Complex crater

Above a certain threshold size, which varies with planetary gravity, the collapse and modification of a transient cavity is much more extensive, and the resulting structure is called a complex crater. The collapse of the transient cavity is driven by gravity, and involves both the uplift of the central region and the inward collapse of the rim. The central uplift is not the result of elastic rebound, which is a process in which a material with elastic strength attempts to return to its original geometry; rather the uplift is a process in which a material with little or no strength attempts to return to a state of gravitational equilibrium.{{cite book

| last = French

| first = Bevan M

| title = Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures

| title-link = Traces of Catastrophe

| publisher = Lunar and Planetary Institute

| date = 1998

| location = Houston, Texas

| pages = [https://archive.org/details/tracescatastroph00fren/page/n129 120]

| id = [http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html LPI Contribution No. 954]

}}

Complex craters have uplifted centers, and they have typically broad flat shallow crater floors, and terraced walls. At the largest sizes, one or more exterior or interior rings may appear, and the structure may be labeled an impact basin rather than an impact crater. Complex-crater morphology on rocky planets appears to follow a regular sequence with increasing size: small complex craters with a central topographic peak are called central-peak craters (e.g., Tycho); intermediate-sized craters, in which the central peak is replaced by a ring of peaks, are called peak ring craters (e.g., Schrödinger); and the largest craters contain multiple concentric topographic rings, and are called multi-ringed basins (e.g., Orientale). On icy as opposed to rocky bodies, other morphological forms appear which may have central pits rather than central peaks, and at the largest sizes may contain very many concentric rings—Valhalla on Callisto is the type example of the latter.

File:Eddie crater.jpg, a central peak-ring crater on Mars]]

A central-peak crater is the most basic form of complex crater. A central-peak crater can have a tightly spaced, ring-like arrangement of peaks, thus be a peak ring crater, though the peak is often single.{{Cite book |chapter=Central Peak Crater |last=Bray |first=Veronica J. |date=November 20, 2015|doi=10.1007/978-1-4614-3134-3_37 |title=Encyclopedia of Planetary Landforms |pages=249–256 |isbn=978-1-4614-3133-6 }} Central-peak craters can occur in impact craters via meteorites. An Earthly example is Mistastin crater, in Canada.{{Cite web |url=https://www.lpi.usra.edu/exploration/CLSE-landing-site-study/ScienceConcept6/ |title=Science Concept 6: The Moon is an Accessible Laboratory for Studying the Impact Process on Planetary Scales}} Many central-peak craters have rims that are scalloped, terraced inner walls, and hummocky floors.{{cite book |doi=10.1007/978-1-4614-9213-9_37-2 |chapter=Central Peak Crater |title=Encyclopedia of Planetary Landforms |date=2014 |last1=Bray |first1=Veronica J. |last2=Öhman |first2=Teemu |last3=Hargitai |first3=Henrik |pages=1–9 |isbn=978-1-4614-9213-9 }}

Diameters of craters where complex features form depends on the strength of gravity of the celestial body they occur on. Stronger gravity, such as on Earth compared to the Moon, causes rim collapse in smaller diameter craters. Complex craters may occur at {{convert|2|km|mi}} to {{convert|4|km|mi}} on Earth, but start from {{convert|20|km|mi}} on the Moon.{{cite book

| last = French

| first = Bevan M

| title = Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures

| title-link = Traces of Catastrophe

| publisher = Lunar and Planetary Institute

| date = 1998

| location = Houston, Texas

| pages = [https://archive.org/details/tracescatastroph00fren/page/n37 27]

| id = [http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html LPI Contribution No. 954]

}}

If lunar craters have diameters between about {{convert|20|km|mi}} to {{convert|175|km|mi}}, the central peak is usually a single peak, or small

group of peaks. Lunar craters of diameter greater than about {{convert|175|km|mi}} may have complex, ring-shaped uplifts. If impact features exceed {{convert|300|km|mi}} of diameter, they are called impact basins, not craters.{{Cite web |url=https://lunar.gsfc.nasa.gov/lessonkit/LROC-LOLA-Mapping%20the%20Surface%20of%20the%20Moon_2014.pdf |title=Mapping The Surface of the Moon |last=Millham |first=Rosemary}}

Lunar craters of {{convert|35|km|mi}} to about {{convert|170|km|mi}} in diameter possess a central peak.

There are several theories as to why central-peak craters form. Such craters are common, on Earth, the Moon, Mars, and Mercury.{{Cite journal |title=Central peaks in lunar craters |journal = Moon|volume = 12|issue = 4|pages = 463–474|last=Allen |first=C. C. |date=April 12, 1975 |bibcode = 1975Moon...12..463A|doi = 10.1007/BF00577935|hdl = 10150/622036| s2cid=120245830 |hdl-access = free}}{{Cite web |url=https://pubs.usgs.gov/pp/1534/report.pdf |title=Atlas of Volcanic Landforms on Mars |last=Hodges |first=Carroll Ann |date=1992 |website=pubs.usgs.gov}}

On the Moon, heights of central peaks are directly proportional to diameters of craters, which implies that peak height varies with crater-forming energy. There is a similar relationship for terrestrial meteorite craters and TNT craters whose uplifts originated from rebound.{{cite journal |last1=Wood |first1=Charles A. |title=Moon: Central peak heights and crater origins |journal=Icarus |date=December 1973 |volume=20 |issue=4 |pages=503–506 |doi=10.1016/0019-1035(73)90023-7 |bibcode=1973Icar...20..503W }}

• {{annotated link|Impact crater}}
• {{annotated link|Impact structure}}
• {{annotated link|Multi-ringed basin}}
• {{annotated link|Peak ring (crater)}}
• Traces of Catastrophe, 1998 book from Lunar and Planetary Institute - comprehensive reference on impact crater science

{{Portal|Astronomy}}

{{reflist}}

{{Impact cratering on Earth}}

Category:Impact craters

Category:Impact geology