Astronomy Paper
8. Meteorites
What are impact craters?
Impact Craters are depressions on many planetary surfaces.
How are craters formed?
They are formed by the impact of chunks of interplanetary matter (meteorites) striking the surface.
Upon first impact, shock waves dissipate the energy of the meteorite through the ground. The ground is compressed rapidly and severely, and may be fractured, melted or vaporized
Next, the surface is decompressed, and the material is flung violently out from the impact site to form a crater
The materials displaced are collectively called ejecta.
As the shattered rock is flung out as ejecta, producing a crater, the crater rim is also pushed up.
The number of craters that form depend on the number and velocity of incoming fragments
Velocity is moderated by the presence of an atmosphere
Planets or moons that lack an atmosphere have the most energetic impacts
Evidence to identify old impact crater sites:
Area is sparsely vegetated and tectonically stable
Spotted in satellite photographs
Do not mistaken certain circular features like calderas, or plutons as impact craters
If a crater is young, meteorite fragments may be found nearby
Passage of shock wave induces shock metamorphism in the rocks and minerals of an impact crater
Shock metamorphic effects include features like:
Production of distinctive fracture patterns in the rocks
Disruption of crystal structures
Production of ultra-high pressure polymorphs
Impact melts may have been produced
Distinguished from volcanic glasses
May incorporate shock minerals, high-press polymorphs, or bits of nickel-iron from the meteorite itself
They tend not to be very homogenous, and have bulk compositions similar to those of country rocks.
Fortunately, few large meteorites strike Earth
Only a few reports have been confirmed about meteorites striking people
No one has been killed
When did dinosaurs become extinct?
Mass extinctions occurred during the Mesozoic/Cenozoic era
What are impact craters?
Impact Craters are depressions on many planetary surfaces.
How are craters formed?
They are formed by the impact of chunks of interplanetary matter (meteorites) striking the surface.
Upon first impact, shock waves dissipate the energy of the meteorite through the ground. The ground is compressed rapidly and severely, and may be fractured, melted or vaporized
Next, the surface is decompressed, and the material is flung violently out from the impact site to form a crater
The materials displaced are collectively called ejecta.
As the shattered rock is flung out as ejecta, producing a crater, the crater rim is also pushed up.
The number of craters that form depend on the number and velocity of incoming fragments
Velocity is moderated by the presence of an atmosphere
Planets or moons that lack an atmosphere have the most energetic impacts
Evidence to identify old impact crater sites:
Area is sparsely vegetated and tectonically stable
Spotted in satellite photographs
Do not mistaken certain circular features like calderas, or plutons as impact craters
If a crater is young, meteorite fragments may be found nearby
Passage of shock wave induces shock metamorphism in the rocks and minerals of an impact crater
Shock metamorphic effects include features like:
Production of distinctive fracture patterns in the rocks
Disruption of crystal structures
Production of ultra-high pressure polymorphs
Impact melts may have been produced
Distinguished from volcanic glasses
May incorporate shock minerals, high-press polymorphs, or bits of nickel-iron from the meteorite itself
They tend not to be very homogenous, and have bulk compositions similar to those of country rocks.
Fortunately, few large meteorites strike Earth
Only a few reports have been confirmed about meteorites striking people
No one has been killed
When did dinosaurs become extinct?
Mass extinctions occurred during the Mesozoic/Cenozoic era