A world of impact craters

Posted by on July 10, 2013 in Igneous Rocks | 0 comments

If you want to talk about a relatively new topic to investigate pick the subject of impact craters.  Impact craters have been talked about for many years but it wasn’t until the second half of the 20th century that several were located in North American and research began.  After all, we have great examples of impact craters to study for comparison as seen on the Moon, Mercury, Venus and Mars.  A view of the Moon through a pair of binoculars or telescope will allow you to see the composition of an impact crater.

The Moon only has 1.6th of the gravity as we have on Earth so meteors or asteroids falling toward the Moon are not burned up in the atmosphere as here on Earth.  Although we have not observed any new craters formed on the Moon, impact crater history is well established.  If you take a look at a crater, you will see the crater with a ridge around the outside of the rim.  You may also see a central peak in the middle, caused by a rebound of material splashing up after the impact.  With many craters, you may be able to also observe material thrown outward of the crater known as ejecta.  Not observed unless you were able to visit the area and explore the material in and around the crater would be brecciated (broken and later rehealed) rock and shocked quartz.

Ok, let’s put the above paragraph into perspective to an event that we all have witnessed or created.  How many of us have thrown a rock into a lake or pond?  What a treat to see how far we could throw it or perhaps how many times could we skip the rock across the water.  But, did you observe the reaction with the water as the rock hit the surface.  A “crater” forms in the water at the instant the rock hits the surface; water is   rebounded outward next to the “crater” and a small dimple of water is shot into the air where the rock entered the water.  The size of the rock would influence how high and far the ripple marks went across the water (shock waves).  Perhaps the angle that the rock entered the water influenced some of the above features.  This principle also pertains to impact craters on any planetary body including the Earth.

As geologists began to recognize the present-day features of earthly impact craters, more of these structures began to be identified.  Scientists began to realize that these events left more scars on our landscape more than earlier thought.  Suddenly there were a somewhat lengthy list of confirmed crater and another list that have yet to be confirmed.  Any yes, how those impact crater that are yet to be discovered.  At least on the East Coast, any craters that were formed  more than 350 million years ago may have been altered or destroyed by later continental collisions or riftings.

A list of known and unconfirmed craters in North America is listed at http://en.wikipedia.org/wiki/List_of_impact_craters_in_North_America.  Links to the individual crater with more information can be followed by clicking on the crater name.

I would like to highlight several of the confirmed craters.  Tennessee actually has two craters.  The most famous is lynnCreek Crater located near Gainesboro.  The crater was created about 360 million years ago (mya).  The  crater is a shallow saucer-shaped depression measuring 3.8 km in diameter.  Depth is 150 m with  a large central cone.  The limestone and dolomite laying in the center of the crater have been thrown upward about 300 m.  It is estimated that the body striking the crust here was traveling 11.2 – 72 km/second.  The Geological Society of America has published a report about this crater.

The second impact crater in Tennessee is the Wells Creek Crater.  The crater is 12 km in diameter and formed during the Jurassic Period (200 mya or another surrounding geologic period.  What this crater is known for is the shatter cones found in the rocks within the crater.  The finest examples of these in the world have been identified from here.  These cones as they appear in the rock were created by the shock waves passing through the bedrock, fracturing the rock in a patterned “cone” shape.

Moving to Ohio, the Serpent Mountain Crater is located in Adams, Highlands and Pike counties.  The crater is about 8 km in diameter and the best date it was formed is placed less than 320 mya.  There is a central peak, transition zone and a trough-shaped rim present.  The bedrock has been brecciated and rehealed with minerals due to the impact.

The closest impact crater to us is known as the Chesapeake Bay Crater.  Located in the southern reaches of the Chesapeake Bay in Virginia, this crater was formed 35.5 mya.  The crater is 90 km in diameter and 1.3 km deep.  The crater was discovered in  1983 when ejecta material was observed in well borings off the coast of Atlantic   City.  In 1993, oil exploration in the area of the crater confirmed its presence.  Presently, researchers are drilling into the center to establish if the cosmic body is still present or if it vaporized upon contact.  Certainly if you were in York County 35.5 mya, that would have been the leading  headlines in the newspaper the following day.

Finally, the leading theory to why the dinosaurs became extinct was a asteroid collision.  It is believed that the 170 km diameter Chicxulub Crater in the Yucatan was the main focus of the extinction.

Sometimes, the body striking the Earth is referred to as bolides.  There are several different definitions of bolides in both astronomy and geology.  In astronomy, a bolide is a meteor that breaks into smaller fragments.  In geology, a bolide is a body that strikes the Earth (a meteorite).

Watch out for a future blog on a local cosmic body known as the Shrewsbury Meteorite.

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