Sky & Telescope - February 2019 - 53

CREDIT

p Left: Hadley Rille was formed when large volumes of lava flowed for a week or more, eroding a
sinuous channel in the underlying strata. Right: Some phase 4 eruptions produce dikes that spread
laterally beneath impact craters, forcing their surfaces upward and fracturing the floors like that of
Vitello crater located in the southwestern lunar quadrant.

as Hadley Rille seen at the Apollo 15
landing site.
In phase 3, more than half of the
dike's magma is now located in the
crust, decreasing its buoyancy and
drastically slowing its eruption. Additionally, the magma starts to cool, and
the dike sidewalls begin to contract,
because the magma's pressure is not as
high as earlier. All of this slowing of
movement gives time for gas bubbles
within the magma to coalesce into single voids that grow as wide as the dike.
This stops the continuous eruption of
magma, breaking it up into individual
pulses, each erupting a discrete layer of
pyroclastic material. Small, abundant
pyroclastic cones form, much like those
found in the Marius Hills.
The dike continues to close during
phase 4, restricting the eruption of
pyroclastic particles, but lava ﬂows are
still pushed out, often rich with vesicles
- hollows that once contained gas
bubbles. Some of these ﬂows are short,
building up low domes around the
vent, such as the well-known ones near
Hortensius and Milicius craters. In
one case, a small circular lava pond was
built up and the escape of gas in lateral
feeding dikes caused collapse pits along
rilles, and a caldera collapsed at the
summit - this, of course, is Hyginus.
Some dikes intrude into brecciated
spaces beneath impact craters and
spread laterally. In craters smaller than
about 40 km across, the center of the
lava pile tends to thicken (a laccolith),
pushing up and deforming the ﬂoor of

the crater. In larger craters, the inﬂowing lava forms a sill - a ﬂat slab of lava
- elevating the entire ﬂoor but with
less central deformation. An example
of a smaller ﬂoor-fractured crater is
Vitello in southern Mare Humorum;
Humboldt near the eastern limb is
larger and has a ﬂat, shallow ﬂoor. In
cases such as Plato, Archimedes, and
Hercules, rising lava ﬁnds an easy path
to the surface, submerging the craters'
ﬂoors and central peaks with smooth
ponds of lava.
Earlier I mentioned that the period
in lunar history in which a dike forms
was critical for its future. Evidence suggests that during its ﬁrst billion years,
heat left over from the accretion of the
Moon, in addition to heat generated
from the decay of radioactive minerals,
put the early Moon into an extensional
stress pattern, stretching the lunar
crust. This made it easier for cracks to
propagate from diapirs to the surface.
Additionally, the warm Moon could
partially melt portions of the mantle
more readily, generating voluminous
amounts of magma. This theoretical
conclusion is consistent with the fact
that most lunar mare lavas were erupted
between 3.9 and 3.1 billion years ago,
and most sinuous rilles formed before 3
billion years ago.
As the early Moon cooled, its interior
contracted slightly, creating a compressional environment that inhibited
mantle fracturing and dike formation.
Small amounts of lava did reach the
surface between 2 to 3 billion years ago,

p The Marius Hills formed when the eruption
rate of a dike's magma slowed sufficiently to
result in the building of discrete pyroclastic
cones like the ones seen here.

and even tinier amounts erupted until
about a billion years ago, testifying that
despite magma source regions becoming smaller and deeper, lithosphere
thickening, and a more compressive
mantle structure, a few dikes did reach
the surface in special circumstances.
The paucity or even complete absence
of younger volcanic features makes it
doubtful that transient lunar phenomena or irregular mare patches are associated with young volcanism. Perhaps
we will be surprised someday, but the
Moon seems to be volcanically dead. For
an observer spotting traces of ancient
volcanism, however, the moment is
alive with awe.
¢ Contributing Editor CHUCK WOOD is
sorry to have missed seeing the surges
of lava flows spreading across Mare
Imbrium. For more on lunar dikes see
Wilson and Head paper at https://is.gd/
lunardikes.

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Sky & Telescope - February 2019

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