America's Volcanic Past -
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"Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Volcanic Highlights and Features:
[This list is just a sample of
various Delaware volcanic features or events and is by no means inclusive.]
Delaware:
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The Appalachians:4
The Atlantic Plain:4 The Atlantic Plain is the flattest of the provinces. It stretches over 2,200 miles in length from Cape Cod to the Mexican border and southward another 1,000 miles to the Yucatan Peninsula. The Atlantic plain slopes gently seaward from the inland highlands in a series of terraces. This gentle slope continues far into the Atlantic and Gulf of Mexico, forming the continental shelf. This region was born during the breakup of the supercontinent Pangea in the early Mesozoic Era.
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Delaware's Piedmont |
Delaware Piedmont:2
Piedmont means foothills, hence these hills
are a part of the foothills of the
Appalachian Mountains which lie to the west.
The rocks exposed
in the Delaware Piedmont are metamorphic and igneous rocks
that are approximately half a billion to 1.2 billion years old. They have
an interesting story to tell of the
collision between the ancient North American continent
and offshore volcanoes,
the upheaval of
an enormous mountain range, prolonged erosion, and the deposition
of sediments onto the continental shelf as the mountains
became hills.
Offshore Volcano Collides with North America:2
The rocks at the surface
in the Piedmont today are
old, deformed, metamorphic rocks
that were once buried
in the core of an ancient
mountain range. This range
formed early in a series of tectonic
events that built the
Appalachians between about
543 and 250 million years
ago. During an early event,
called the
Taconic orogeny, an offshore chain of volcanoes
collided with the ancient
North American continental
margin to push up a gigantic
mountain range
that was as
tall as the Alps or the Rockies
of today. Geologists date the
Taconic orogeny between 470
and 440 million years ago.
The Taconic orogeny is
important to our understanding
of the geology of
Delaware, because during this
event, the rocks of Delaware's
Piedmont were deeply buried
under miles of overlying rock
and metamorphosed by heat from the underlying mantle. Since
that time, rivers and streams have carried the erosional products,
mostly sand, clay, and gravel, from the mountains onto the
Atlantic Coastal Plain and continental shelf. As the mountains
wear down, the buried rocks rebound and rise to the surface. Thus
what we see in the Piedmont today are old deformed metamorphic
rocks that were once buried deep within an ancient mountain
range.
Delaware's Volcanic Rocks |
Delaware's Amphibolite:2
A rock composed primarily of amphibole and feldspar. The
amphibole grains are commonly elongated with long axes parallel.
In the Delaware Piedmont most amphibolites are formed
by the metamorphism of igneous rocks.
Delaware's Blue Rocks:2
The most extensive and well-known rock in the Wilmington
Complex is a light-dark banded gneiss,
known locally as the "blue rocks."
The amphibolites
and "blue rocks" of the
Wilmington
Complex were
formerly a volcanic
island that
existed seaward
of the ancient
North American
continent about
500 million years
ago.
Wilmington's original minor league
baseball team thought
they were as solid and resilient as the
local rocks and called themselves
the Blue Rocks. Recently the name has been revived, and
Wilmington's new baseball team is also called the Blue Rocks.
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Delaware's Coastal Plain Units -- Triassic to Holocene:1
Sediments, includes Triassic to Jurassic
diabase dikes
Delaware's Gneiss:2
Gneiss is a course-grained rock
commonly having imperfect, but prominent
light-dark layering. In the Delaware Piedmont the light
layers are composed of feldspars and quartz and the dark layers
of mica, garnet, sillimanite, amphiboles, and pyroxenes.
Gneisses are formed by the high-grade metamorphism of
either igneous or sedimentary rocks.
Rocks of the Delaware Piedmont:2
Within Delaware's Piedmont, five distinct rock units can be
recognized:
(1) rocks of the volcanic arc, (2) rocks formed from the
mud and sand deposited in the deep ocean that existed between
the volcanic arc and the ancient continental margin, (3 & 4) rocks
that were once sand and carbonates (calcite and dolomite) lying
on the shallow shelf of the ancient continental margin, and (5)
rocks of the ancient North American continent.
