 |
| To view the
entire Appalachian chain from Maine to
Georgia, click
on the image above.
(1.3 KB file) |
The
Appalachians are a complex mix of mountains
formed by a series of continental collisions that
took place over a period of more than 1 billion
years.
| This page focuses on important mountain-building and
landscape altering milestones in the
history of the Northern Appalachians and
New England. It utilizes a set of illustrative cross sections
to depict the cumulative effects of these
milestone events. The example cross-section
runs parallel to,
but slightly north of the present
southern coasts of Connecticut, Rhode
Island and Massachusetts, and
extends westward into New York state and
eastward beyond Cape Cod. Each geological event mentioned on this
page is described more
fully on other pages on this site; and those
pages, in turn, have links to external resources. Each
of these mountain building events (orogenies)
included incorporation of new land masses seaward of the
former coast line,
distortion of existing land forms and
metamorphosis of existing rocks. Erosion of the each
newly-formed mountain chain resulted in deposition of
massive amounts of sediment - both inland and along the coast.
Over the more recent past (tens of
thousands of years) the region
has been further shaped by two major glacial advance and
retreat cycles.
To assist in visualizing the
impacts of these cumulative events, a consistent color
code has been used to indicate the ages of rocks
depicted in each sketch. (See description near bottom of
page.)
|
The Process Begins More than 1
Billion Years Ago with Creation of Grenville Mountains
Creation
of the Appalachians began more than 1 billion
years ago when an extensive chain of
mountains was formed as a result of a series of collisions between the
largest land masses of the time. Approximately 85% of
the world's current land mass had formed by this time in the
Earth's history; however, the various pieces were arranged quite
differently than they are now (see sketch below-left).
A series of
collisions known as the Grenville events occurred over a prolonged period -
about 350 million years. They resulted in
formation of an early supercontinent called Rodinia
with Laurentia occupying a central position. The core of the present
North American land mass was hit from all sides
during formation of this
"supercontinent"; and the grand scale
of the Grenville collisions resulted in the
creation of colossal mountain ranges on many
continents.
The large, roughly
triangular-shaped land mass (labeled
"Laurentia") located in the center of the sketch of the
components of Rodinia is the continental core of present North America.
It includes much of the present United States, Canada and
Greenland. Many parts of the United States - most of the
land west of the Rocky Mountains and east of the Appalachians -
were not part of this early land mass (see sketch on right).
"Laurentia" or "Proto-North
America" (two names commonly used
to refer to predecessors to
our familiar continent) had been built up over
the previous 1 1/2 billion years by agglomeration
of many smaller land masses (see the purple, dark
blue and dark teal-green areas of the map
below-right.) Between each of these early collisions, there were periods
of crustal stretching, marine
sedimentation, and erosional
deposition.
 |
|
Proto-North
America (at the center of the
drawing of Rodinia on the left) was located at the equator
and rotated
almost 90 degrees from its current
orientation.
Impacts with parts of Africa, South America
and Europe, pushed a huge wedge of
continental margin and ocean sediment
onto Proto-North America. This resulted in a wide
chain of mountains from northern Mexico
through Canada to Greenland (red band
at right.)
|
|
 |
During these
collision, the
"Northeast US" collided with Amazonia, which is part
of present-day South America. The impact was against the western
side of present-day South America - where Peru is currently
located. The "Northeastern US" coastline was near the
present Hudson Valley.
| Circa 1.3 Billion
Years Ago - Proto-North America is
Surrounded by Sediment - Amazonia
Approaches |
 |
| Proto
- North America (Coast Near Hudson
Valley) |
Part
of Present-day South America |
| Please
visit our Prelude
page for more information on what was
happening at this time. |
| Circa 1.2 to 1.0
Billion Years Ago - The Grenville
Orogenies - Focus on the Present "East Coast" of the
United States - Part of Present-day South America
Collides with Laurentia |
 |
| Proto
- North America |
Mountains
Within Rodinia Supercontinent |
Part
of Present-day South America |
| Please
visit our Prelude
page for more information on what was
happening at this time. |
Rodinia
is believed to have existed as a coherent mass
for about 350 million years. The lands to the
north of Laurentia (Antarctica, Australia, India
and parts of southern Africa) began to rift away
around 750 million years ago, beginning formation
of the Panthassa (later Pacific) Ocean. About 650
to 575 million years ago, the lands to the south
rifted as well, as the Iapetus Ocean (predecessor
to the Atlantic) formed. (Scroll back up to see the rift zones shown in
white in the depiction of Rodinia, about 1 billion years ago.)
About
575 Million Years Ago:
Rifting Creates the Iapetus Ocean
The Iapetus Ocean (a forerunner to the Atlantic
Ocean) was born as Laurentia separated from components of
present-day Africa and South America and from
Baltica (Proto-Western Europe).
| Circa 575 Million
Years Ago - Rifting Separates Laurentia and Southern Rodinia
- Iapetus Ocean Forms |
 |
| Proto-
North America (Coast Re-forms Near Hudson
Valley) |
Part
of Present-day South America |
| Please
visit our Prelude
page for more information on what was
happening at this time. |
For the next
several hundred thousand years, Laurentia drifted
further north until the current eastern seaboard
area was close to the Equator.
Most of the North
American continent that is now south of Canada
was covered by shallow tropical seas.
