My first Magazine Sky & Telescope - 02.2019 | Page 54

It All Depends on Dikes
The appearance of volcanic lunar features has everything to
do with what was happening below the surface eons ago.
olcanic landforms, such as sinuous
rilles, domes, cones, dark-halo cra-
ters, fl oor-fractured craters, pyroclastic
deposits, and even young lava fl ows are
often favorite targets for lunar observers.
Scientists have published many studies
describing the morphologic character-
istics and possible origins for these vol-
canic 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 differ-
ent 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.
V
Dark mantle deposits
such as those found east
of Mare Aestuum (above)
are the result of a phase 1
explosive gas eruption.
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FE B RUA RY 2 019 • SK Y & TELESCOPE
Magma is generated deep within
the lunar mantle, and large blobs of
it known as diapirs ascend to within a
few hundred kilometers of the surface.
The continuing upwelling of additional
magma into the diapir increases the
pressure 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 dia-
pir 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 penny, 60 to 100
kilometers (37 to 62 miles) high and
wide but less than 100 meters thick. If
a dike from a diapir is stalled relatively
near the surface, it can maintain a con-
nection 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 Moon quakes!
Wilson and Head propose that once
the dike reaches the surface, there are
four phases of eruption, and the result-
ing 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 ris-
ing through the mantle.
The fi rst phase occurs when the top
of a dike fi rst penetrates the surface,
and gas explosively erupts into space.
Without wind, the entrained micro-
scopic 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 sur-
face at a single point, but as it continues
to ascend it creates a fi ssure 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 fi re foun-
tains discharge magma as rapidly as one
million cubic meters per second. When
the individual droplets of lava hit the
surface, they are still extremely hot and
coalesce into a lava lake that ultimately
fl ows over the lowest topographic bar-
rier and races downslope, sometimes for
hundreds of kilometers. If the eruption
is short-lived, it produces a single lava
fl ow complex. But if it’s longer, say,
a week or more in duration, the lava
erodes down into underlying material
and rapidly cuts a sinuous rille, such
FEBRUARY 2019 OBSERVING
Exploring the Moon by Charles Wood