IMPACT GEOLOGY, CHEMISTRY AND PHYSICS

Contents

Introduction

The Physics of Impacts

The Chemistry of Impacts

The Geology of Impacts

A Brief History of an Impact

Suggested Reading

Some Interesting Websites

Introduction

When the Shoemaker-Levy 9 comet hit the planet Jupiter in 1994 the heat released could be detected by astronomers on Earth. It is thought that throughout Earth's history, there have been similar impacts by various comets and asteroids. The best documented case of recent years occured in 1908, when the Tunguska comet exploded just prior to an impact in Siberia, causing destruction over an area approximately 30km in diameter.

In photographs of the moon it is often quite easy to pick out the circular depressions which are thought to be a result of impacts. On Earth, such craters are more difficult to identify due to the effects of erosion and weathering, cover by vegetation or younger rocks and the destructive effect of plate tectonics. However, impact craters can be identified by consideration of not only their shape, but also by unique geological, chemical and physical characteristics.

To date, approximately 150 impact structures have been identified on Earth's surface. These have often been identified using remote sensing techniques and borehole evidence which allow us to see those rocks which lie below the surface.

Crater Locations

Impacting bodies range in size from a few centimetres up to a thousands of kilometres. It is only the largest bodies however, which lead to global catastrophes, initiating large scale changes in Earth's climate.

This document aims to summarise the various characteristics of impact craters, and show how these features can be used as evidence to identify an impact structure. It is also hoped that readers will gain an insight into what they could expect if a large extra-terrestrial body were to crash into Earth today.

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The Physics of Impacts

A disaster would begin to occur as soon as the comet, or asteroid entered Earth's atmosphere. These bodies, which have been calculated to travel at velocities of between 20km per second and 60km per second have so much energy that they create a huge fireball even before impact. As these hot bodies land they create a depression in the Earth, which is often far greater than the size of the objects themselves.

Crater Dimensions

The size and shape of the crater formed depends a number of factors:

From our knowledge of craters on the moon, an equation has been developed which allows us to estimate the the size of the crater that would be produced by an impacting body if we know details such as the the Kinetic Energy of the body.

Click on this star to see the equation

Comet and Asteroid Dimensions

Comets and asteroids range in diameter from a few centimetres, up to around a thousand kilometers - the size of a small planet! Small bodies landing on Earth generally only cause localised damage. A body would need to have a diameter of at least 1km to produce global effects.

Impact energies

Wherever the comet or asteroid lands, a vast amount of energy is produced, causing the meteorite to vaporise, and shock waves to move rapidly away from the impact site. Geologists believe that an impact such as that which formed the Chicxulub crater would be seismically equivalent to a magnitude 10 or 11 earthquake, and that the energy released would be at least 10 billion times greater than that produced by a nuclear bomb.

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The Chemistry of Impacts

Because the energy of the impact usually causes the impacting body to vaporise, fragments of the meteorite or comet are not generally preserved in the geological record. However,because these impacting bodies have a composition which is exotic compared to the rocks found on Earth, the meteorite can leave a unique chemical signature which can be detected in contemporaneous sediment deposits.

Element Anomalies

Iridium is an element which is usually barely detectable in rocks on Earth. However, it occurs in meteorites in in much greater concentrations. When a meteorite vaporises as it impacts with Earth, Iridium is dispersed around the planet. When geologists later study the rocks which were at the surface at the time of the meteorite impact, they can often detect levels of Iridium which are much greater than those usually seen in rocks. It is important to remember however, that levels of Iridium can vary greatly between asteroids, and comets in particular contain little if any Iridium. This means that impacts of extra-terrestrial bodies are not always associated with this chemical anomaly.

Isotope Ratios

In nature, there is often a discrimination between the heavy and light isotopes of elements such as Carbon, C, Oxygen, O, and Sulphur, S. The ratios between these light and heavy isotopes are often used to give chemists an insight into relative temperature changes and relative biomass changes. The Carbon-isotope ratios in marine rocks deposited at the time of a large comet or asteroid impact often change negatively. This would suggest that there was a loss in biomass ie. many organisms were dying. The negative change seen in the Oxygen-isotope ratios suggests that temperatures on Earth were suddenly increasing ie. Global warming was occuring. Changes in the Sulphur-isotope ratios indicate that the oceans were containing less oxygen - they were becoming anoxic.

Element Ratios

Iridium is not the only element which occurs in significantly different concentrations in comets and asteroids. In particular, the Rare Earth Elements are thought to occur in meterorites in certain ratios which are different to the ratios of these elements when seen on Earth.

