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.
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 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).
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).
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)