GEOLOGY 422/522

The Moon - Part 2

by Scott Hughes

Volcanic Features on the Moon

Volcanic features can best be seen on the near side of the Moon, among and around the multiring basins that have been filled with mare basalt lava flows. But there exist numerous other lunar volcanic features that are interesting to study.  Lunar samples returned by the Apollo missions proved that the chemical composition and mineralogy of mare basalt are similar to those that characterize basalts on the Earth.

Radiometric ages and relative geologic dating show that the flows probably occurred several hundred million years after the formation of the impact basins, thereby indicating that they were not impact related melt caused by the craters. Perhaps some great thermal event occurred within the Moon between 4 to about 3 billion years ago. Some mare flows fill their craters completely and even spill out onto the surrounding landscape.

Lobes and flows of lava indicate high rates of basaltic extrusion on the Moon. Some of these flows have been measured at over 600 kilometers long, a value that is consistent with major flood basalt eruptions on Earth. These long flows are useful in determining the relative ages of the tectonic features on the Moon, such as ridges or troughs.

Meandering, sinuous rilles are found along the flat edges of the maria.

Hadley Rille – Image courtesy of NASA

The Hadley Rille, an example of a sinuous rille, is seen at the top of this image. Rilles probably formed by the collapse of lava tubes. Some rilles have flat bottoms with v-shaped profiles that may indicate that they are fault-bounded troughs produced by extensional tectonics that have been modified by lava flows.

Surface picture (with caption) of astronaut, rover and rim of Hadley Rille on the Moon, provided by the National Space Science Data Center, Goddard Space Flight Center, Greenbelt, MD

 

Color image of Vallis Schroteri on the Moon (Clementine Mission), provided by the National Space Science Data Center, Goddard Space Flight Center, Greenbelt, MD.

 

Lava tubes, lava channels, domes, and basaltic plains are other features seen on the Moon. You should be able to define and recognize each of these. Domes are usually small features seen within the lunar maria and are often mistaken for impact craters. They differ from craters, however, because they have positive relief, often have lava flows coming out of the sides or tops, and can occur in groups on the surface a broad mound.

 

Basaltic plains volcanism is evident on most terrestrial planets covering much of the surface with basaltic lava flows such as those found on the lunar maria. The lava flows coalesced into a plain of low, shield-type domes and flows. This style of volcanism was recognized by Ron Greeley and colleagues on the Snake River Plain of Idaho where small amounts of lava were extruded from many central vents.

Close-up color image of the orange (volcanic glass, Apollo 17) soil at Shorty Crater on the Moon, provided by the National Space Science Data Center, Goddard Space Flight Center, Greenbelt, MD.

 

 

 

 

 

Tectonic Features of the Moon

There is little evidence for tectonism on the Moon from about 2.5 billion years ago. The youngest mare basalts show only slight compressional modification and the youngest craters remain undeformed, indicating a tectonically inactive surface since that time. Pre-2.5 Ga, both extension and compression occurred on the Moon, as evidenced by ridges and graben seen within the older mare. Extensional features are long, straight, high angle faults and grabens that were probably formed by subsidence of the basin floors after impact. These linear rilles were formed not from plate tectonic extension of the crust, but rather by local subsidence or expansion, possibly related to regional internal heating.

Compressional features on the Moon were probably formed by buckling of the crust due to the weight of accumulating lava in the basins. This would have caused vertical adjustment of the crust to compensate for the overburden and would have compressed the crust to form wrinkle ridges. Another way the older compressional features could have formed is by global cooling about 3.5 billion years ago, after the formation of the youngest extensional linear rilles.

Examine this image of Mare Tranquillitatis that illustrates the surfaces of numerous mare basalt lava flows and their associated structural features. Note that the linear rilles in this image are grabens, elongate fault-bounded structural basins formed during extension.