The names given
to these units indicate the geographic
area where they were first
identified.
The rocks within the volcanic arc geologic
setting are known as the Wilmington Complex and were
first identified in the City of Wilmington, Delaware.
Delaware's Piedmont Units -- Precambrian:1
Baltimore Gneiss
Delaware's Piedmont Units -- Cambrian, Ordovician, Silurian:1
Wilmington Complex:
metagabbro,
gneiss,
granite,
pegmatite,
tonalite,
marble
Delaware's Wilmington Complex:2
The Wilmington Complex is a diverse association of metamorphic rocks
that formed in an offshore volcanic setting.
Although originally
igneous and sedimentary, most of the exposed rocks have
been buried, heated, and changed into metamorphic rocks.
Geologists estimate these rocks were buried to depths of 11 to 13
miles, and heated to temperatures as high as 1,600 degrees F. These are
among the highest metamorphic temperatures recorded in the
entire Appalachian system.
Mixed with the metamorphic rocks are
various intrusive igneous rocks, such as
gabbros, diorites, and granites.
These igneous rocks may represent the crystallized remains of
magma chambers or vents, or pockets of rock melted
during metamorphism.
The age of the Wilmington Complex is controversial; however,
a large mass of granitic rock, exposed in the community of Arden,
was radiometrically dated and found to be approximately 500 million
years old. This date suggests the Wilmington Complex may
represent the root zone of the volcanic arc that existed off the
ancient North American continent during Cambrian and Early
Ordovician time, between 543 and 480 million years ago.
Delaware's Wissahickon Formation Gneisses:2
The gneisses
of the Wissahickon
Formation
represent sediments
deposited
in a deep ocean
basin between
the volcanic
island and the
continental shelf.
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Brandywine Springs Park |
Brandywine Springs Park:2
In Brandywine Springs Park, you will notice there are many
large round boulders in the stream bed and along the banks.
These boulders are gneisses and amphibolites,
and if you could see them
up close, you would find that many of them are spotted with large
olive-green grains of pyroxene, a mineral commonly found in
igneous and high-grade metamorphic rocks.
Red Clay Valley |
Red Clay Valley Igneous Rocks:2
The igneous rocks
exposed in the Red Clay Valley are mostly
coarse-grained, intrusive rocks
that are named
granites, granitic pegmatites, diorites, and
gabbros.
These rocks form in large masses
usually without the layering that is characteristic of
sedimentary and
metamorphic rocks.
White Clay Creek Preserve |
White Clay Creek Preserve, Delaware and Pennsylvania:3
White Clay Creek forms a scenic valley
incised in the rolling Piedmont terrain of southeastern
Pennsylvania and northwestern Delaware.
Some 600 million years ago, the Preserve was part
of a large continental area that subsided
and was covered by a shallow sea. Through time,
sediment composed of sand, silt, and mud
spread over the sea floor.
At various intervals,
volcanoes poured lava onto these deposits.
Gradually the sediments hardened into
sedimentary rocks.
About 460 million years ago, an immense
mountain-building episode folded and heated the
rocks and completely changed their character.
The rocks in the Preserve "cooked" at elevated
temperatures and pressures for some 70 million
years, long enough for the new minerals to
develop. Approximately 390 million years ago,
the Preserve was uplifted and cooled, which
halted the metamorphism.
Since then, the minerals have remained largely
unchanged.
The lava flows became very dark gray amphibolites.
Nearly black hornblende dominates these
rocks; interspersed feldspar grains tend to be
medium gray to white.
Excerpts from:
1) Delaware Geological Survey Website, 2002
2) Margaret O. Plank and William S. Schenck, 1998, Delaware Piedmont Geology:
Delaware Geological Survey, University of Delaware, Delaware Geological Survey Website, July
2001;
3) Pennsylvania Geological Survey Website, July 2001
4) USGS/NPS Geology in the Parks Website, August 2001
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