structure, volcanology

Meteorite Impact or Maar?

Looking at the two images above, they appear quite similar: both are of very large (~1 mile wide), nearly perfectly circular craters in the ground.

However, one was formed by a meteorite impact and the other is a maar formed by an explosive volcanic eruption. Think you can figure out which one is which?

From just looking at photos like these, it’s actually quite hard to tell.

The top photo is of Meteor Crater in northern Arizona. As the name gives away, it was formed by a meteorite impact ~50,000 years ago. Due to the immensely high shock pressures produced during a meteorite impact, the target rocks undergo a shock-metamorphic overprint. Shock-metamorphism allows for the unambiguous identification of impact craters, as there are no other natural phenomena capable of producing such high pressures. Rocks like shatter cones, microscopic features like planar deformation features (PDFs) and minerals such as coesite and stishovite are therefore diagnostic of an meteorite impact.


The bottom photo is of Crater Elegante at the Pinacate Volcanic Field in Sonora, Mexico. Crater Elegante is a maar—which is volcanic crater formed by a phreatomagmatic eruption (when groundwater comes in contact with hot lava or magma)— formed ~32,000 years ago. The schematic cross-section below shows the difference between a magmatic [left] and phreatomagmatic [right] eruption.

image

This water-magma interaction produces a steam explosion, which crushes the overlying rocks and launches them into the air. Ejecta fall back down and produce a layered tuff or tephra ring around the crater. Due to the high water table, the resultant crater will often be filled in as a lake. Pressures during these eruptions are not high enough to produce a shock-metamorphic overprint, so maars do not have the same diagnostic criteria as impact craters. Maars will often be associated with other volcanism in the area. For example, the Pinacate Volcanic Field has over 400 cinder cones and 8 maars total.

All images and illustrations by author

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