Impact sites

Thousands of large objects, meteors and comets, orbit the Sun. Meteors are made of stone or metal, and comets are made mostly of ice (which vaporizes when the comet is close to the sun, forming a tail). Most meteors and comets do not normally approach the Earth, but the gravitational pull of other objects in space can upset their orbits, causing them to move into the inner solar system. From time to time, large ones strike the Earth, and they are moving so fast that they leave holes or craters in the Earth that can be miles across. One of the most recent craters is near Flagstaff, Arizona. The crater, more than a kilometer (4000 feet) across and "only" 50,000 years old, was caused by a house-size iron-nickel meteor that hit the earth going about 18,000 meters per second or 40,000 miles per hour.

Students: Find out how much energy is released when a meteor strikes the Earth!

Erosion (the action of wind and rain) and geologic forces usually destroy impact craters that form on the Earth's surface. This is why we do not have as many visible craters as, for example, the Moon. Some craters are destroyed by the action of the Earth's tectonic plates, as mountains form or seafloor descends beneath the Earth's crust. Others are eroded away by gulleys and streams. Some, however, are buried in sediment (sand, gravel, or mud deposited by rivers). One of the oldest visible craters on the Earth is located in New York's Catskill Mountains, only 150 kilometers (100 miles) from New York City.

Panther Mountain

In the early part of the 20th century, scientists noticed an unusual circular pattern, 10 kilometers or 6 miles across, in New York's Catskill Mountains. The circular feature is formed by the valleys of two streams, the Esopus and Woodland Creeks, each of which traces a part of the circle. The streams, made more visible by fields, roads, and buildings along their banks, can be seen in the accompanying true-color satellite photograph. The streams flow into New York City's Ashokan Reservoir.

The other two images give us different ways of looking at the circular feature. One is an infrared picture, which shows warmer (valley) areas as lighter shades of red. The third image is a digital elevation model of the Pather Mountain area, in which the color indicates how high the ground is. Most people think impact sites are big holes. Because of events that happened after the impact, Panther Mountain is just the opposite! This picture also shows the contrast between the circular feature and the surrounding drainage patterns.

The fact that the streams follow a circle is very unusual and led scientists to seek an explanation. Circular drainage patterns can be caused either by volcanoes, domical uplift (in which geologic forces gradually cause the ground to rise in the shape of a low dome), or by impact craters. Panther mountain, because it is made of sedimentary rock (the kind of rock that forms from layers of sediment deposited by a river), could not be a volcano. Also, there is no evidence for domical uplift. And on the surface, the site doesn't look like an impact crater.

Scientists Yngvar Isachsen, Stephen Wright, and Frank Revetta (reference note) think the circular feature might be the result of an impact crater buried under the ground. Their hypothesis (a scientific explanation that can be tested) is that a meteor or comet struck the Earth about 400 million years ago. At that time, there were no Catskill Mountains, just a shallow sea that was slowly filling up with sediment. There was very little life on the land, and dinosaurs had not yet appeared.

Because sediments were being deposited in the shallow sea, the crater was soon buried and preserved, like a fossil, instead of eroding away. Each layer of silt or sand turned to rock as more layers piled up on top of it. Over time, the fractured rock beneath the crater settled a bit, causing the layers of sedimentary rock above the crater to form cracks, or joints, around the edge of the circle. These joints made it easier for streams to wear away the rock. Thus, the streams have carved out a circular outline even though the crater itself remains completely buried.

Where is Panther Mountain?
Evidence for the buried crater
The Catskill Center's virtual hike of Panther Mountain

The scientists speculate that the fractured rock in the crater itself could act as a reservoir for natural gas. The Panther Mountain crater may intersect rock layers that produce natural gas in other parts of the state, and similar craters have been tapped for fuel. A circular feature on the Yucatan Peninsula was extensively drilled by researchers at Petroleos Mexicanos, and subsequent research revealed that the 150-mile-wide structure is a buried impact crater. The Chicxulub crater, as it is called, formed 65 million years ago. Much more recent than the hypothesized Panther Mountain crater, it could be the long-sought explanation for the extinction of the dinosaurs.

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