My first Magazine Sky & Telescope - 02.2019 | Page 55

p Left: Hadley Rille was formed when large volumes of lava fl owed 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 fl oors 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. Addi-
tionally, 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 sin-
gle 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 inflow-
ing 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 sug-
gests 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 com-
pressional 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 suffi ciently 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 becom-
ing 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 phenom-
ena or irregular mare patches are asso-
ciated 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 fl ows spreading across Mare
Imbrium. For more on lunar dikes see
Wilson and Head paper at https://is.gd/
lunardikes .
sk yandtele scope.com • FE B RUA RY 2 019
53