Lunar Rocks

Basalt Photomicrograph of lunar basalt from Apollo 16 (1 mm view)

Volcanic Glass

Apollo 15 green volcanic glass (1 mm squares)

Apollo 17 orange volcanic glass(~0.7 mm view)

The Lunar Geologic Time Scale

NOTE: See Chapter 4, "The Moon"by Paul D. Spudis

During its formational period the Moon was accreted from materials ejected from a large, Mars-sized impact with the Earth which had already begun differentiating into core-mantle-lithosphere. Thus, the bulk Moon is comprised of material very similar to Earth’s mantle, i.e., it is relatively depleted in core-forming materials.

As gravitational and radiogenic energy was released the Moon heated internally and differentiated. As you might expect, the bulk density of the Moon is less than that of the Earth and the Fe-Ni core of the Moon is proportionately smaller than typical terrestrial planet cores relative to the entire planetary mass.

A computer has simulated the effects of a giant impact on the Earth 4.6 billion years ago. The mantles of both the Earth and the impactor are vaporized; some of this material ends up in Earth orbit and forms a circumterrestrial disk from which the Moon coalesces shortly afterward.

See Don Wilhelm's Geologic History of the Moon, US Geological Survey Professional Paper 1348.

Also visit the following brief, yet elegantly simplified version of Lunar Geologic History.

See the Wikipedia Lunar Geologic Timescale and compare it to Earth's Geologic Timescale.

Figure 14, p. 51, Chapter 4 in the textbook:

The outer several hundred kilometers of the lunar mantle became molten yielding a Lunar Magma Ocean (LMO) that began to cool and differentiate like a thick intrusion of mafic magma. Dense ferromagnesian minerals, such as olivine and pyroxene, crystallized and sank to the deeper mantle. Less dense calcium plagioclase feldspar minerals accumulated into anorthosite masses (along with minor amounts of ferromagnesian minerals) that rose and concentrated near the surface. These masses formed the early lunar crust of which the highland regions are made.

KREEP basalts, some of the oldest on the Moon, were possibly erupted from pockets of magma trapped beneath the forming crust which eventually led to the development of the ridged lithosphere.

There are five main stages in the development of the Moon (see Table 1, Chapter 10 in the text) following its initial formation. Compare to Earth’s geologic history.

Pre-Nectarian (4.6 to 3.92 billion years ago) This stage consisted of a period of intense meteorite and asteroid bombardment, and the formation of the lunar crust and densely cratered lunar highlands. Includes crater and basin deposits and many other units formed before the Nectaris basin impact.

 

Nectarian (3.92 to 3.95 billion years ago) Named for the Nectaris Basin, a huge multiring basin on the nearside that sent ejecta almost to the opposite side of the Moon. Includes almost four times as many large craters and basins as the Imbrian system and may contain some volcanic deposits.

 

Imbrium (3.85 to 3.15 billion years ago) Formation of Orientale and the Imbrium basins and voluminous outpourings of mare basalt. Remember, most of the maria are found on the near side of the Moon, possibly due to the offset of the lunar core towards the Earth.

 

Eratosthenian (3.15 to 1.0 billion years ago) Formation of Eratosthenes crater and includes craters that are slightly degraded and without visible rays (compared to Copernican deposits). Youngest mare deposits.

 

Copernican (1.0 billion years to present) Youngest level of the Moon’s stratigraphy; includes the freshest craters, many of which were formed on top of mare deposits. Most have bright rays, such as the Copernicus crater just south of the Apennine Mountains on the edge of Mare Imbrium (see Figs 2 and 3, Chapter 4).

 

NOTE: Volcanism and tectonism appear to have ceased since about 2 billion years ago.

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Other web sites to visit:

USGS Lunar Homepage

http://astrogeology.usgs.gov/Projects/Clementine/index.html

Books About the Moon

http://www.tsgc.utexas.edu/everything/moon/links.html

Planetary Display System, Jet Propulsion Laboratory, Caltech

(scroll down to lunar features on this page)  http://pds.jpl.nasa.gov/planets/welcome/earth.htm

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