GEOL 106 Lecture Notes - Lecture 41: Meteor Crater, Tunguska Event, Impact Crater
26 May 2018
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14.2 Airbursts and Impacts
- in atmosphere, asteroids, comets, meteoroids travel at 12 – 72 km/s
- composition varies
o some contain carbonaceous material
o others have native metals like iron and nickel
o others are stony, consisting of silicate minerals like olivine and pyroxene (common minerals
in igneous rocks)
- differentiated: have undergone igneous, and sometimes metamorphic processes as part of their
geologic histories
o stony meteoroids and asteroids: derived from asteroid belt between orbits of Mars and
Jupiter
- comets come from Oort Cloud
- he the iterset earth’s orit ad eter our atosphere, eteoroids, asteroids, oets udergo
changes as they heat up due to atmospheric friction during descent and produce bright light
o meteoroid entering atmosphere at 8k aoe earth’s surfae: ill eoe eteor ad eit
light → will then explode in airburst in atmosphere at between 12 and 50 km or collide with
earth
- meteorite: when meteoroid actually strokes earth
- many meteorites in Antarctica where they are concentrated because of particular meteorological
conditions
o e.g. 1908 large airburst in Tunguska event
CASE STUDY 14.2 THE TUNGUSKA EVENT
IMPACT CRATERS
- impact craters: more than 175 identified worldwide
- e.g. 49000 year old Barringer Crater in Arizona (Meteor Crater), well preserved, bowl shaped
depression with upraised rim, 1.2 km in diameter, depth of 180m
o rim is overlain by ejecta blanket (layer of debris)
o material fragmented by impact has fallen back into crater and some of walls have collapsed
o rocks that the asteroid hit were shattered and deformed, and angular broken pieces have
been naturally cemented or fused together to form a rock type called breccia
o asteroid that hit it was probably about 25 – 100m in diameter
- features that form at time of impact differentiate impact craters from craters from other processes
o impact involves extremely high velocity, energy, pressure, temperatures
o most of energy is form of kinetic energy → transferred to surface through shock wave that
propagates into earth → compresses, heats, melts, excavates earth material
o transfer of kinetic energy → produces crater
o shock can metamorphose some rocks in impact area while others are melted and mixed with
materials of impacting object itself
o most of metamorphism consists of high pressure modifications of minerals like quartz
o these modifications are characteristic of meteorite impact so they are extremely helpful in
identifying one
- two types of impact craters:
1. simple craters: small, few km in diameter, e.g. Barringer crater
a. experience vaporization, melting, ejection of material, formation of ejecta rims, later
infilling typical of simple craters
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