Huge rocks at Mons Hansteen Options

[wildespace]

Greetings. What is your opinion on the origin of big rocks that are visible at this location at Mons Hansteen (also known as the "Arrowhead"). One of the rocks casts a particularly long shadow.

Could they be volcanic plugs, or simply hard rocks exposed through erosion? Would love to hear your take on this. Thanks.

[Juramike]

Hansteen rocks are different than most normal basaltic moon rocks.
Check out: Hawke et al., JGR 108 (2003), E7,5069. http://www.spudislunarresources.com/Bibliography/p/78.pdf

This region has rocks that are unusual in that they are made of dacite, which is much more silica-rich lava than basalt. Dacite is a superfine-grained form of granodiorite. Since there is less manganese and iron in the chemical makeup of silica-rich lavas, the silicate-molecule tetrahedrons aren't as bound up by metal ions, so the silica units cling to each other more. (At least that is the simple explanation I have in my head). As a result, the silica-rich lavas are lumpy and flow like toothpaste. The more basaltic lavas are more runny and flow like maple syrup. Dacite and the even more silica-rich rhyolite (superfine-grained granite) form steep-sided plug domes. The plug dome in Mt. St. Helens formed in the 1980's was dacite (http://vulcan.wr.usgs.gov/Imgs/Jpg/MSH/Ima...9-13-84_med.jpg)

Other good candidates for silica-rich volcanic constructs on the Moon are : Marian domes, Oceanus Procellarum [41.4 N, 48 E]; Hansteen alpha, Oceanus Procellarum [12.3 S, 51 W]; Mons Gruithuisenen, Mare Imbrium [36.3 N, 40 W], and Compton-Belcovich Farside [61 N, 99.5 E].

If you poke around those areas, you'll also find some features that look like lumpy big-bouldered plug domes remnants. There is a great example of a big boulder pile just S of Beta dome in the Compton-Belkovic complex.