LAB 3:    Campus Geology Field Trip

The following pages contain reference material on the geology of the UCSC campus and
questions to be discussed and answered at each stop of the in-lab hike around campus.
WEAR COMFORTABLE WALKING SHOES, CLOTHES YOU DON'T MIND
GETTING DIRTY, AND BRING FLASHLIGHTS.

Caves

	Most caves are formed by ground water solution and are found in marble and
limestones (which are composed of calcite (CaCO3) and/or dolomite (CaMg(CO3)2). 
Pure water has very little effect on these minerals but when CO2 is added to water,
carbonic acid (H2CO3) is produced; this greatly increases the dissolving power of ground
water.  The ground water receives its CO2 from two sources, a small amount is absorbed
by rain falling through the atmosphere but most is derived from the decay of organic
matter.  The reasctions that cause solution/precipitation of calcite/dolomite are as follows:

			1.   H2O + CO2 == H2CO3
			2.   H2CO3 == H+ + HCO3-
			3.   2H+ + CaCO3 == H2O + CO2 + Ca++

	These equations show that addition of CO2 to ground water favors solution of
calcite, conversely, removal of CO2 favors precipitation of calcite.
	Ground water seeps into fractures (usually joints or faults) in the rock and, if the
CO2 content is high enough, solution occurs.  The original fracture is gradually enlarged
until caves and passageways are eventually formed.  A variety of factors dictate the size of
any specific cave.  Some important variables are: CO2 content of the ground water, length
of time rock is exposed to ground water solution, variations in rock solubility (dolomite is
less soluble than calcite), etc.
	Most students of solution caves believe they are formed below the water table
(surface below which the soil and rock is saturated with water).  They reason that
maintenance of a sufficiently high concentration of CO2 in the ground water for a time
period long enough to dissolve a large volume of carbonate material requires that the CO2
not be allowed to equilibrate with the much lower concentration of CO2 in the atmosphere. 
(This is what happens when you leave your ginger ale unstoppered overnight - loses it's
charge but saves your teeth).  Therefore, effective solution only takes place below the water
table where there is virtually no air.  This then also explains the formation of flowstone or
dripstone structures such as stalactites, stalacmites, etc.; if ground water with a high
concentration of Ca++ ions is exposed to air, the calcite will precipitate.
	A cautionary note should be mentioned; under certain circumstances it is possible
to dissolve carbonate in the presence of air.  If the H2CO3 is sufficiently concentrated, it
could dissolve carbonate before the CO2 could equilibrate with the atmosphere.
	Limestones and marbles are almost never pure carbonate minerals but contain
variable amounts of clays and other insoluble impurities.  When the carbonate is dissolved,
the insoluble residue is either carried away in suspension by the ground water or settles out
and is deposited on the walls and floor of the solution cavity.

Sinkholes

	Sink holes are solution cavities at the earth's surface; they form in two ways, the
collapse of an underground cave or by solution at the surface (in contact with the
atmosphere).