America's Volcanic Past
|"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:
|[NOTE: This list is just a sample of various Delaware features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Delaware.]|
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.
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|
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.
Delaware's Coastal Plain Units -- Triassic to Holocene:1
Sediments, includes Triassic to Jurassic diabase dikes
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
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.
|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.
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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