This is the first of several virtual geology “geocaches” since they
all describe some aspect of the unique geology of Staten Island.
This rock outcrop is one of several exposures on Staten Island
and forms part of the northern and central upland of this island.
At this waypoint, you are standing approximately 209 feet above sea
level. This rock was formed when an island arc system, similar to
that of the Aleutian Islands of Alaska, collided with an eastward
moving North American continent. The resulting collision or
mountain building event known as the Taconic Orogeny formed the
Appalachian Mountains.
During this collision, slivers of the Earth’s upper mantle rock
known as peridotite were injected into the crust. This variety of
peridotite rock is known as a harzburgite and it typically contains
an abundance of the minerals olivine and pyroxene with minor
magnetite and chromite. During this collision and injection
process, heat, pressure, and chemically active fluids, a process
known as metamorphism occurred. As a result, the olivine and
pyroxene minerals became transformed into a metamorphic rock,called
the serpentinite.
During and after its emplacement during the Taconic Orogeny,
this serpentinite body became highly fractured by tensional forces
that occurred as it expanded during its metamorphic alteration.
These fractures exist as three separate but intersecting sets and
decrease in abundance with depth below the surface. The existence
of these fractures or joints allows mineral-rich groundwater to
flow preferentially along these natural pipelines. Look for
fracture-filled veins of small aragonite crystals on the hillside,
just below the GPS waypoint.
Another similar serpentinite body appears under Steven’s
Institute in Hoboken, New Jersey. These serpentinites are part of
the Appalachian Ultramafic Belt that stretches from Newfoundland to
Alabama. On the whole, this rock is jet black in color when fresh,
but chemical weathering from the atmosphere rapidly turns it from
black-to-green-to-yellow-to brown, due to the oxidation of the iron
and magnesium-rich minerals present. Looking more closely at this
rock, it contains many unique minerals and some rare types as
well.
The serpentine group of minerals are known in order of abundance
on Staten Island as lizardite, chrysotile, and antigorite. The
lizardite occurs as apple-green to dull green-white grains or
masses, which will stick to your tongue when licked! Chrysotile
forms short, soft and flexible white cross-fibers in narrow veins,
while the antigorite forms brittle waxy-green or bluish-green sharp
and splintery (painful) fibers found in small fault or shear zones.
Associated with the minerals are others known as magnetite,
chromite, brucite, and talc. The magnetite occurs as small but
sharp 8-sided black crystals or octahedrons, which are attracted to
a magnet when freed from their host rock, while the chromite occurs
similarly but is not as magnetic. Brucite exists as flat masses of
silvery-white but clear flexible sheets, while the talc occurs as
silvery white or greenish-white masses and flakes that are very
slippery to the touch in cracks and fractures within the
serpentinite. In other parts of the world, talc is used to make
talcum powder. Also, look for the reddish-brown and fibrous
anthophyllite, which resembles petrified wood. Feel free to search
and collect some of these minerals!
Surficial weathering involves two processes: chemical
dissolution and physical breakdown. Rain water charged with carbon
dioxide forms a weak but natural acid known as carbonic acid
(H2CO3). At and below the surface of the ground, this acid reacts
with the minerals of the serpentinite, olivine, pyroxene, and
brucite. Weathering products derived from their breakdown weaken
the rock by creating new, less dense and hydrated minerals such as
artinite, hydromagnesite, pyroaurite, and iron ores such as
limonite and hematite.
Physical weathering involves the downward percolation of
rainwater and circulating groundwater that can enter cracks and
fractures within the rock. Upon freezing, the water expands 9% in
volume to further enlarge existing cracks and create new ones,
therefore further weakening the rock. Thus, both chemical and
physical weathering serves to eventually destroy the exposed rock.
Highly weathered rock is fractured, greenish-white in color,
relatively soft, and has a low density.
Exposed rock weathers relatively fast here as evidenced by the
present exposure and the larger outcrop exposed along Route 278,
near the Petreides School. Eventually insoluble parts of this rock
will crumble into small fragments at the surface and form a
relatively infertile serpentine soil, upon which will grow only a
few species of grasses (bluestem), shrubs, and trees. The
serpentinite also has low chromium content from remnant chromite
grains, which many plants cannot tolerate; thus plants and shrubs
become stunted in their growth. Good soil profiles are rare above
outcrops and any soil covering is usually very thin.
In 1970, this hillside was scheduled by a developer to become a
residential area with a great overlook of the lower portion of New
York Harbor and the Narrows. Once the serpentinite bedrock was
uncovered, construction and excavation costs became problematic and
the project was abandoned. It was later turned into an unofficial
park by local residents who maintain this area today.
Visitors are encouraged to explore this hilly area. To get
credit for this Earthcache, read the signs along the fence and tell
me how the serpentinite rock type got its name.