Feature

Presto, Change-o!

Extraterrestrial impacts transform Earth's surface in an instant

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7:33pm, June 11, 2002
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Most geological processes unfold at less than a snail's pace. The tectonic plates that cover Earth's surface slog along, crashing into and sliding over one another at rates of only a few millimeters per year. Over millions of years, however, these unhurried liaisons raise mountain ranges. Wind, rain, and natural chemical erosion gradually rework the mountains into silt, clay, and dissolved minerals. Slowly, this inorganic detritus wends its way to the sea, where it joins a languid rain of dead marine organisms to form thick layers of ocean-floor ooze.

Every now and again, however, things happen in a flash. Asteroids, comets, and smaller objects smack into the planet at clips of thousands of kilometers per hour. When this happens, the impacts can gouge sizable holes in Earth's outer crust. Within milliseconds, rocks at the impact site vaporize. The rapid expansion of this superheated gas blows melted and pulverized material into the atmosphere or back into space.

The immense seismic vibrations from an impact can create temperatures high enough to melt or demagnetize some rocks in and near the crater. Farther away, the sudden changes in pressure triggered by shock waves shatter and otherwise transform mineral crystals as no other geological process does.

Although these planetary bruises and black eyes have significantly shaped the planet's surface, many have remained hidden. Scientists are taking advantage of the magnetic and gravitational scars of these impacts to identify the sites of the most dramatic bombardments this planet has ever experienced.

When worlds collide

Many of the smallest objects on a collision course with Earth burn up in the atmosphere before they reach the surface. A meteoroid–an interplanetary object ranging in size from a dust grain up to a mountain–needs to be at least the size of a child's marble to blaze all the way to Earth's surface. Anything that survives the fall is, by definition, a meteorite. The kinetic energy of the meteorite when it strikes the ground–a function of the mass of the space rock and its velocity–strongly influences the size of the hole or the splash it creates.

Tiny meteorites are slowed by the atmosphere so much that they simply drop to the ground, sometimes making no more than a dent. When these dark objects fall on frozen, snow-covered terrain, they're particularly easy to find. Residents of Canada's Yukon Territory recovered pieces of a rare carbon-rich meteorite soon after it fell in January 2000 (SN: 4/8/00, p. 235), and scientists visiting Antarctica routinely use snowmobiles to hunt for the extraterrestrial rocks.

More-massive meteoroids are slowed less by air resistance and therefore pack a bigger punch when they land. They typically gouge out classic, bowl-shaped craters. Arizona's Meteor Crater–also known as Barringer Crater, after the Philadelphia mining engineer who began studying the site in 1902–is the best-preserved terrestrial example of such a so-called simple crater.

The impact scar, located about 20 kilometers west of Winslow, Ariz., was formed nearly 50,000 years ago when an iron-nickel meteorite about 45 meters in diameter punched through the region's rocky plain. The impact energy of 20 million tons of TNT was roughly equivalent to the power of a hydrogen bomb. The sudden collision vaporized the meteorite, pulverized rocks at ground zero, and heaved large blocks of limestone, some the size of small homes, out of a 200-m deep, 1.2-km-diameter hole. That debris formed an elevated rim that still rises above the Arizona plain.

On Earth, craters that range up to about 5 km across have this simple structure, says Harrison H. Schmitt, a geologist and retired astronaut who trained at Meteor Crater before walking on the moon during the Apollo 17 mission.

Meteoroids larger than 200 m or so across create a different type of impact scar when they slam into Earth, says Thomas Kenkmann, a geologist at Humboldt University in Berlin. These complex craters have a flat floor marked with a central uplift, which typically is either a single or ring peak. This uplift forms as the rocks beneath the deepest portion of the crater floor rebound from the compressive shock of the meteorite's impact.

Complex craters also have terraced rims, which form when the initially steep walls of the crater collapse downward and inward. An analysis of twisted rocks taken from the central uplift of the 7-km-wide Crooked Creek crater in Missouri suggests that this collapse is very quick, says Kenkmann.

