The entrance to the Empire Cave, as seen on the left, partially hidden
behind the overgrown
thicket of bushes, and protected by a cement wall. The view is looking up
Cave Gulch, with the small stream on the left and Kresge and Porter
colleges up the hill to the right. The University has tried to cover the
entrance to the cave several times with cement and steel structures, only
to have it dynamited each time by irate
cave enthusiasts. Several other
caves occur in the area, with entrances across the creek.
Caves are common worldwide, wherever karst terranes are found.
As discussed at
the sinkole site, caves form when acidic groundwater disolves the CaCO3
marble. This process can occur most easily below the water table because
the CO2 is needed to make carbonic acid in the groundwater:
The image on the right looks down the entrance to the Empire Cave. Most
of the caves along Cave Gulch were probably formed as acidic waters
percolated along faults and fractures, as can be seen from the
north-south orientation of the
Empire Cave passage way.
Many of the canyons on campus and
joints in the quarry walls
are oriented in
this direction as well.
These directional features are controlled by a
persvasive north-south fracture system
in the marble.
A number of characteristic cave features, such as stalactites and
stalagmites, are found in the Empire Cave. These formation grow by the
deposition of CaCO3 from waters that percolate through the rock.
Presumably, as the waters reach the air of the cave, dissovled CO2
escapes, decreasing the ability of the water to hold the calcium carbonate
in solution. This process enables CaCO3 to precipitate as the water drips
down the stalactites, which grow down from the ceiling, and onto the
stalagmites, which grow up from the floor.
The last rock type found on campus is represented here by this
granitic boulder photographed in the Cave Gulch creek.
A
closer look reveals an igneous rock with interconnecting grains
of quartz, plagioclase feldspar, amphibole, and biotite. This rock is a
member of the Ben Lomand Quartz Diorite. It is exposed in Cave Gulch and
in many roadcuts on Empire Grade. In many places, heavy rains have
decomposed the amphibole
and biotite, leaving a sandy soil composed of quartz and feldspar.
the Ben Lomand Quartz Diorite is part of a number of intrusions that
occur within the Salinian Block and shown on this
map in red. The granitic rocks of
the Salinian Block are similar in composition and to those of the Sierras
and are believed by some to be a
southerly extension of the Sierras that
has been transported northward along the San Andreas Fault. Both the
Sierran batholith and the plutons of the Salinian Block were probably
represent the deep roots of volcanoes formed along the over-riding plate
of a subduction zone. Some magmas reached the surface and formed
volcanoes like
Mt. St. Helens or
Mt. Ranier, whereas some magmas were
somehow trapped beneath the surface and cooled slowly to form the
plutonic rocks we observe today.
