Crater Classification

On this planet, impact craters are divided into basic morphologic subdivisions:

simple: The transition size between simple to complex craters is 2km in sediments and 4km in crystalline rocks (Dence 1972).
complex: The transition size between complex to ringed basin craters is 10 to 50 km (Osinski, G. 2008).
peak ring: With increasing diameter, impact structures become proportionately shallower and develop more complicated rims and floors, including the appearance of central peaks and interior rings.
Multi-Ring Crater: Sudbury is a possible example.

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.

CLASSIFICATIONS

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”.

(A) HiRISE image of an unnamed simple crater on Mars (38.7° N, 316.1° E) displaying an elevated crater rim and steeply dipping upper cavity walls. The mid and lower parts of the wall are covered by talus deposits. (B) Kaguya/SELENE image (S0000001616_1906) of the complex impact crater Aristarchus on the Moon, showing a central peak, a fl at crater fl oor with isolated hummocks and an extensive slump-terrace zone. Note the different scale bars in the two images. (Collins et al 2012)

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 ~5 km diameter Gow structure with a **central peak,** is one of the smallest currently known complex impact structures on earth (Grieve 2006).

Lake Mistastin impact structure with a diameter of 28 kilometres is a ‘central peak basin’ structure.(French 1998)

The exposed core of uplifted rocks in complex meteorite impact craters. The central peak material typically shows evidence of intense fracturing, faulting, and shock metamorphism.

RADARSAT radar image of the Mistastin impact crater.

CENTRAL UPLIFT: Structural elevation (central peak) in complex impact structures. Originates from elastic rebound and transient-crater collapse in the modification stage of impact cratering.

Horseshoe Island, the **central peak** of the Mistastin impact crater.

This false-color image shows a green ring depression that surrounds a central peak. The ring depression contains the Manicouagan Reservoir. A fracture halo, which extends out to -150 km from the center, was first noted on Skylab photography. This halo is best developed in the west and south.

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).

Reflection seismic cross-section of Chicxulub along Chicx-A and -A1 (Bell et al. Forthcoming). The post-impact Tertiary sediments are clearly identifiable as high-frequency reflections from 0 to ~1 sec two-way travel time (TWTT). A topographic **peak ring**, with draped sediments, is identifiable on the floor of Chicxulub and separates the central basin from a surrounding annular trough. (GRIEVE et al 2003)

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.

CRATER SIZE vs PLANET/MOON

The depth to diameter ratio of craters smaller than a certain size is a constant, as predicted by the Maxwell Z-model. Below a break point (10 km for the Moon), the ratio follows a power law, decreasing as size increases [Hiesinger, 2006, Sharpton, 1994]. Source: [Hiesinger, 2006].

CRATER vs SIZE OF METEOROID

The cosmic projectile, meteoroid, asteroid, comet, or other celestial object which causes an impact event. The kinetic energy of an object of mass m traveling at a speed v is = mv²/2, provided v is much less than the speed of light.

The Earth is immersed in a swarm of Near Earth Asteroids (NEAs) capable of colliding with our planet, a fact that has become widely recognized within the past decade. The first comprehensive modern analysis of the impact hazard resulted from a NASA study requested by the United States Congress. This Spaceguard Survey Report (Morrison 1992) provided a quantitative estimate of the impact hazard as a function of impactor size (or energy) and advocated a strategy to deal with such a threat (Morrison, 2007).

Typical extraterrestrial impactor entering Earth’s atmosphere. Image from Igor Zh/Shutterstock.

HYDROTHERMAL

The origin and emergence of life under impact bombardment (Cockell 2006)

Craters formed by asteroids and comets offer a number of possibilities as sites for prebiotic chemistry, and they invite a literal application of Darwin’s ‘warm little pond’. Some of these attributes, such as prolonged circulation of heated water, are found in deep-ocean hydrothermal vent systems, previously proposed as sites for prebiotic chemistry. However, impact craters host important characteristics in a single location, which include the formation of diverse metal sulphides, clays and zeolites as secondary hydrothermal minerals (which can act as templates or catalysts for prebiotic syntheses), fracturing of rock during impact (creating a large surface area for reactions), the delivery of iron in the case of the impact of iron-containing meteorites (which might itself act as a substrate for prebiotic reactions), diverse impact energies resulting in different rates of hydrothermal cooling and thus organic syntheses, and the indiscriminate nature of impacts into every available lithology—generating large numbers of ‘experiments’ in the origin of life.

Darwin’s warm little pond—the impact crater as a prebiotic reactor. Some of the diversity of characteristics of impact structures that make them favourable sites for prebiotic reactions are shown.

Theories on the pathways of prebiotic evolution and the formation of the first complex self-replicating macromolecules have to take into account several common requirements, including: (i) a source of energy to drive molecular and macromolecular synthesis, (ii) a mechanism for the localized concentration of reactants to favour the required chemical reactions, (iii) suitable catalysis and (iv) a suitable geochemical environment for these reactions and their products to be sustained for sufficiently long periods to lead to the origin of life.