The roughly 320-million-year-old impact occurred in sediments composed of mineral grains 10 to 100 micrometers in diameter bound into rock. As many as 40 percent of the boundaries between individual grains were fractured, and rock deformation typically took place in bands between 10 and 500 micrometers wide. None of the grains seem to have been stretched before they broke. All these clues point to the crater collapsing in less than 30 seconds, says Kenkmann. His analyses of several complex craters between 5 and 15 km in diameter suggest that their rims collapsed within a minute of the impact. He reports his findings in the March Geology.

The pressure's off

Thick sheets of melted rocks line the bottom of many large meteor craters. Some of these impact melts derive from the kinetic energy of the impact, a large part of which is converted to heat when the meteorite smacks Earth and grinds to an abrupt stop. However, the sudden excavation of a large crater probably plays a bigger role in forming impact melts, says Schmitt.

Rocks lying kilometers deep within Earth are often on the verge of melting but are prevented from doing so by the immense pressure of all the material above them. When meteorites blast that weight away, the pressure in the rocks beneath the crater floor drops precipitously and the underlying minerals melt. The impact melts may not fully cool for hundreds of thousands of years. In the meantime, water from the environment and the heat from the newly exposed rocks can combine to form hydrothermal systems in the heavily fractured rocks in and around the crater. Scientists believe such warm, mineral-rich venues could have played a role in the early development of life on Earth (SN: 3/9/02, p. 147: Available to subscribers at Space Rocks' Demo Job: Asteroids, not comets, pummeled early Earth).

The 200-m-thick impact melts found within an ancient crater surrounding the town of Sudbury in central Ontario are more than a sign of extraterrestrial impact: They're a treasure trove of minerals. More than $1 billion of metal ores including those bearing nickel, platinum, and copper are mined from the melts each year, says Richard Grieve, a geologist at Natural Resources Canada in Ottawa. Isotopic analyses show that the metals come from Earth's crust, not from the meteor that fell from space. Before the impact melts solidified, the deep, thick blend of light silicates and dense metal ores–which didn't mix well with each other–separated into two layers, according to density, just like oil and vinegar do. This ancient segregation makes mining today much easier.

The hydrothermal system created by the Sudbury impact also dissolved minerals containing copper and other metals from a broad area and then concentrated them in rich veins. One large outcrop of ore alone holds minerals valued around $100 billion, says Grieve. The economic interest in the area has proven a boon to scientists, who have attained access to deep rock cores originally extracted to determine the best locations to sink mining shafts.

Radioactive dating of the melts and the hydrothermal deposits indicates the Sudbury impact occurred about 1.85 billion years ago. The original crater probably was between 250 and 300 km across, says Grieve. It's tough to tell because erosion, including the ravages of several ice ages, has scraped away up to 4 km of Earth's surface from the crater site. That has erased many of the impact's effects.

A somewhat older impact crater provides a deeper view. The Vredefort impact structure, named after the city in South Africa that was built in the center of the ancient bull's-eye, was created by a collision about 2.02 billion years ago. The rocks now at Earth's surface there were once between 7 and 10 km belowground, says Roger Gibson of the University of the Witwatersrand in Johannesburg. That much overlying material, including all of the crater's impact melts, has eroded away since the crater formed.

However, that loss is science's gain: The erosion has made it easy for geologists to get samples of rock that formed deep within the crater's central peak, now a dome of exposed material.

Most of the crystalline mineral grains in the dome's rocks measure between 1 and 5 millimeters across, which matches the grain size for similar rocks in the area.

However, rocks found within 5 km of the center of the Vredefort dome typically have grains no more than 100 micrometers across. Because grain size is related to the length of time that the crystal took to grow, Gibson contends that the rocks in the center of the dome experienced a short burst of terrific heat before they rebounded toward Earth's surface.

His analyses indicate that the rocks were between 15 and 20 km below ground, at around 400C, before the impact occurred. Then, during the strike from space, temperatures in the rocks directly beneath the impact briefly rose to between 1,000C and 1,400C, primarily due to intense shock waves. At sites about 25 km from the impact, shock waves had dissipated somewhat, and the rocky material there got only a small boost in temperature, Gibson says. His team's analyses appear in the May Geology.

