That portion of the earth’s surface known as the Big Bend has
often been described as a geologist’s paradise. In part this
is due to the sparse vegetation of the region, which allows the
various strata to be easily observed and studied. It is also due to
the complex geologic history of the area, presenting a challenge to
students and researchers from all over the world.
Not all field geologists, however, refer to the Big Bend as a
paradise. For some, this land of twisted, tortured rock is a
nightmare. The abundance, diversity and complexity of visible rock
outcrops is staggering, especially to first-time observers. From
500 million year old rocks at Persimmon Gap to modern-day windblown
sand dunes at Boquillas Canyon, geologic formations in Big Bend
demonstrate amazingly diverse depositional styles over a vast
interval of time.
The spines of dark rock you see marching along the desert floor
here are called dikes.
These outcroppings are evidence of the most recent igneous activity
in the area. Like great stone fences, dikes can be traced for miles
across the park. They resemble spikes on the back of a Triceratops.
They are reminders of the molten world beneath the quiet desert
surface.
Dikes are tabular or sheet-like bodies of magma that cut vertically
or almost vertically through and across strata, though some dikes
are steeply inclined. Hundreds of dikes can invade the cone and
inner core of a volcano. Dikes may occur in swarms of parallel
dikes, particularly where there has been crustal extension. In
regions of crustal extension, fracturing may open the route for
filling by magma from a deep source, or intrusive magma may promote
the fracturing and extension of the crust. Outcrops of dikes can
range from a few metres to many kilometres in length, and can
spread lateral distances from a few centimetres wide to over 100 m.
Very thin dikes or dikelets are sometimes called veins.
Most dikes are made of magma, but sediments can form dikes too. The
sandstone dike was emplaced as hydrocarbons and gases moved into
the thick sand bed soon after it was buried, and the sand dike rose
into the overlying mud while all of the material was not yet
hardened into stone. Igneous dikes are common in many bodies of
plutonic or highly metamorphosed rock, where molten or fluid
materials have invaded preexisting rock formations. What defines a
dike is that it cuts across the bedding planes of the rock it
intrudes. When an intrusion cuts along the bedding planes, it is
called a sill. In a simple set of flat-lying rock beds, dikes are
vertical and sills are horizontal.
To claim credit for this cache, answer the following
questions...
1. What do you think is beneath the surface here?
2. According to the sign, how long ago were these dikes
created?
3. How do you think the spike-like formations were created?
4. Bonus points for pictures although this is not mandatory.
Park superintendent Bill Wellman is aware of the creation of this
earthcache.