We reconstruct the abundance of thorium near the Compton-Belkovich Volcanic Complex on the Moon , using data from the Lunar Prospector Gamma Ray Spectrometer .
We enhance the resolution via a pixon image reconstruction technique , and find that the thorium is distributed over a larger ( 40 \mathrm { km } \times 75 km ) area than the ( 25 \mathrm { km } \times 35 km ) high albedo region normally associated with Compton-Belkovich .
Our reconstructions show that inside this region , the thorium concentration is 14 - 26 ppm .
We also find additional thorium , spread up to 300 km eastward of the complex at \sim 2 ppm .
The thorium must have been deposited during the formation of the volcanic complex , because subsequent lateral transport mechanisms , such as small impacts , are unable to move sufficient material .
The morphology of the feature is consistent with pyroclastic dispersal and we conclude that the present distribution of thorium was likely created by the explosive eruption of silicic magma .