text: compton – belkovich thorium anomaly the compton – belkovich thorium anomaly is a hotspot ( volcanic complex ) on the moon . it is on the far side of the moon and was found by a gamma - ray spectrometer in 1998 . it is an area of concentrated thorium , a radioactive element . lunar rock samples from the apollo missions reveal that most lunar volcanism occurred around 3 to 4 billion years ago , but could have been as recent as 1 billion years ago due to the unknown history of the moon 's far side . description the compton – belkovich thorium anomaly was found in 1998 by the gamma ray spectrometer ( grs ) instrument on board the lunar prospector ( lp ) and subsequently identified as a hotspot , located around 61 . 1 ° n 99 . 5 ° e ﻿ / 61 . 1 ; 99 . 5 . the estimated thorium concentration reaches 5 . 3 µg / g ( 5 . 3 micrograms per gram ) while the surrounding highland basalts only contain between 0 and 2 µg / g . compared to the earth 's thorium concentration of 0 . 06 µg / g , the compton – belkovich 's is very high . it has unusually high reflectance , identified by a visible imaging study that was carried out later by the clementine spacecraft in a clementine visible images study . high resolution images from the lunar reconnaissance orbiter lro made it possible to analyze the surface features of the compton – belkovich thorium anomaly in 2011 . location the anomaly is between the bel crater , which is 214 kilometres ( 133 mi ) wide , and the compton crater , which is 162 kilometres ( 101 mi ) wide . the region as a whole is 32 kilometres ( 20 mi ) wide and 18 kilometres ( 11 mi ) long . the center of the region is a volcanic complex , 25 kilometres ( 16 mi ) to 35 kilometres ( 22 mi ) across , between the bel and the compton craters . it is 900 kilometres ( 560 mi ) from the extent of the northeastern kreep terrane ( an area which has high abundances of kreep , a geochemical component of some lunar rocks ) . features in the center of the elevated region is a depression ; this is bounded by scarps and may be some kind of caldera . just to the north is a feature called little dome , 500 metres ( 1 , 600 ft ) in diameter . further north is an elongated dome , oriented north - south , called middle dome . it is 2 . 5 kilometres ( 1 . 6 mi ) long and 0 . 6 kilometres ( 0 . 37 mi ) wide . both little dome and middle dome have boulders on top that may be volcanic blocks . big dome is further to the north at the edge of the anomaly . it is 2 . 5 kilometres ( 1 . 6 mi ) in diameter with a depression in the top . an extension of the reflective material extends to the south - east from the elevated region by about 7 kilometres ( 4 . 3 mi ) . this may be a pyroclastic flow . this more highly reflective area also matches an area that shows a christiansen feature with shorter wavelength . it reflects more strongly in the 7 . 1 to 7 . 5 μm range , which indicates quartz or alkali feldspar is the major constituent . explosive remains also appear scattered to the east for about 300 km covering an area of 70 , 000 km2 . volcanic slope volcanic features provide information about the composition of the lava that formed the compton – belkovich thorium anomaly . on average , many volcanoes on the moon have slopes of lower than 7 degrees . however , the compton – belkovich thorium anomaly has a slope which reaches 25 degrees at the highest . this suggests that the region was formed by more viscous lava . composition using infrared reflectance data from clementine at 750 nm and 950 nm , the level of iron oxide was determined to be about 3 % by mass . formation a direct analysis of apollo program samples revealed that most lunar volcanism occurred around 3 to 4 billion years ago . however , volcanic activity on the lunar back side could have occurred around 1 billion years ago . the smoothness of the surface associated with the anomaly indicates that it could possibly have been formed in a more recent event . as the lava cooled , it would have crystallized to produce a silicate structure ; incompatible elements such as thorium would have been excluded from the process and formed thorium - rich pockets in the remaining liquid rock . the eruption associated with the thorium anomaly could have created the elevated features to the west and the low and broad area to the east . the latest possible eruptions of lava would have made domes with steeper slopes , and also would have caused small bulges , as they would barely reach the surface .