Crater Classification

by: Charles O’Dale

On this planet, impact craters are divided into three basic morphologic subdivisions: simple craters, complex craters, and peak ring craters. The transition size between simple to complex craters is 2km in sediments and 4km in crystalline rocks (Dence 1972). The transition size between complex to ringed basin craters is 10 to 50 km (Osinski, G. 2008). With increasing diameter, impact structures become proportionately shallower and develop more complicated rims and floors, including the appearance of central peaks and interior rings. While a single interior ring is required to define a basin, basins have been subdivided, with increasing diameter, on other planetary bodies, into central-peak basins, with both a peak and ring; peak ring basins, with only a ring; and multi-ring basins, with two or more interior rings (Wood and Head 1976). It is not known if there are examples of true multi-ring basins equivalent to those observed on the moon on Earth (Grieve 2006). A possible exception to this may be the Hudson Bay Arc, also known as the Nastapoka Arc, which I describe in a later article.

  • Simple Crater
A simple crater is a “transient” crater that has kept its bowl shape after the impact with minor slumping.
In the case of terrestrial simple craters, the true depth of the crater is measured to the bottom of a layer of shattered or “brecciated” rock under the floor of the crater. This layer is called a “breccia lens”. The depth to the base of the breccia lens (i.e., the base of the true crater) is roughly twice that of the depth to the top of the breccia lens (Grieve et al, 2002).

The Pingualuit Crater from 1500′ AGL. The simple crater is 3.44 km in diameter with a depth of 400 metres.
The 49 thousand year old Barringer Crater in Arizona has a diameter of 1.19 km and an apparent depth of 170 metres and is an excellent example of a simple crater. The true depth of the Barringer crater (to the base of the breccia lens) is approximately 300 metres (Melosh and Ivanov, 1999). The Barringer Crater’s “square” shape sub-classifies it as a “Jointed Crater”.
Another Jointed Crater was documented on the asteroid Eros.
  • Complex Crater

The central peak of the complex crater is formed as a result of uplift of material beneath the crater. Complex craters on Earth first occur at diameters greater than 2 km in layered sedimentary target rocks but not until diameters of 4 km or greater in stronger, more coherent, igneous or metamorphic, crystalline target rocks (Dence 1972).

The central peak of the complex crater is a rebound in response to impact compression and the release of a pressure overburden (Melosh 1989).
The 36.4 million year old Lake Mistastin impact structure with a diameter of 28 kilometres is a ‘central peak basin’ structure.(French 1998)
RADARSAT radar image of the Mistastin impact crater.
Tycho, a complex lunar crater, is approximately 160 million years old and has a diameter of 85 km (image courtesy of NASA).

There are examples of complex impact structures with central uplift on Earth, e.g., the Steinheim Basin in Germany or Jebel Waqf as Suwwan in Jordan; the latter is eroded by some tens to hundreds of meters but still exhibits the classical smashing morphology (Schmieder 2010). Since these craters are a bit “out of range” of my airplane, I will use the lunar crater Tycho as a complex crater example.

  • Peak Ring Crater

Peak ring craters develop within the rim of larger complex craters. The ring structure forms as the central peak collapses and creates a peak ring before all motion stops (Melosh 1989).
The 290 million year old Clearwater West Crater (illustrated to the LEFT of the complex crater, Clearwater East) is also an example of a surviving peak ring crater on this planet. The rim diameter is 36 km and the internal “peak ring” has a diameter of 10 km. An annular trough surrounds the ring.
This is the “peak ring” structure within the Clearwater West Impact Crater. The ring structure is 10 km diameter and 36 km circumference.
A side-note about the twin Clearwater Craters; there is a pair of craters, Ritter and Sabine, visible on the moon in the south west corner of Mare Tranquillitatis at 2°N latitude 19°E (lunar coordinates for Ritter Crater). Observing these lunar craters will give you an excellent perspective of the physical size of the Clearwater Craters as these twin craters are “almost” the exact dimension and orientation of the Clearwater Craters. In other words, an observer on the moon would see the twin Clearwater Impact Structures almost exactly as we see the Ritter/Sabine twin craters on the moon from our planet.

Anther peak ring basin on Earth is Ries crater (crater diameter 24 km, diameter of the crystalline ring 12 km), the ring is not visible very well due to the Ries lake sediments that cover large parts of the crater. It is a major structural feature that outcrops, e.g., beneath Nördlingen city.(Schmieder 2010)

  • Multi-Ring Crater
The Sudbury Structure comprises a 200-250 km multi ring impact basin formed at 1.85 Ga.