Though the Moon has no plate tectonics, the most common cause of quakes on Earth, the Moon still experiences many small quakes. A seismometer placed on the Moon by one of the Apollo missions measured 3,000 quakes per year, though each was orders of magnitude less powerful than normal quakes on Earth.The quakes seem to occur at depths of 375 to 500 miles (600 to 800 km). Many of these quakes occur on a monthly cycle, and are therefore caused by tidal stresses. The seismometer also recorded between 70 and 150 meteorite impacts per year. By measuring the length of time it takes a laser to bounce off reflectors left on the Moon by the Apollo missions, and come back to Earth, scientists recently discovered that the Moon expands and contracts by 3.9 inches (10 cm) every 27 days. This distortion is caused by tides from the gravity of the Earth and Sun. This information, along with evidence that moonquake waves lose energy at depth, indicates that the inside of the Moon is still partly molten, or at least close to the temperature required for melting. Only cold, brittle material can create quakes, which are the result of energy being released when rocks break in a brittle fashion, as they do on the Earth's surface. Warm rocks will flow in response to stress. The expansion and contraction of the Moon is allowed by flowing warm interior rocks, while the quakes occur in cold, brittle regions.The crust of the Moon has been investigated using gravity signatures. Where gravity is strong, the crust must be thinner and the mantle material closer to the surface, because the mantle material is denser and creates a larger gravitational pull.The Clementine orbiter mission recorded the Moon's gravity field in detail. Based on modeling from this gravitational data, the Moon's crust appears to be between about 13 and 75 miles (20 and 120 km) thick. The crust is thickest on the far side by an average of about eight miles (12 km) (which is perhaps part of the reason there are so few mare basalt flows on the far side), thinner on the near side, and thinnest under the basins.There is no clear explanation now for the difference in crustal thickness between the near and far sides.
Gravity data also reveals that there is a strong gravity increase over each of the mare basins, as well as a few craters without mare fill.The gravity highs, as they are called, correspond to areas of high density that are known as lunar mascons. Mascons and their attendant high gravity signature are thought to be the result of the giant impacts:The giant impact thinned the crust by ejecting mass from the site of impact. After thinning, the crust at the site of impact rose in elevation to its level of buoyancy, as a ship rises in the water when its freight is removed.When the crust rose the mantle flowed upward to fill in the new dome in the bottom of the crust. The mantle material is denser than the crust it replaced, and causes the high gravity.The Moon today has a crust consisting of anorthosite and the mafic and alkali suites of highlands rocks, along with mare basalts. Beneath the crust is a deep mantle of silicate minerals, mainly olivine and pyroxene, but apparently divided into regions with different compositions both by depth and laterally at the same depth. Judging from the laser ranging and seismic evidence, the upper mantle is cold and brittle, but at some depth the mantle becomes warm enough to flow, and perhaps to be partly molten.
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