New finds, old tools

Extraterrestrial impacts leave distinct calling cards. For instance, when a rock's temperature rises above its so-called blocking temperature, any magnetic fields in the minerals are disrupted and then realign to match the strength and direction of the magnetic fields in the rock's environment. This phenomenon takes place in molten rocks spewing from volcanoes and undersea ridges, but it also takes place in the wake of meteor strikes. If the magnetic field at the location of an extraterrestrial impact is significantly different from the one in place when those rocks last cooled, then the cosmic bruise will produce magnetic anomalies.

Those irregularities can be quite extensive, says Jasper Halekas, a geophysicist at the University of California, Berkeley. He and his colleagues have analyzed data collected from lunar craters during the Apollo moon missions and the more recent Lunar Prospector probe. Those studies show magnetic anomalies that often extend up to several crater radii from an impact site. That finding implicates temperature boosts from seismic shock rather than exposure to vaporized material from the meteorite. The team presented its results last December at a meeting of the American Geophysical Union in San Francisco.

Impacts also can produce gravitational anomalies. Even long after an impact scar becomes heavily eroded, the pulverized rock that fills the crater bottom is much less dense than the solid rock from which it's derived. The precise force of gravity at any location depends, in part, on the density and amount of material in the neighborhood.

Impact melts and a central uplift, if any, also can affect local gravitational patterns.

Other geological processes can produce magnetic and gravitational anomalies, but when these two hallmarks occur together, or are backed up with other geologic evidence, it's a strong hint that scientists may have found an ancient impact site. At the meeting in San Francisco, Dallas Abbott and her colleagues at Lamont-Doherty Earth Observatory in Palisades, N.Y., described a possible impact crater southeast of Hawaii. They found two strong magnetic anomalies, possibly related to impact melts, inside an unusually shallow, 150-km-diameter crater that lies in water about 3.8 km deep.

The team also found small spherules of glassy material in sediments all around the proposed impact site. The tiny orbs ranged up to 200 micrometers in diameter, a size characteristic of those produced by meteorites that create craters 55 km or more across. The crater may be uncharacteristically shallow for a couple of reasons, the researchers say. First, the deep water probably cushioned the blow of the meteorite.

Also, chemical analyses of the spherules, which are high in potassium and low in silicon, suggest that the impact landed on an undersea mountain rather than flat ocean floor.

A group of scientists from the University of South Carolina in Columbia says that they've used geological anomalies, as well as clues from rock samples, to identify an ancient crater buried beneath the piedmont sediments of their state. A magnetic anomaly about 10 km across is nearly superimposed on a 12-km-diameter gravitational irregularity near the town of Johnsonville, says geologist Christopher D. Parkinson. A 290-m borehole, drilled when other scientists were studying the area's aquifers, shows that sediments at the proposed impact site are about 275 m thick. The deepest sediments were laid down about 90 million years ago, and they lie directly on top of basement rock that is a little less than 300 million years old.

Shocked quartz and other metamorphic changes in the basement rocks indicate the minerals were subjected to the intense pressures and strong seismic waves generated by a meteorite impact, says Parkinson. Some of the changes suggest that temperatures in the rocks rose to at least 1,300C.

Other boreholes drilled in the area during the aquifer study were spaced 20 to 50 km apart and, like the Johnsonville borehole, extended all the way to the basement rocks. All these other sediment cores include a layer of volcanic basalt, dozens of meters thick, that was laid down about 200 million years ago.

Parkinson suggests that the Johnsonville core doesn't contain this basalt because it was blown away by an impact that occurred between 90 million and 200 million years ago. The team is now conducting detailed analyses of the melt glasses in the sediments, which should provide a more specific date for one of the Piedmont's worst days in the last few thousand millennia.

So far, scientists have identified fewer than 200 impact craters on our planet. However, one look at the pockmarked moon–which shares Earth's orbit around the sun–suggests that many of our planet's scars have faded or remain hidden. Finding ancient craters and unveiling their geophysical histories will help fill in the blanks of Earth's continuing story.


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