T(l-r): Basalt/Diabase, Mica Schist, Gneiss
B(l-r): Limestone w Stromatolites, Pumice, Scoria
Jetties
are mostly volcanic rocks like basalt and diabase - not
obsidian!
Obsidian
is a sought-after block
in Minecraft that forms when magma rapidly cools in air or water.
There's no obsidian or volcanoes at Sandy Hook, but most of those
dark gray boulders in the jetties were also formed by volcanoes
(slide
20).
Like
obsidian, diabase
is formed from magma in the earth's crust, while basalt
is formed from lava,
which is magma that has flowed to the earths surface.
Basalt
and diabase were formed as the supercontinent
Pangaea started breaking apart, according to the theory of plate
tectonics. The Appalachian Mountains and Anti-Atlas mountains
that are now in Africa were once a single range higher than the
Himalayas (Gallagher, 2003). NJ
shared a border with Morocco about 245 million years ago!
When
Pangaea started rifting apart about 200 million years ago, lava
formed into the basalts of the Watchung
Mountains, the pillow lava (Figure
94) of the Orange
Mountains, and the Great
Falls in Patterson). Diabase cooled into the columns
of the Palisades. The water surrounding Pangaea rushed in to fill
the rift between the new continents, creating the Atlantic Ocean
(Gallagher, 2003). A lot going on in the back-story of those bland
jetty rocks and the gravel in your driveway.
Basalt
and diabase are also used as traprock and gravel are mined in the
Piedmont
province of NJ. Chunks of the jetties worn smooth as sea
glass can be found in the wrack line.
Other
Jetty Rocks
Schist
and gneiss boulders like those found in the Highlands
province of NJ can be found in jetties at Sandy Hook or in the
seawall
extending to Sea Bright that was built by the Army in 1898
(scroll
down for rock pictures). Several boulders of light gray mica
schist can be seen among the darker basalts in the jetty
protecting the road along Horseshoe Cove south of Parking Lot K. When
pieces are found in the wrack line they look like this.
Parts
of the granite
seawalls the Army built around the tip of Sandy Hook in the 1890s
to protect the gun batteries still line the shoreline of the
freshwater pond past Nine
Gun Battery near North Beach. Granite in NJ is from the Highlands
province (map,
p. 1)
Limestone
boulders that could be half a billion years old from the Valley
and Ridge province (start
at slide 37) are also in the seawall near the entrance to Sandy
Hook. The chalky white boulders that formed from
shells accumulating in warm shallow seas stick out among the
dark, volcanic basalts in the seawall near the steps from the
Highlands Bridge. Some have banded swirls that may be fossils of
stromatolites
- the Earth's oldest fossil (3.7
billion years old) - that gradually changed the “atmosphere
from a carbon dioxide-rich mixture to the present-day oxygen-rich
atmosphere” 2.5 billion years ago.
Other
Volcanic Rocks
Other
volcanic rocks in the wrack line more likely washed up from a
storm-flooded landscaping
project rather than from drifting in on the Gulf
Stream from a Caribbean volcano. Pumice (scroll
down for picture) looks like quartz but is much lighter, almost
airy; when you break it open you can see crystals similar
to obsidian. The unusual foamy look comes from being rapidly
cooled and depressurized after it is ejected from a volcano.
Scoria forms from
cinder cones around
volcanoes and is used to make lightweight concrete.
Bedrock
The
gneiss and schist like the boulders found in the jetties and seawall
make up the bedrock (pdf-p
21) beneath the aquifers of the coastal
plain in NJ. They are metamorphic rocks formed from shale and
sandstone and are found in the Highlands
province of NJ. (The Wanaque
Tonalite Gneiss is part of an outcrop that runs from Wanaque to
Ringwood with the oldest rocks in NJ - 1.35 billion to 1.37 billion
years old.
In
Monmouth County, this “basement rock” lies about 700 feet below
sea level, beneath the deepest aquifer in Sandy Hook, the
Potomac-Raritan-Magothy (map).
Bedrock is more than 1500 feet below sea level in Manasquan, and over
a mile deep in Cape May (map
on pdf-p 19 ).
Selected
References
Forman,
Richard (Ed.). 1998. Pine Barrens: Ecosystem and Landscape
Gallagher,
W. 2003. When Dinosaurs Roamed New Jersey. Rutgers University Press.
New Brunswick, NJ
https://www.amazon.com/When-Dinosaurs-Roamed-New-Jersey/dp/0813523494/ref=sr_1_fkmr0_1?s=books&ie=UTF8&qid=1472587810&sr=1-1-fkmr0&keywords=When+Dinosaurs+Roamed+New+Jersey.+Rutgers+University+Press.+New+Brunswick%2C+NJ.
The
other four parts of this blog are at http://pehealthnj.blogspot.com/
.
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