the equivalent of 10 oceans' worth of water dissolved in minerals in the mantle.

Unique in all the solar system, oceans on Earth are fed by rivers carrying surface and ground water off the continents, by melting ice caps and glaciers, and by precipitation from clouds. Rivers erode and redistribute sediments, in addition to transporting water to oceans. Rivers are classified as meandering when they run mainly in a single channel which winds back and forth across the river's floodplain, and as a braided river when many channels combine and split across a floodplain. The image of the Nile River on page 84, taken from the

International Space Station, clearly shows its floodplain, as well as the pattern of meanders in the current river channel.

Rivers drop sediment along their beds where the water slows. When the beds build up to a certain level, the river flows out of its banks during a flood and forges a new channel.The patterns of channels and distribution of sediments make patterns unique to water transport, distinct from the other materials that can create channels (for example, lava on Venus).

The Mississippi River is the longest river in North America and one of the longest in the world, at 2,350 miles (3,750 km) (the Nile is the world's longest river). It begins at Lake Itaska in Minnesota and ends in the Gulf of Mexico. Along its length it displays all the characteristic forms of water transport, including the meander bends (curves in the river) and oxbow lakes (meander bends that have been abandoned when the river leaped its banks during a flood and assumed a new channel). Meander bends and oxbow lakes are shown in the image on page 85. Along the course of a river smaller rivers, called tributaries, empty into the main channel. When the river comes to its end, it slows as the slope it is flowing down becomes less steep and the river drops its sediment. As the sediment builds up in the river's bed the flow eventually splits and forms a new bed at a lower elevation. A river tends to split into many channels at its end, called distributaries, shown in the lower color insert on page C-3 for the Mississippi River.

Mars is the only other planet in the solar system now known to show evidence for having had flowing surface water at some point in its past.The image on page 86 shows meander loops and distributaries from an ancient water river on Mars, similar to those shown for the Mississippi River.

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