From Davis Mountain Vistas
by William MacLeod
B
ooks on geology always have to
begin with a table such as the one
pictured here to introduce the gen-
eral reader to the geological time scale
and
the
terminology
involved. The time scale or
geological column begins
with the creation of the
Earth, about 4.6 billion years
ago and ends at the present
day. The basic unit of time is
one million years or m.y.
Events are described as hav-
ing occurred or taken place so
many millions of years ago,
abbreviated as Ma.
The time scale is divided
into
the
Precambrian,
Paleozoic, Mesozoic and
Cenozoic eras, which in turn
are divided into periods.
Periods are further divided
into epochs. Rocks are tabu-
lated into formations, bodies
of rock that can be identified
in the field by their physical
characteristics and position in
the geologic time scale. A for-
mation is sometimes subdi-
vided into members or com-
bined with other formations
to form a group.
Volcanic activity in the
Trans-Pecos of Texas devel-
oped in two episodes, one
between 47 and 27 Ma, the
other between 25 and 18 Ma,
as part of a volcanic arc
roughly parallel to the west
coast of northern Mexico and
the southern United States.
The arc developed because
the North American plate,
the section of the Earth’s crust
on which North America sits,
began over-riding its neigh-
boring Farallon plate at the
west coast of the continent.
As the Farallon plate descend-
ed into the Earth’s interior
below North America, it car-
ried with it vast amounts of
water and water-saturated
sediments from the sea floor.
Once this water-laden mate-
20
rial reached 75 miles or so below the
Earth’s surface, if began to melt from the
combination of high temperature and
pressure.
The molten rock or
magma rose in the crust
under the force of gravity
and formed pools or reser-
voirs in magma chambers,
typically 20 to 60 miles
underground. Eventually,
magma reached the Earth’s
surface through vents and
fissures, creating volcanic
eruptions.
Geologists have identi-
fied several vents in the
Davis Mountains, none
unfortunately visible from
the highways, but you can
see rock-filled fissures in the
Paisano Volcano between
Alpine and Marfa, where
erosion has uncovered
many vertical rock bodies
or dikes that originated as
magma in fissures.
Some magma, especially
the magma first erupting,
contained water vapor and
exploded into clouds of
small droplets with the drop
in pressure at the Earth’s
surface. The droplets
instantaneously solidified
into volcanic glass and,
depending on their size and
the strength of the explo-
sions fell to the ground
locally or rose up into the
atmosphere. All eventually
fell to earth to form vol-
canic ash.
Over time, minerals
such as calcite, silica and
iron oxide came in with
groundwater and cemented
the volcanic ash deposits
into a soft rock called tuff.
In some cases, the hot ash
partially re-melted as it
accumulated and created a
rock called welded tuff. All
degrees of welding, from
slightly-welded rocks to
Cenizo
Third Quarter 2018
rocks almost identical to lava, occur in
the Davis Mountains.
Magma that lacked water vapor or
other volatile matter oozed out as lava on
to the Earth’s surface. Some were basalts
that flowed freely but most were silica-
rich rhyolites or trachytes that were vis-
cose when hot, having the consistency of
oatmeal, and that tended to form thick,
stumpy flows near their vents or fissures.
In places, a circular or oval depression
on the surface known as a caldera devel-
oped as the of a magma chamber col-
lapsed after some or all magma had
erupted. Some calderas, such as the
Infiernito Caldera, are visible at the
earth’s surface. Geologists have identi-
fied several calderas in the area, marked
on the geological maps by black dashed
lines.
A large number of volcanic and intru-
sive entities have been identified in the
Davis Mountains volcanic field; some are
area-wide, others more local. Younger
volcanic rocks are absent south of Alpine;
volcanism died away there much earlier
than in the high mountains.
Volcanic action began with the erup-
tion of a thick blanket of volcanic ash
with occasional basalt lava layers. The ash