Page 219 - Fundamentals of Physical Volcanology

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196 CHAPTER 13

Fig. 13.4 The lunar sinuous rille called
Vallis Schroeteris (Schroeters Valley),
interpreted to be a channel formed
when a hot turbulent lava flow melted
the ground beneath it, meanders for
∼180 km across the lunar surface with
an average width of ∼3 km and an
average depth of ∼1000 m. (Apollo
15 metric frame #AS15-M-2611;
NASA image.)

rock was incorporated into the flow and so the flow
began to melt a channel into the surface, eroding it
at a rate of a few centimeters per hour to begin to
form a sinuous channel. In many cases the eruption
went on for several months, so that typically a 10 m
thick flow was eventually traveling down the floor
of a 100 m deep channel. About 30 such channels
can be traced for many tens of kilometers across the
lunar surface. The lengths of these channels, just
like the lengths of more normal lava flows, can be
used to deduce the volume eruption rate, and imply
3 −1
rates of up to 10,000 m s , similar to the rates
implied by the longest lava flows.
Not all eruptions on the Moon involved large
magma volumes or took place at high eruption
rates. In an area called the Marius Hills there are
more than 250 small (up to ∼20 km in diameter),

low (50–500 m high) features (Fig. 13.5), com- Fig. 13.5 The area of the Moon known as the Marius Hills,
monly called domes but actually more reminiscent a region in Oceanus Procellarum. Each “bump” on the
of shield volcanoes on Earth. One possible explana- surface is a small shield volcano, typically 5 to 10 km wide
tion for these features is that they represent erup- and 100 to 200 m high. (Part of Lunar Orbiter IV image
157H2; NASA image.)
tions from a reservoir of magma that accumulated
at the base of the crust. This magma cooled and
crystallized to some extent before erupting with a Deposits from explosive eruptions on the Moon
higher viscosity and hence at a lower effusion rate are very much rarer than on Earth. This is mainly
than magma rising directly from the mantle to the because the Moon is completely depleted in vola-
surface. Elsewhere on the Moon, near the craters tile compounds such as water and carbon dioxide
Gruithuisen and Mairan, there are six small (∼10 km which are common in the Earth’s mantle. However,
diameter and 1 km high) domes that appear to be there was a source of gas available to magmas erup-
the result of the extrusion of small amounts of ting on the Moon but it had an unusual origin. The
an even more evolved type of magma. Although of lunar mantle contains small amounts of free carbon,
great interest because the magma is so different and carbon atoms can react with the oxides of cer-
from that elsewhere, these eruptions represent a tain metals to form carbon monoxide. The reaction
vanishingly small fraction of the Moon’s total vol- only works at low pressures, so it would only have
canic activity. occurred in the magma near the tip of a new dike