The next time you are
enjoying the spectacular views of Manhattan from the North Beach at
Sandy Hook, NJ, shift your gaze across the Verrazano Bridge to Staten
Island. The crest of that bland, dark ridgeline is Todt Hill. Todt is
Dutch for
“dead”.
At 410 feet, it is the
highest natural point on the eastern
seaboard south of Maine. The Dutch settlers probably called it
“Dead Hill” because of its rocky, treeless outcrops. One of the
bedrock exposures is serpentinite:
grayish-green, about 430 million years old, and consisting mainly of
the mineral serpentine. It contains such high levels of magnesium
that plants
can't grow in its thin soils.
That was then. Today, Todt
Hill is a verdant, upscale community, that was once home to Paul
Castellano of the Gambino crime family.
An average sample of this
serpentinite also contains about 27%
chrysotile asbestos. In 1858, the H.W. Johns Manufacturing
Company began mining chrysotile asbestos in Staten Island for
manufacturing fire-resistant shingles (Powell, 2005). They eventually
merged with Johns-Manville, which went on to manufacture a variety of
asbestos-containing products in numerous factories, including one in
Manville,
NJ. It was litigated into bankruptcy in 1982, by “class action
lawsuits based on asbestos-related injuries such as asbestosis,
lung cancer and malignant mesothelioma.”
The west coast has
outcrops of serpentinite as well, but the serpentine
in the San Andreas Fault is about 400
million years younger than in Staten Island. The University of
California has published a factsheet
about how to recognize and reduce risks from the asbestos in its
landscape.
That's all well and
good but how the hell did it get there?
0.5 bya
Half
a billion years ago, North America was a continental land mass
near the equator called Laurentia.
It was across the Iapetus
Ocean (an early version of the Atlantic Ocean) from Gondwana
- the ancient continent of Africa, South America, Australia,
Antarctica, and India (Benimoff and Ohan, 2005), according to the
theory of plate
tectonics. The plates containing these continents were
converging and would eventually form the single supercontinent of
Pangaea about 200
million years later.
Converging means
colliding. About 470 million years ago, as the Iapetus Ocean was
closing during the slow-motion collision of Laurentia and Gondwana,
a string of volcanic islands in between the two continents (map)
called the Taconic
Arc collided with Laurentia (Powell, 2005).
As the ocean floor between
Laurentia and the islands was driven under Laurentia (subduction),
a “slice”of it buckled
onto the continent. This fragment was composed of peridotite,
which over time “metamorphosed to form the string of serpentinite
pods that occur across the New York City area, including the Staten
Island serpentinite” (Powell, 2005).
It's the oldest
rock on Staten Island. NJ's serpentine deposits are discussed on
page
5 of this 2014 NJGS
newsletter.
Cameron's Line
Not only is the Taconic
Orogeny the source of this asbestos, as well as the
first of three mountain -building
events that eventually formed the Appalachian mountains, it
created a crack in the earth's crust - a
fault - called Cameron's Line.
Cameron’s
Line is an 80
to 180 foot wide band of crushed rock originating in New England
that lies about 600’ below NY Harbor. It crosses the Bronx, follows
the East River, and passes under Staten Island to the west
of Todt Hill. This map (pdf-p
2, Figure 1) shows it (CL) entering NJ's
Coastal Plain in Middlesex County by western Raritan Bay. This
is near the Fall
Line - the geologic boundary between the the rocks of the
Piedmont
and sandy outwash of the Atlantic Coastal Plain – that more or less
follows Rt 1 in NJ.
NJ's Coastal Plain is
deposited on a bedrock
of gneiss and schist in Monmouth County (pdf-p
21) – metamorphic rocks formed from shale and sandstone – and
other rock (pdf-p
12). In Monmouth County, this “basement rock” slopes steeply
under the deepest aquifer in the County, the Potomac-Raritan-Magothy
aquifer system (map).
The basement rock is about 500 feet below sea level in Aberdeen, and
more than 1500 feet BSL in Manasquan – and over a mile in Cape May
(map on pdf-p 19
).
Cameron's Line cuts
through the basement rock beneath the northern Coastal Plain and may
eventually join with the Huntingdon Valley Fault zone (HVF on the map
in Figure 1) near Philadelphia and Trenton (pdf-p
17).
Earthquakes in NJ are
mostly caused by the Ramapo
Fault along the Ramapo
mountains, as well as Cameron’s Line. The Ramapo Fault is
about twice as old as Cameron's Line, and
at 1 billion years is one of the oldest faults in the US. This is
one of the reasons the earthquakes on the East Coast are less active
than on the West Coast - because the geology of the East Coast is so
much older. The crust on the East Coast is also cooler and more rigid
than on the West Coast, so
seismic waves disperse further and earthquakes are not as intense
locally.
The New Jersey Department
of Environmental Protection's webpage on the Earthquake
Risk in New Jersey, states that “the presence or absence of
mapped faults (fault lines) does not denote either a seismic hazard
or the lack of one ...”
Okay. In any case, what's
beneath those rolling hills in Staten Island is anything but bland. Or as Job
said: “As for the earth, out of it cometh bread: and under it is
turned up as it were fire.”
Selected References
(no longer posted)
Benimoff, A. and Ohan, A.
Accessed 12/23/05. The Geology of Staten Island. Formerly posted at
www.library.csi.cuny.edu/dept/as/geo/sigeo.htm
Powell, Wayne. Accessed
12/23/05. The Staten Island Serpentinite. Formerly posted at
http://academic.brooklyn.cuny.edu/geology/powell/NYCgeology/staten%20island/staten_island.htm
