by Fred Bervoets

Some 65 million years ago the Yucatan Peninsula was hit by a metorite creating a 180-300 km diameter crater. Enormous ammounts of material were thrown into the atmosphere, blocking the sunlight for more than a year, because of that the earth\'s temperature drastically cooled and photosynthesis decreased or was even prevented.

As a result of the schockwave tsunamis hammered Atlantic shores. This impact happenend at a time that drastic climate changes marks the abrupt demise of a large number of biological species including dinosaurs. An interesting fact about the Chicxulub crater is that you cannot see it. Its circular structure is nearly a kilometer below the surface and was originally identified from magnetic and gravity data.

Impact craters are geologic structures formed when a large meteoroid, asteroid or comet smashes into a planet or a satellite. The explosion and excavation of materials at the impacted site created piles of rock, termed ejecta, around the circular hole as well as bright streaks of target material, termed rays, thrown for great distances. Another basic method that form craters in nature is when the top of a volcano collapse, termed caldera.

All the inner bodies in our solar system have been heavily bombarded by meteoroids throughout their history. The surfaces of the Moon, Mars, Mercury and Venus record this bombardment clearly because unlike those on Earth, there has been very little deformation of the craters due of the minimal geological activity.

The Magellan spacecraft found about 150 impact craters on Venus (about the same number as on Earth) the range of crater diameters is also similar. It is now clear from planetary bodies that have retained portions of their earliest surfaces that impact was a dominant geologic process throughout the early solar system.

There are many more impact craters on Mars than there are on Earth or Venus, but not nearly as many as there are on the Moon which far side shows an enormous contrast with the Earth as it is saturated with impact craters. Many of the craters on Mars have been partially eroded or filled in with lava, volcanic ash, windblown sediments, or sediments carried by ancient streams. One of the larger impact craters on Mars has a diameter of 1,100 km!

Many surface features on Earth may look like impact craters, however their origin is different and sometimes even unknown. On the coastal plains of Georgia and Virginia, eastern United States, is a group structure of up to 500,000 elliptical bogs and bodies of water, called the Carolinas.Many theories for their origin have been proposed among which a theorie that the bay was formed from the impact of numerous meteorite fragments. This hypothesis however is verry unlikely because the fact that there are so many bays and not a single meteorite fragment has been recovered.

Volcanic structures may also have the same appearance as impact craters, a good example is Crater Lake (in Oregon) a bowl-shaped depression that was formed when the volcanic peak collapsed inward.The more than 150 impact craters on Earth are almost all recognized since 1950. The first recognized terrestrial crater was the Meteor Crater also called Barringer Crater, with a diameter of 1.186 km, in the 1920\'s in Arizona, U.S.A and is only 49.000 years old.

The oldest known crater on Earth is called Vredefort located near the center of the Witwatersrand Basin, about 100 km from Johannesburg. Its estimated age is 2-billion-years. One estimate of its original diameter is 300 km, which would make it one of the largest craters on the planet. Other estimates place it at 140 km. This crater has a central core about 40 km in diameter, composed of old crystalline rocks that is surrounded by a deformed collar of uplifted and overturned younger sediments and lavas. Much of the structure is buried by younger flat-lying sediments.

Onother large still preserved impact crater on Earth is called Manicouagan, its located in Quebec, Arizona. This crater is approximately 210 million years old, with a diameter of 70 km there are however indications that approximately its orginally diameter was 100 km. Not every impact creates a crater. A 60 to 90 meters large metorite exploded in 1908 eight kilometers above the ground of the Tunguska region of Siberia. Trees were incinerated in a 914.4 km radius from ground zero and were knocked over in a 40 km radius. On Earth twin impact craters are verry rarely, a good example of such an impact are the Clearwater Lakes caters in Quebec, Canada, aged 290 ± 20 million years old. They are called Clearwater Lake West with an original rim diameter of 32 km, and Clearwater Lake East with an original rim diameter of 22 km.

The smallest impact craters are bowl-shaped formed, but the bigger craters shows a slumping of the inner walls and rebounding of the depressed crater floor they have a progressively larger rim terracing and central peaks. At larger diameters, the single central peak is replaced by one or more peak rings, resulting in what are generally termed impact basins. This same progression in crater morphology is observed throughout the solar system, including on the Earth; although, terrestrial craters are less well-preserved and more challenging to classify. One notable difference between lunar and terrestrial impact craters is the lower diameter range for each morphological type on Earth. This difference is due to the higher gravity on Earth.

Since the 1960s, numerous studies have uncovered another physical marker of impact structures, shock metamorphism. Certain shock metamorphic effects have been shown to be uniquely and unambiguously associated with meteorite impact craters; no other earthly mechanism, including volcanism, produces the extremely high pressures that cause them. They include shatter cones , multiple sets of microscopic planar features in quartz and feldspar grains, diaplectic glass , and high-pressure mineral phases such as stishovite . All known terrestrial impact structures exhibit some or all of these shock effects.

Impact craters on Earth are divided into two groups based on morphology, the ones with a diameter up to 4 km, these have uplifted and overturned rim rocks, surrounding a bowl-shaped depression and partially filled by breccia, fragments of meteortes are found only at the smallest craters and they are quickly destroyed in the terrestrial environment.

The second and more complex impact structures are generally 4 km or more in diameter, and have a distinct central uplift in the form of a peak and/or ring, an annular trough, and a slumped rim. The interiors of these structures are partially filled with breccia and rocks melted by the impact. In massive events caused by a large impactor, tremendous pressures and temperatures are generated that may vaporize the meteorite altogether or may completely melt and mix it with melted target rocks. The diameter at which craters become complex forms depends on the surface gravity of the planet: The greater the gravity, the smaller the diameter that will produce a complex structure. On Earth, this transition diameter is 2 to 4 km (depending on target rock properties); on the Moon, at 1/6 Earth\'s gravity, the transition diameter is 15 to 20 km.

An impact event with energy greater than the world\'s nuclear arsenal occurs on a time-scale of less than a million years. From estimates of the terrestrial cratering rate, the frequency of K-T-sized events on Earth is of the order of one every 50-100 million years. Smaller, but still significant impact events, occur on shorter time scales and will affect the terrestrial climate and biosphere to varying degrees.

The formation of impact craters as small as 20 km could produce light reductions and temperature disruptions similar to a nuclear winter. Such impacts occur on Earth with a frequency of two or three every million years. The most recent known structure in this size range is Zhamanshin in Kazaksthan, with a diameter of 15 km and an age of 1 million years. Impacts of this scale are not likely to have a serious affect upon the biosphere and cause mass extinctions

Earths impact craters form an extremely important resource in understanding the impact phenomena. They provide important data on the structure of such landforms in all three dimensions. Two of the larged sized terrestrial impact craters requires orbital imagery and observation to provide an overall view of their structure and large-scale context.

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