50 AUSTRALIAN SKY & TELESCOPE April 2019
EXPLORING THE MOON by Charles WoodDikes on a dead Moon
The appearance of volcanic lunar features tells us what was happening below the surface eons ago.
V
olcanic landforms, such as
sinuous rilles, domes, cones,
dark-halo craters, floor-fractured
craters, pyroclastic deposits, and even
young lava flows are often favourite
targets for lunar observers. Scientists have
published many studies describing their
characteristics and possible origins for
these features. Recently, Lionel Wilson
(Lancaster University, U.K.) and James
Head (Brown University) have proposed
a conceptual model that links all these
lunar volcanic landforms with different
stages in the rise and eruption of
magma. While their formal publications
are detailed and massive — Head calls
one a “doorstop” — a general awareness
of their model will enrich your observing
with understanding and wonder.
Magma is generated deep within
the lunar mantle, and large blobs of
it known as diapirs ascend to within a
few hundred kilometres of the surface.
The continuing upwelling of additional
magma into the diapir increases thepressure at its top until the overlying
brittle crustal rocks crack, and a dike
of magma fractures its way towards
the surface. Whether this dike reaches
the lunar surface and what kind of
eruption it produces depend on several
factors, such as the volume of magma,
its eruption rate, duration and gas
content, as well as the period in lunar
history that it occurred.
A dike is much smaller than the
diapir it ascends from, but it may still be
enormous. Wilson and Head calculate
that a dike feeding a large eruption may
be shaped like a giant coin, 60 to 100
kilometres high and wide but less than
100 metres thick. If a dike from a diapir
is stalled relatively near the surface, it
can maintain a connection to its source
region, but dikes that rise from greater
depths detach from their feeder diapirs
and fracture their way upward in just a
few hours — imagine the moonquakes!
Wilson and Head propose that once
the dike reaches the surface, thereare four phases of eruption, and the
resulting landforms directly relate to
how much of the dike is in the mantle
and how much is in the crust. Magma
in the mantle is less dense and more
buoyant than the surrounding rocks.
The crust is likely to be lower density
than the dike magma, which tends not
to rise upward but is still pushed up by
the magma rising through the mantle.
The first phase occurs when the
top of a dike first penetrates the
surface, and gas explosively erupts into
space. Without wind, the entrained
microscopic magma droplets follow
ballistic paths, creating a thin circular
rain of pyroclastic debris around the dike
breech. Some of this forms dark mantle
deposits, like those seen east of Sinus
Aestuum, and also widely dispersed glass
beads. The dike penetrates the surface
at a single point, but as it continues to
ascend it creates a fissure typically 15 km
long, with pyroclastics and, later, lava
streaming out.
Following the explosive degassing,
phase 2 consists of rapid eruption of
large volumes of lava as the entire
dike continues upsurging. Giant
fire fountains discharge magma as
rapidly as one million cubic metres per
second. When the individual droplets
of lava hit the surface, they are still
extremely hot and coalesce into a lava
lake that ultimately flows over the
lowest topographic barrier and races
downslope, sometimes for hundreds
ofkilometres.Iftheeruptionisshort-
lived,itproducesasinglelavaflow
complex.Butifit’slonger,say,aweekor
more in duration, the lava erodes down
into underlying material and rapidly
cutsasinuousrille,suchasHadley
Rilleseen at the Apollo 15 landing site.
Inphase3,morethanhalfof
thedike’smagmaisnowlocatedin
thecrust,decreasingitsbuoyancyDark mantle deposits
such as those found east
of Mare Aestuum are
the result of a phase 1
explosive gas eruption.
NASA / GSFC / ARIZONA STATE UNIVERSITY (4)