Floor-Fractured Craters     

The majority of lunar craters are small and simple impact craters. But for some craters larger than 20-30 km the impact morphology is modified by volcanic-related activity. Posidonius is an excellent example and it and similar craters share many of these characteristics: shallow, mare-flooded floor, concentric and radial rilles, dark halo craters, and location near a mare. In 1976 Pete Schultz recognized these commonalities and named such craters floor-fractured craters.

Schultz detected 206 FFC and proposed that they formed by magma which rose up basin fractures and ponded under the crater floor. The pressure of the magma lifted the crater floor (accounting for FFC shallowness) and produced the fractures. In many FFC craters, lava escaped to the surface, creating lava ponds, sinuous rilles and dark halo craters. Because of such volcanic modifications, FFC are among the most interesting craters on the Moon for observers.

The left diagram illustrates the process Schultz proposed that uplifted and modified impact craters. Drawing A shows an impact crater - both its original transient cavity (dashed line) and final shape after rim collapse and central peak uplift. The second drawing shows the rise of mare magmas up a fault and into the breccia (impact fractured) zone under a crater. As the magma zone grows (drawing C) the crater floor is uplifted and fractured, allowing some magma to erupt onto the floor (bottom drawing). Not all craters progressed thru the entire sequence of disruption and modification.

Schultz classified FFC into 6 types based on their crater depth, fracture pattern and floor type. On the right above is Schultz' illustration showing examples of each FFC class, and below is a summary of class characteristics:

Class I: Fractured-floor impact craters. Have most characteristics of fresh large impact crater (central peaks, wall terraces or slumps and ejecta blanket) but also floor fractures (rilles), mare patches and dark halo craters. Examples are Atlas, Petavius and Humboldt.

Class II: Shallow, with fractured hummocky (hilly) floor. Smaller craters tha Class I with sharp wall scarp and shallow, hummocky floor cut by concentric rilles. Examples include Encke and Vitello.

Class III: Wide moat floor. A wide annular moat filled with mare or light-hued plains separates the inner floor from the crater wall. Examples are Gassendi, Posidonius and Warner.

Class IV: Narrow moat floor. Overall smaller craters than Class III, lacking terraces. Floor looks like domed loaf of bread with ridged radial fractures. Illustration above includes two sub-types. IV A & B. of this type of FFC. Examples include Gaudibert and Bohnenberger.

Class V: Fractured light plains. Appear to be older large craters that are little modified except for smooth floors (typically light-hued) cut by linear rilles. Some may have dark halo craters. Examples are Alphonsus and Lavoisier D.

Class VI: Fractured mare floor. Broad, shallow mare flooded craters with fractures/rilles. Examples are Pitatus and Tsiolkovsky.

As you observe the Moon and look at images you may notice other FFC craters and you should consider which of these classes they fit into.

Schultz's map of FFC distribution shows that they are mostly near mare and that many occur in clusters.

Schultz has not published his extensive list of FFC but here is a short compilation to guide your studies of these fascinating craters. 

CRATER

DIAMETER/km

LAT

LONG

FFC CLASS

Airy

37

 

 

IV

Alphonsus

119

 

 

V

Atlas

85

 

 

I

Bohnenberger

33

 

 

IV

Briggs

37

 

 

II

Cardanus

50

 

 

I

Davy

35

 

 

II

Doppelmayer

64

 

 

III

Encke

28

 

 

II

Einstein A

51

 

 

I

Gassendi

110

 

 

III

Gaudibert

36

 

 

IV

Haldane

38

 

 

III

Humboldt

207

 

 

I

Krieger

22

 

 

 

Lavoisier

70

 

 

III

Lavoisier D

62

 

 

V

Lavoisier E

49

 

 

I

Petavius

177

 

 

I

Pitatus

110

 

 

VI

Repsold

110

 

 

V

Runge

39

 

 

III

Schluter

89

 

 

I

Taruntius

56

 

 

 

Tsiolkovsky

180

S20.4

E129.1

VI

Vitello

42

S30.4

W37.5

II

Warner

35

S04.0

E87.3

III

Note that some of these FFC are relatively small (40 km is small when you are trying to see details on the floor) or close to the limb so that observing floor features will be difficult.