<P> The samples of pyroclastic glasses are of green, yellow, and red tints . The difference in color indicates the concentration of titanium that the rock has, with the green particles having the lowest concentrations (about 1%), and red particles having the highest concentrations (up to 14%, much more than the basalts with the highest concentrations). </P> <P> Rilles on the Moon sometimes resulted from the formation of localized lava channels . These generally fall into three categories, consisting of sinuous, arcuate, or linear shapes . By following these meandering rilles back to their source, they often lead to an old volcanic vent . One of the most notable sinuous rilles is the Vallis Schröteri feature, located in the Aristarchus plateau along the eastern edge of Oceanus Procellarum . An example of a sinuous rille exists at the Apollo 15 landing site, Rima Hadley, located on the rim of the Imbrium Basin . Based on observations from the mission, it is generally believed that this rille was formed by volcanic processes, a topic long debated before the mission took place . </P> <P> A variety of shield volcanoes can be found in selected locations on the lunar surface, such as on Mons Rümker . These are believed to be formed by relatively viscous, possibly silica - rich lava, erupting from localized vents . The resulting lunar domes are wide, rounded, circular features with a gentle slope rising in elevation a few hundred meters to the midpoint . They are typically 8--12 km in diameter, but can be up to 20 km across . Some of the domes contain a small pit at their peak . </P> <P> Wrinkle ridges are features created by compressive tectonic forces within the maria . These features represent buckling of the surface and form long ridges across parts of the maria . Some of these ridges may outline buried craters or other features beneath the maria . A prime example of such an outlined feature is the crater Letronne . </P>

What are the dark spots on the moon made of