Check the locations of the elements on this periodic table

Acid Rain

The shock waves produced as a result of the impact would reverberate both through the land and the atmosphere. It has been suggested that atmospheric shock waves could create large amounts nitric oxides. These are acids which when mixed with rainwater, could produce Acid Rain. The production of acid rain could also be enhanced if the impacting body were to land on carbonate containing rocks. If this were to happen, the impact may result in the release of carbon dioxide, which again could combine with rainwater producing acid rain.

Ozone Depletion

The atmospheric shock waves may also result in the production of forms of nitric oxides within the stratosphere, which may attack and rapidly destroy the ozone layer.

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The Geology of Impacts

As the impacting body forces its way into the Earth's crust, it brecciates and melts the rocks in its way. The largest impacts may result in atmospheric blow-out, where a hole is created in the atmosphere above the impact site. This means that impact-related material, which may become incorporated into the fireball, can be dispersed relatively quickly all around the globe.

Coarse ejecta blanket

As material is pushed away by the body it is ejected into the atmosphere. Larger blocks of material are too heavy to be carried far, and form a deposit known as an ejecta blanket surrounding the crater.

Tektites

Tektites are simply small glassy spheres of material. The glass is produced when rocks which are melted on impact, cool so quickly that crystals of minerals do not have enough time to form. The melted rock which forms the tektites is produced as a result of the high pressure and temperatures of impact. The energy of the impact causes some of the melted rock to be ejected from the crater where it coalesces into small spheres and then falls back down to Earth. The thickest deposits of glassy spherules are therefore found in locations closest to the impact site, however they can be transported as far away as 1000km from the crater.

Shocked minerals

The high pressure of impact, affects crystals in different ways. While some crystals are melted other crystals, in particular quartz and feldspar crystals, simply become shocked. Shocked crystals are identified by the fact that they contain sets of parallel lamallae. These lamallae are formed as a result of sudden changes to the atomic structure of the crystal.

High pressure minerals

Stishovite, a high pressure polymorph of the mineral Quartz, can only be formed at relatively high temperatures and pressures, (approximately 130,000 times the pressure at Earth's surface today). It has been identified at a number of impact sites, and is thought to form as a result of the sudden high pressures associated with an impact.

Suevite

Suevite is a particular rock type found in the impact crater itself. It is a brecciated rock which consists of fragments of local surface and basement rocks, mixed in with fine, ground up material. It is likely to contain examples of all the high pressure and temperature mineral features described above, such as shocked quartz, melted rock, impact glasses, aswell as deformed rock fragments. with

Tsunamites

Since approximately two-thirds of Earth's surface is covered by water, there is at least a 2:1 probability that any body would impact somewhere in the oceans. This would result in the displacement of huge amounts water and probably create tsunamis. These tsunamis, or high energy tidal waves, are likely to disturb coastal sediments leaving in their place, a highly chaotic and jumbled deposit called a tsunamite, which is much thicker than similar coastal deposits in other areas which are not affected by the tsunamis. Tsunamite deposits are usually only seen in locations proximal to the impact site. impact site.

The Creation of a Tsunami

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A Brief History of an Impact

These various physical, chemical and geological characteristics provide us with a good indication of what we could expect to happen if a large comet or asteroid were to land on Earth today.

Initially as the meteorite burns, there would be a huge fireball, which would probably cause local wildfires. These would be followed by earthquake tremors and tsunamis as the meteorite or comet actually impacts on Earth.

Enormous amounts of materials are ejected from the impact site, and due to the atmospheric blow-out effect, soon circulate around the globe. The thickness of this atmospheric layer, which consists of dust, ash and other debris causes the Earth to be plunged into darkness. A Nuclear Winter effect occurs as global temperatures fall due to the sun's heat being unable to penetrate this layer of material above the Earth.

As the dust settles, temperatures on Earth begin to rise again. However, due to the presence of excess Carbon Dioxide and various Nitrous Oxides in the atmosphere, the Earth undergoes a Greenhouse Effect. This is made worse by the depletion of the ozone layer allowing harmful UV rays to reach Earth's surface.

It has been estimated that these changes would affect Earth's climate for tens of years. The effect of these changes on Earth's flora and fauna appear obvious - a fight for survival.

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Suggested Reading

Benton, M.J. & Little, T.S., Impact in the Caribbean and the death of the dinosaurs,Geology Today,13,222-227, 1994.

Rampino, M.R., Haggerty, B.M., Pagano, T.C., A Unified theory of Impact Crises and Mass Extinction: Quantitative Tests, Annals of the New York Academy, 822, 403-431, 1997.

Gehrals, T. (ed.), Hazards due to Asteroids and Comets, University of Arizona Press, Tuscon, 1994.

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Some Interesting Websites

Terrestrial Impact Craters brought by the SML

Terrestrial Craters

Asteroid and Comet Impact Hazard

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