Sinkholes, like this one located directly outside the Applied Sciences
Building, form in carbonate terranes when acidic groundwater dissolves
the underlying rock. They are typically closed depressions, in which water
drains down into the underlying rock rather than over a surface stream or
gully. Sinkholes are common in a type of topography called
karst, which is
characterized by abundant sinkholes, caves, springs, and disappearing
streams. Although common in many parts of the world,
such as the southeast U.S., karst is uncommon in the west. It occurs on
the UCSC campus, however, a feature which makes its
natural histroy all the more unusual and interesting.
Sinkholes, caves, and other karst features form when carbonate rock
disolves in acidic groundwater. Normal rainwater becomes acidic as it
percolates through the soil and picks up carbon dioxide (CO2) produced by
organisms in the soil. The CO2 dissolves in the water and forms carbonic
acid:
CO2 + H2O = H2CO3 (carbonic acid)
which disassociates into a hydrogen cation and bicarbonate anion to form
carbonic acid:
H2CO3 = H+ + HCO3- (bicarbonate anion)
The hydrogen of the carbonic acid then attacks the calcium carbonate of
which the marble is composed:
CaCO3 (calcium carbonate) + 2H+ = Ca++ + 2HCO3-
(the two +'s near the Ca
refer to the double positive charge of the Ca cation)
The Ca++ and HCO3- ions then flow away in the groundwater. This process
can form underground caves and passageways. If one of these underground
cavaties collapse, a sinkhole forms. On the UCSC campus groundwater flows
along joints and fractures dissolving
the marble and forming sinkholes, caves and other karst features. With
time, the joints and fractures widen and
turn into cracks and canyons. As a result, the deep
canyons of the UCSC campus align with the
general north-south orientation of the joints observable in the marble of
the upper quarry.
Karst terranes can cause enginerring challanges. These large cement
pillars, photographed on the ground floor of the Applied Science Building,
are part of the building's foundation and go all the way to the top floors.
They were incorporated during early phases of construction
when construction workers noticed cement kept disappearing into the
earth, presumably into subsurface karst features. The pillars, which are
essentially long support stakes, extend deep into the ground and comprise
an integral part of the building's foundation.
This outcrop, found along the path on the way to the quarry, is an
example of the campus marble. The most notable feature is an isoclinal
fold visible in the faint banding. Click on the image to see
the larger
version and see if you can see the noses of these folds. They point down to
the lower right. Folds like this are formed when stress from tectonic
forces cause the rocks to compress and fold. The layers, or bands, are
probably of metamorphic origin
and visible because some layers of the marble weather more easily than
others. This process, called differential weathering, occurs because some
layers contain small amounts of minerals like graphite, which weather
differently than the layers without the graphite.
The deformational event that caused the isoclinal folding
probably occured during or after the metamophic
event that produced the marble; metamorphic banding, not
sedimentary banding, is preserved.
These kilns, along the path between the Applied Science Building and the
quarry, are similar to those found at the base of campus and throughout
the Santa Cruz Mountains. They were used to bake marble to separate the lime
used in cement,
morter, and plaster. More information about quarrying and the use of
these kilns is given at
The Quarry
and
History of the Quarry stops.
