As is not the case with any other world in the Solar System, we have a pretty clear idea of what the Moon is made of and how it was put together, because we have an extensive suite of rock samples brought back from the Apollo Lunar landings. The images below are photomicrographs of thin sections made from Lunar rocks, breccias, and regolith. Through a careful examination of these images, we can gather some first-hand information about what has happened over the last several billion years to create and modify the Lunar crust and surface.
Link to Map of Apollo Landings on the Moon
Lunar Mare Basalts: The lavas that solidified to form these Volcanic Igneous Rocks were generated by shock-induced heating associated with the series of gigantic meteor impact events that formed the mare basins. The most recent mare impact occurred approx. 3.0 Ga; the oldest, around 3.8 Ga. Lava erupted in the crater basins and filled them, and because of the vacuum of space, cooling proceeded relatively slowly, permitting more-or-less diabasic textures to develop. Some Mare basalts preserve poikilitic textures, indictaive of progressive crystallization with cooling, while others suggest more rapid cooling. Mare basalts show little evidence of subsequent shock metamorphism, as is fitting given their relatively "young" lunar ages and the lack of cratering in the mare basins.
Question 1: When describing basalts on the earth via thin section, we use several different terms to describe the textures:
a) Intersertal: Mineral grains completely surrounded by glass (Remember, glass is black in the colorful crossed-polar images!)
b) Intergranular: The mineral grains touch, but there is glass in the spaces between grains.
c) Variolitic: No glass, and plagioclases crystals (long, white grains that are gray to white under crossed polars) form radiating aggregates, with other minerals (usually pyroxenes and olivines - blocky grains that are highly colored under crossed polars - or iron and titanium oxide minerals - opaque, so they look black in both plain light and with crossed polars - in the spaces between plagioclases.
d) Poikiliitic: One kind of mineral grain (in Moon rocks, olivine or pyroxene, usually) is completely surrounded by another kind of mineral (usually plagioclase, or an oxide mineral
So, take a look at the thin section pictures for the three lunar basalts below, and give these three rocks textural names!
Question 2: Intersertal and Intergranular textures occur on the Earth in Mid Ocean Ridge Basalts, which are erupted on the bottom of the sea. Variolitic and Poikilitic textures on Earth occur in basalts erupted subaerially, like from Kilauea in Hawaii. The difference relates to the fact that water can "pull" heat out of a material much more effectively than air, cooling it faster, which makes the rock solidify more rapidly (i.e., the same reason you run water on your hand if you get a burn while cooking!). Look at the texture terms you used to describe the Lunar baslts, and tell me: based on their textures, did they cool slower or faster in the vacuum of space than basalts on the Earth's seafloor did?
Helpful hint:to see an Intersertal texture, look at the Basalt pictures in the Rocks in Thin Section Activity.
Question 3:Does your answer to Question 2 seem like "common sense" to you? Whatever your response, give your reasons.
Other types of impact-related melts are found in the Lunar regolith: distinctive, spherical glass droplets formed during impact events. Lunar Brown Glass was a significant component of the Apollo 17 regolith. When these glass droplets begin to crystallize, they produce fine, spiny to bladed crystals, similar in form to those encountered in Chondrules found in primitive meteorites.
Images of Lunar Brown Glass from Apollo 17 Regolith
Plutonic Igneous Rocks of the Moon: The original crustal rocks of the Moon, which constitute most of the Lunar Highlands, were rich in the mineral Feldspar, consisting of Anorthosites (Feldspar-only rocks), Norites (Feldspar-pyroxene rocks) and Troctolites (Feldspar-olivine rocks). These rocks are VERY old (4.0 to 4.4 Ga) and show substantial evidence for Shock Metamorphism due to a long history of meteor impacts on the lunar surface. Most of these rocks show a fractured, Brecciated texture, and some contain Maskelynite, a diaplectic glass formed by the shock metamorphism of feldspar.
The old Lunar crust was feldspar-rich because when the Moon differentiated internally, and its interior was molten (a Lunar Magma Ocean) feldspar which crystallized from this melt was buoyant and floated toward the surface, while denser, iron-rich minerals along with most of the olivine and pyroxene sank to the bottom. After the Lunar magma ocean crystallized, it was heating due to later huge impacts that remelted its now iron-rich interior and produced the Mare Basalts.
Question 4: Look at the pictures of the Anorthosite and Norite thin sections, and compare them to the pictures of the Mare Basalts. Then make a short list of reasons why we think that these rocks of the Lunar Highlands have undergone substantial Shock Metamorphism: what do YOU see that might make you think so?
Question 5:Do you see any iron or titanium oxide minerals in these Highlands rocks? Given that most oxide minerals are significantly denser than Plagioclase feldspar and even Pyroxene, what might have happened to these minerals when they crystallized if, as is thought, the entire interior of the Moon was molten, in a magma ocean?
Images of Lunar Anorthosite 60025, Apollo 17 landing site
Images of Lunar Norite 78235, Apollo 16 landing site
Lunar Impact Breccias: Perhaps the most common rock type on the Moon today, however, are Impact Breccias - rocks made from the debris of the many meteor impacts which have occurred in the Moon's history. These rocks form when fragment of shattered rock either are smashed together by the collision, sintered together by the heat of collision, or are cemented together by infiltrating impact melt. Breccias are said to be Monomict if they contain pieces of only one kind of rock, and Polymict if they contain more than one kind of rock fragment. Most Lunar breccias are polymict by strict definition, but they are typically dominated by fragments of the local country rock.
Some breccias cover large regions of the lunar crust, and can be mapped on the Moon via telescope. These Lunar "formations" were early targets for exploration in the Apollo program.
Question 6: Can you see pieces of Mare Basalts or Lunar Highlands rocks in any of the breccia pictures below? If so, identify the sample, and the kind(s) of rock in it.
Images of Lunar Impact Breccia 14305, "Fra Mauro Formation", Apollo 14 landing site
Images of Lunar Melt Breccia 65015, Apollo 16 Landing Site
Images of Lunar Breccia 72275, Apollo 17 landing site
For further insights into rocks through the microscope, check out the following online exercises:
Link to Meteorites under the Microscope.
Link to Terrestrial Rocks under a Microscope.
Link back to the Moons, Planets... Homepage
Link back to Jeff Ryan's Homepage
