Meteorite Impacts Ms. Nelda Breedt GLY 162 Environmental Geology 2 Meteorite Impacts Meteorite Fragment of extra-terrestrial material that strikes the surface of the Earth. Meteoroid Before hitting the ground On entering the atmosphere: They are heated by friction Surface become ionized and glows Meteors: the visible path of a meteoroid (falling stars) Played an important role in the geological history of the Earth 1
3 Meteorites Traditional classification scheme: Stony igneous rocks Irons Alloys of nearly pure Ni & Fe. Stony-irons mixtures of stony & metallic material. Origin: Fragments of larger bodies. Meteorites 4 Stony meteorites: Chondrites: has not been modified by melting or differentiation formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids c. 80% of all meteorites on earth are chondrites. Achondrites: similar to terrestrial basalts or plutonic rocks. has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies. 2
Meteorites 5 Iron meteorites: thought to be the fragments of the cores of larger ancient asteroids that have been shattered by impacts. metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans. 5.7% of all meteorites on earth. Meteorites 6 Stony-iron meteorites (Pallasite): Olivine crystals in an iron-nickel matrix. Probably impact-generated mixtures of core and mantle materials of an asteroid 1.8% of all meteorites on earth 3
7 Asteroids Belt of 100,000 small, irregularly shaped rocky bodies orbiting the Sun Remnants of a broken up planet or rather fragments that failed to come together into a planetary mass Total mass not even equal to the Moon Some have Earth crossing orbits Largest known = 8 km in diameter Asteroids 8 4
Asteroids 9 Asteroids 10 5
11 Sources of Meteorites Are Asteroids parent bodies of Meteorites? Parent bodies of iron meteorites must be large enough for minerals to cool slowly in the core Collisions & fragmentation of asteroids provide mechanism for breaking a differentiated body apart Collisions serve as mechanism to kick out asteroid fragments from normal orbit, yielding the Earthapproaching group of Apollo asteroids; trajectories of recovered meteorites originate in/near asteroid belt 12 Sources of Meteorites Asteroids are the main source of most meteorites Other sources: Identical samples of the Moon and Mars Broken off pieces from impacts of other bodies Unusual group originating from comets 6
Comets Bright object with long, wispy tail pointing away from the Sun. Not visible until they are as close as Mars. Head: central nucleus (rock, dust, water ice, and frozen gases such as CO, CO 2, methane and ammonia) surrounded by bright coma (temporary atmosphere). Near the Sun icy material in nucleus is vaporised It streams away driven by the solar wind Carrying dust particles along. Orbit highly eccentric Source of some objects striking the Earth s surface 2 tails: gas dust 13 14 Comets Halley s comet Dust tail = white Gas tail = blue 7
15 Comets Impact Events 16 Impact crater: bowl shaped depression Surface deformation due to transfer of energy 8
180m GLY 162 17 Meteorites Meteorite flux = 10 7 to 10 9 kg/a 100 s of tons per day! Atmospheric friction cause small objects to disintegrate and burn before reaching the surface. This and erosion processes causes the Earth s surface to be less cratered than on the Moon 18 Meteorites Larger objects (>1m) strike Earth at 1/a Meteorites > 100m to km s: < 1 in 10 6 years. Small meteorites measured at 4 40 km/s. 1500m If 30 m meteorite strikes Earth at 15 km/s the energy released would be equal to 4 million tons of TNT (Hiroshima: 13,000 tons of TNT) Resulting crater 1200m across, 200m deep. Equal to Meteor Crater in Arizona 9
19 Terrestrial Impact Craters > 200 identified on Earth Must be more Moon heavily cratered Early in history a period of intense meteorite bombardment occurred. Few craters remained on Earth due to weathering and erosion and erasure by the rock cycle processes. Only very large, very recent craters are preserved 20 10
21 Impact Cratering Very rapid process; at high-speed impact: Jet of rock and dust (ejecta) away from point of impact Blanket surrounds crater - thinning out from rim Compression of underlying rock Intense shock wave exceeds rock strength: Large volume of crushed & brecciated material Decompression takes place: Decompression may cause melting. Molten rock may flood crater floor Layers adjacent to rim are overturned Impact Cratering 22 11
Impact Cratering 23 24 Impact Cratering 12
Impact Cratering 25 Aorounga impact crater, Chad Mistastin Lake, Canada Pseudotachylitic breccia 26 13
27 Tektite: natural glass up to a few centimeters in size formed by impact of large meteorites. 28 Effects of Impacting In a single human lifetime there is a 1:10,000 chance that the Earth will be hit by a meteorite large enough to cause worldwide climatic changes Same as dying from anesthesia during surgery or dying in a car crash in a six month period. Major impact would destroy the immediate area, but will also have devastating regional and global effects 14
29 Effects of Impacting Mass Extinctions: There may be a direct link between major impacts and the mass extinctions of species during the Earth s history Events will leave characteristic signatures such as soot (impure carbon particles) or Ir layers - such as at the K-T boundary Many pieces of evidence have been located About 12 extinctions during the past 250 Ma 30 Effects of Impacting Mass Extinctions: Well known at 65 Ma dinosaurs + 25% of animal species became extinct Impact crater at Chicxulub Yucutan Peninsula, Mexico 180 km diameter, 65 Ma Meteorite: 5 10km diameter Initial impact survivors died later from cold and hunger 15
Effects of Impacting 31 32 Effects of Impacting Global darkness Fine dust will cause darkness across the entire Earth for several months. Collapse of the food chain. Worldwide deposition of thin blanket of sediment 16
33 Effects of Impacting Cold Intense cold accompany darkness. Dust will block out radiation from Sun An ocean impact will cause a large amount of water vapour to be distributed throughout the atmosphere Water vapour is an important greenhouse gas. This will cause a period of extreme global warming following the cold period 34 Effects of Impacting Other effects: Energy and shock heating may cause N and O in the atmosphere to combine to create nitric acid results in acid rain and acidification of surface waters Tsunamis may also occur Earthquakes This will be devastating to life 17
35 Effects of Impacting 36 Near Misses 14-06-1968: 1566 Icarus passed within 60 km 10-04-1972: Rocky object approached the Earth at 10km/h, accelerated to 15 km/h due to gravitational attraction, travelled over North America and dipped to as low as 50-60 km above surface before bouncing back into outer space There may be more than 300 000 Earth approaching objects with ø > 100 m 18
RSA Tswaing Crater 37 38 RSA Tswaing Crater Diameter - 1.13 km Well preserved rim of up-tilted granite Rim is elevated 60 m above surrounding plains Elevation of rim above crater floor 119 m Preserved rim indicate a young age for the crater (220,000 years) Consist of pinkish Nebo Granite (Bushveld Complex 19
39 RSA Tswaing Crater 40 RSA Tswaing Crater For much of this century a controversy raged over the origin of the crater: Volcanic Meteorite impact Described in 1868 as a unique feature of volcanic origin In 1933, German geologist Rohleder, suggested an impact crater 20
RSA Tswaing Crater 41 42 RSA Tswaing Crater Crater age as determined by radiocarbon dating of the sediments and the age of volcanic glass in the breccia indicate an age of 220 000 years. 21
43 RSA Vredefort Dome 3-3.4 Ga 70 Km Diameter Meteorite ~10 km in diameter Original crater ~300km in diameter 70 km 3 of rock vaporized on impact 22