 |
|
|
Although the campus marble is too
highly fractured for use in
sculpture or
building material, it was
sufficiently pure for the extraction of lime (CaO). The sheer cliffs seen in
these
photographs remains from use of explosives to extract the marble from
these quarries. The marble of the UC
Santa Cruz campus is part of a group of metamorphic and plutonic rocks
called the Salinian Block. These rocks are located west of the San Andreas
Fault and are sliding northward along coastal
California as part of the Pacific Plate. Marble is metamorphosed
limestone -- a kind of sedimentary rock. There are many different types
of limestone, but most are composed entirely of the
shells or hard parts of marine organisms. Limestone usually forms in
warm shallow seas, where many types of organisms
live. Coral reefs, for example, build up layers of coral skeletans that
eventually turn into limestone. Coral reefs form anywhere there are warm
shallow seas. Old volcanoes like Tahiti or Bora Bora in the
South pacific form common nucleation points.
Thus, these rocks started their
story in a place where folks might like to go
SCUBA diving
or snorkling.
After limestone formation, tectonic
forces took these particular rocks to high temperatures and pressures
deep within the Earth,
and turned them into metamophic rocks. The limestones turned into marble
while the the pelitic rocks turned into schists and gniesses. During
metamorphism, nearly all original sedimentary structures were
obliterated. The campus marble is massive, which means it lacks structure
that was formed during metamorphism. After metamophism, the roks must
have been uplifted, and erosion occurred to reveal the rocks as we see
them today. Joints, which formed after the main
period of metamorphism, are one type of structure that is well developed
in the marble.
The vertical cracks on the right and left photographs are good examples
of joints in the campus marble. On the left, the large vertical joints
used to form popular
climbing routes, whereas on the right, the same joint orientation can be
seen on the cliff in the foreground and (less easily) on the cliffs in
the background. The photograph on the right was taken from the spot in
the quarry that is now filled in with boulders. The orientation of the
joints is constant and is related
to the direction of stress. These joints have a regional orientation on
campus and play an important role in groundwater flow and the
orientation of the canyons on campus. Since the rock deformed brittly
and not ductally (i.e., folding), we know that the stress occured at low
pressures and temperatures, perhaps close to the surface. The red color is
oxidation of surfaces that were exposed to air or percolating fluids.
Closer inspection of the marble reveals a coarse-grained rock composed
almost entirely of a single mineral -- calcite. Calcite is easily
recognizable in the field because it can be scratched with a pocketknife
and fizzes readily with HCl. The crystals
are rhombahedral in shape -- sort of like squished cubes. This shape is
characteristic of calcite and reflects its atomic structure. The faces of
the rhombahedron are not crystal growth faces, but fracture planes,
called cleavage. These faces represent three orientations in the calcite
crystal structure that are most likely to fracture when the crystal is
broken. Thus, on the surfaces that we can observe, which must have been
broken from another piece of rock, we always see fractures along these
cleavage planes. The campus marble is about 97% pure calcium carbonate.
One of the most common impurities is graphite, which is pure carbon and
appears as tiny dark, metallic specks. The fitid oder from freshly broken
pieces is hydrogen sulfide -- H2S. At the Kalkar Quarry, just below
campus, a huge variety of over 50 unusual minerals have been found, many
of which can be seen in the Earth Sciences mineral exhibit. These unusual
specimans result from element-rich groundwater percolating
through the joints in the marble and reacting with the carbonate in the
marble.
This little shed housed explosives used to break the rock free during the active mining days. Historical photographs tell the story of the social, economic, and environmental impact that these rocks had on Santa Cruz County and California.
