Terrestrial Impact Craters

Impact craters are common geological structures formed by fast moving micro-meteors, meteors, asteroids or comets crashing into bodies larger than themselves. All the static planets, moons and asteroids in our solar system clearly record heavy bombardment in the past, and that bombardment continues today. On evolving planets such as the earth, craters are continually being buried by sedimentation and volcanic activity, or erased by erosion. That makes Terrestrial Craters a little harder to identify, but there remains far more evidence of impact crater existance [such as Magnetic Field Reversal, and Far Side Lava Flow] than is generally taught.

Please click here for special information on Impact Vocabulary.

Impact results vary with size, velocity, approach angle and surface geometry at the impact site, atmosphere and gravity of the impacted body, and composition [structure, integrity, volume, density, elements present, etc.] of impactor and impactee. Pressure developed in the impact is a product of mass and velocity mitigated by the other factors. As pressure increases a continuum is formed consisting of breccia, breccia with fused edges, tektites, impact melt, and impact condensates [Impcons].

I. Size: Impactors come in all sizes. Even galaxies collide.

II. Velocity: Meteoroids traveling in a contra-earth orbit [as the Leonids] boring straight down may impact at 180,000 miles per hour, or if traveling close to and parallel with the surface of the earth, as slow as 75 mph , or even slower if reduced to dust or smoke. Terminal velocity for most meteoroids is between 200 & 400 Mph. [See Meteorite Faq Sheet]

III. Angle and Surface Geometry: If the meteoroid falls straight down, all the pressure is directed straight down, and the ejecta will fall in and around the crater. A steep angle directs the pressure to one side, and much of the ejecta will fall in an ejecta blanket in the direction of travel. A shallow angle allows much of the pressure to be deflected away from the site.

IV. Atmosphere: Slows down and heats up meteoroids, cools down and changes the disbursement of ejecta, and alters the impact structure and chemical composition of the impact minerals over time.

V. Gravity: Among other things, gravity determines the transition diameter between simple and complex craters. on the moon [1/6th Earth's gravity] the transition diameter is 9 to 12 miles. On the Earth the transition is 1 1/4 to 2 1/2 miles. This progression in crater structure is seen throughout the solar system, with a lower diameter range for each structural type as the size and gravity of the impacted body increases.

VI. Composition: Obviously a hundred tons of solid crystalized copper will react differently than a hundred tons of mixed unconsolidated ices on a long trip through our atmosphere and impact. The unusual minerals in meteoroids [unique to each original body before impacting each other] may be altered by oxidation and other chemical reactions, but only the most extreme erosion will remove all signs of their presence.

On earth, the features left by impacts range from: 1. Skid Marks left by slow moving meteoroids as they slid to a halt. They are not to be confused with wind blown rocks that slide on mud in storms.

2. Simple Craters are bowl-shaped depressions up to 2½ miles in diameter. They are relatively small with depth-to-diameter ratios of about 1:5 to 1:7 and characterized by an outer ring with uplifted inner rim. They are partially filled with and surrounded by small sharp edged rocks from the meteoroid and country rock shattered in the impact called breccia. In high speed impacts the quartz in the breccia can be so severely shocked they will be altered into the high pressure form known as coesite or even stishovite. The ground beneath the crater will be compacted in a Compression Cone. Increasing mass and/or speed will produce progressively larger crater diameters.

3 Complex Impact Craters have a shallower depth compared to diameter [between1:10 and 1:20] and contain central peaks, an outer tipped up rim, and overturned rim rocks. This transition occurs in sedimentary rocks when diameters become greater than 1¼ miles and in crystalline rocks when diameters become greater than 2½ miles. Complex craters in crystalline rocks often contain sheets of impact melt on top of the shocked and fragmented rocks of the crater floor. Gravity causes the initially steep crater walls to collapse, pressing the melt into a lense shape. Fractures in the sides and bottoms of the crater are known as dikes. Material injected between separated layers of sedamentary rocks are called sills. Dikes and sill are filled with breccia and melt from the impact. These are "primary minerals." After weathering, the surface materials alter into "secondary minerals."

4. Impact basins are even larger diameter sites where one or more peak rings, [probably caused by separate fragments of the impacting body] replace the single central peak. The outer rim is slumped and surrounded by an annular trough.

5. Impact Crater Complexes have related features such as multiple impacts, impact chains, associated synclines, rifts, sand jets, percussion cones, etc.

6. Carrot Shaped Craters many times deeper than their diameter are made by ultra-high speed meteoroids going so fast that only the outer shell has time to vaporize as the meteoroid penetrates the earth. The craters are filled with extreme pressure minerals. Examples are the Kimberley Diamond Mines of South Africa.

7. Magnetic Field Reversals and Far Side Lava Flows: Some impacts are so violent that the far side of the target planet bulges outward. Such a collision is assumed to have pressed the Moon's core off center so the same area always faces Earth. Some impacts on Earth have been recorded by reversing the planetary magnetic field, and others were so violent that Sheet lava was forced out of the ground on the far side of the planet from the impact site.

So, how do you recognize an impact crater on earth? Study a couple dozen impact craters. Compare them with volcanoes and salt domes, then you will never confuse them again.

A. ERODED CRATER IDENTIFICATION

On Earth, erosion and weathering alter the surface appearance of the crater as rims and ejecta blankets are washed away. Concentric ring structures emerge as weaker rocks are removed. The more resistant melt sheet and compression cone may be left as a plateau overlooking the surrounding area. The items to look for are:

1. The remaining impact deformed strata: Breccia, Compression Cones, Impact Melt, Geodes, Impact Condensates, Impact Dikes and Sills, Metamorphic Rocks, Granite Plutons, and signs of Hot Springs activity.

2. The presence of unusual minerals. The forces that gave birth to this planet assured an orderly distribution of the elements. The heaviest at the center, the lightest on the outside. ALL the original gold, silver, uranium, etc. is now under hundreds of miles of iron! Anything outside that framework accreted from outer space. All the gold from India, Egypt, the Gold Coast, South America, All of it came later from space. All the tin from the "Tin Islands," all the uranium and plutonium dug from the Chinle Formation, and the Morrison Formation came from space. The gold, silver, platinum, copper, molybdenum being mined by the Kennecott Corporation in Canada, Montana, Nevada, and California came from a chain of meteor impacts.

3. The presence of shock metamorphism. Some shock metamorphic effects have been shown to be unique to meteorite impacts; no other geological mechanism - including volcanism - produces the extremely high pressures that cause them. They include: conical fractures known as shatter cones, microscopic deformation planar features in quartz and feldspar grains, diaplectic glass, and high-pressure mineral phases of quartz such as keatite, coesite and stishovite. rocks melted by the intense heat of impact.

4. The presence of extremely high heat and low pressure minerals such as lussatite, formed immediately after the impact.

B. BURIED CRATER IDENTIFICATION

When an impact take place in a lowland area of soil deposition, the crater bowl may be filled with water and become a lake that fills with sand, or an evaporation pond that fills with salt. It may be surrounded with lush plant life and later fill with gas and oil. It may be deeply buried and covered from sight, but it still exists, and can easily be detected by magnetic mapping. The hardened bowl shape will outlast the newer soils when the area is later uplifted to be eroded away in turn. In the meantime, The material deposited by the meteoroid may have economic value, like the copper-nickel deposits at Sudbury Canada; or the copper-gold-silver of the Kennecott properties in Montana, Utah, and Nevada; or the copper-lead-zinc of Joplin Missouri.