Extraterrestrial Impact Evidence
This presentation discusses the types of evidence that geologists use to confirm extraterrestrial
impact sites.
Some geological features are ambiguous because they can be produced by meteorite impacts
and other geological processes, but some features are reliable indicators of shock metamorphism
and meteorite impact.
In 2010, Bevan French and Christian Koeberl published a paper entitled "The convincing
identification of terrestrial meteorite impact structures: What works, what doesn't and why".
The paper points out that meteorite impact events in the terrestrial geological record
were once ignored and even ridiculed.
They are now generally accepted based on the ability of geologists to unambiguously identify
meteorite impact structures and to distinguish them from similar structures formed by more
conventional terrestrial geological mechanisms.
This image shows a cross-section of half of an impact crater and the location of different
impact-produced lithologies.
Lithology is the study of the general physical characteristics of rocks.
To understand the description of impact processes, we will discuss some of the geologic nomenclature.
Curved lines in the image show the shock pressures in gigapascals produced by the impact on the
rocky target.
The greatest pressures occur at the point of impact in the center of the crater.
For reference, ten gigapascals correspond to a pressure more than 98 thousand times
greater than the pressure of the atmosphere.
These extreme pressures produce unique diagnostic shock effects.
Lithologies with shock effects tend to be restricted to materials in the crater and
in the rocks near the point of impact that experienced pressures of more than 10 gigapascals.
The final crater may be a relatively shallow geological feature that can easily disappear
by wind and water erosion.
Dikes and Breccias
A dike is a type of vertical rock between older layers of rock.
Technically, a dike is any geologic body that cuts across flat wall rock structures.
Breccia is rock composed of broken fragments of minerals cemented together by a fine-grained
matrix.
It is a conglomerate rock containing angular pieces.
Suevite is a rock formed during an impact event.
It consists partly of melted material, typically forming a breccia containing glass and crystal
or lithic fragments.
The word "suevite" is derived from the Latin name of Swabia, a region in southwestern Germany.
The term "Monomict Brecciated Basement" means a foundation layer of broken rocks, all of the same type,
cemented in a fine matrix.
A conglomerate where the clasts represent more than one rock type is termed polymictic,
while one where the clasts are of a single rock type are monomictic.
Here is a quick comparison of three rock types.
A conglomerate is a sedimentary rock formed from rounded gravel and boulder sized clasts
cemented together in a matrix.
Breccia consists of broken fragments of minerals cemented together by a fine-grained matrix.
It is a conglomerate rock of angular clasts.
Suevite is a rock consisting partly of melted material, typically forming a breccia containing
glass and crystal or lithic fragments formed during an impact event.
Pseudotachylite is a cohesive glassy or very fine-grained rock that occurs as veins and
often contains inclusions of wall-rock fragments.
Pseudotachylite is typically dark in color and glassy in appearance.
Pseudotachylite may form by frictional melting of faults, in large-scale landslides, or
by impact processes.
Pseudotachylite resembles tachylite, a volcanic glass formed by the rapid cooling of molten
basalt.
Microspherules are droplets of melted target rock that are generally produced during the
early stages of cratering and are immediately ejected at high velocities from the developing
crater.
Tektites are gravel-sized bodies composed of natural glass formed from terrestrial debris
ejected during meteorite impacts.
Microspherules and tektites can be deposited at great distances from the impact site.
Spherules cannot be used as independent evidence for meteorite impact events because they are
produced by volcanic eruptions and human industrial processes, such as welding spatter and fly
ash from coal-fired power plants.
Planar deformation features, or PDFs, are formed in quartz crystals at pressures from
10 to 25 gigapascals.
This image shows narrow planes of glassy material arranged in parallel sets that have distinct
orientations with respect to the grain's crystal structure.
A shatter cone is a fluted conical structure produced in rock by intense mechanical shock,
such as that associated with a meteorite impact.
Shatter cones are produced at pressures from 2 to 5 gigapascals.
French and Koeberl have a table of the features that are indicators of a meteorite impact
and features that may be produced by other geological processes.
Clear evidence of an extraterrestrial impact is provided by meteorite fragments, shatter
cones, planar deformation features and chemical signatures of extraterrestrial projectiles.
Inconclusive evidence includes circular crater morphology, brecciation and microspherules.
Now you know that breccia is not some kind of Italian pastry.
So, if someone invites you for breccia with cappuccino, you should only take the cappuccino.