Layers of limestone formed in many parts of
Laurentia, including atop the now-greatly-eroded
Grenville mountains.
| Adding the Land
that Became New England was an Extended,
Multi-step Process |
New England was added between 465 to 400
million years ago, when two very
long island arcs, the Taconic
and Avalonian
(which had each formed near the junction
of South America and Africa and then
drifted away) were drawn toward the
southern (now-eastern) coast of
Proto-North America by a subduction zone
that formed near the coast. (A subduction
zone is a zone of sinking ocean crust
that forms along a plate boundary.) As
each island chain collided with
Laurentia, it added land and raised
mountains from Canada to the Carolinas.
(See the graphics and summary descriptions below).The
two-island arc collision model just described is, of course, a
simplification of complex events that occurred over
hundreds of millions of years. Michael
Brown, of the Laboratory for Crustal
Petrology, University of Maryland,
reports that on the basis of geological,
geo-chemical and paleomagnetic evidence,
"it is clear that the tectonic
evolution of the Northern Appalachians
was (more) complex".
Brown cites local
variations in characteristics of land in
the Newfoundland through New
Brunswick-New England segment of the
Northern Appalachians as evidence that
"a number of independent, generally
short-lived arc systems were present in
the Iapetus Ocean, outboard of the
margins of Laurentia and Gondwana during
the Ordivician and Silurian".
(Between about 505 and 408 million years
ago). These small arc systems became
incorporated into Proto-North America,
independently or as part of the
large-scale Taconic and Avalonian
collision events, leaving evidence of their existence in local
geological discontinuities.
|
About 450
Million Years Ago:
Formation of
The Taconic Mountain Chain
| Circa 470 Million
Years Ago - the Taconic Island arc approaches
the eroded, limestone-covered future "East Coast"
of the United States |
 |
| Proto-
North America (Coast Near Hudson Valley) |
Taconic
Island Arc (Volcanic) |
| Please
visit our Taconic
and Acadian Orogenies
page for more on what was happening at
this time. |
The
Taconic Orogeny took place about 450 million
years ago. These collisions between the
island chain and Proto-North America added
the western portions of New England as well as portions of the Maritime
provinces in Canada and land in northern New
Jersey, south-eastern Pennsylvania, Virginia and
North Carolina.
 |
|
The Taconic Mountains were created as
the islands rode up and onto the
Laurentian land mass. In the process,
Grenville Mountain remnants were pushed
up, distorted and overlaid by Taconic arc
materials. Thrusting and folding led to huge amounts of sediment
that washed into a basin that formed behind
the mountain chains resulting in the "Queenstown delta"
near Albany, New York.. The Taconic orogeny
ended about 445 million years ago. |
| Circa 445 Million
Years Ago - the Taconic Orogeny is Ending
- The Avalonian Island Arc Approaches |
 |
| Proto-
North America (Coast East of Hudson
Valley) |
Avalonian
Island Arc (Volcanic) |
| Please
visit our Taconic
and Acadian Orogenies
page for more on what was happening at
this time. |
About 425
Million Years Ago:
Creation of More Mountains: Collision with
Avalonia
The Acadian Orogeny
started about 430 to 425 million years ago when
the eastern end of the Avalonian arc (together
with Baltica, or Proto-Western Europe) impacted
against the northeast coast of Proto-North
America, in what is now Canada. Ongoing
collisions continued for about 50 million years
as more of the island chain impacted against
Laurentia. The impact zone progressed from east to west. Parts
of the Avalonian island chain came ashore as far
south as the Carolinas.
As the Avalonian chain
approached Laurentia, the basaltic bed of the
Iapetus Ocean was pulled under Laurentia
(subducted) and melted. Volcanoes
formed in and near the narrowing ocean.
| The Avalonian
Island Arc Pushes Ocean Crust Toward
Shore - Circa 410 Million Years Ago |
 |
| Proto-
North America (Coast East of Hudson
Valley) |
Avalonian
Island Arc (Volcanic) |
| Please
visit our Taconic
and Acadian Orogenies
page for more on what was happening at
this time. |
 |
|
As Avalonian terrane rode up and onto
the coast, it pushed a wide swath of
seabed material onto the land,
compressing, bending, lifting and pushing
existing rock formations. The new
mountains were initially very high. |
| A large amount of sand
and gravel that had been pushed ashore
washed westward from New England and
southward from Canada into a basin inland
of the mountains, where thick layers of
sedimentary rock built up in the Catskill
Delta region of New York and
Pennsylvania. Additional sediment was
generated by erosion of the new
mountains, and it added to the build up
of the land in this region, which is
often referred to as the Appalachian
Plateau.. |
 |
Most of
Maine, New Hampshire and Vermont plus
major parts of Massachusetts and
Connecticut were created by the large
volumes of volcanic material and Iapetus
Ocean sediments that were pushed
onto the margin of Laurentia. This Iapetus Ocean
Terrane (land created from ocean
bottom materials) is colored
light grey in this sketch.
Avalon Terrane (the original
volcanic arc material) is
shown in dark grey.
All of Rhode
Island is underlain by Avalon Terrane.
|
While the Acadian
orogeny (sometimes also
called the Appalachian or
Avalonian orogeny) is given credit for creating
the Northern Appalachian mountains - the story of
these mountains is far from complete.
About
350 Million Years Ago:
Collision with Gondwana Creates More Mountains
More than 425 million years ago,
subduction zones developed in the oceans on all
sides of Laurentia and around Gondwana (the large
continent that contained most of modern Africa
and South America). As ocean crust was consumed,
the giant land masses were drawn closer together.
By about 390 million years ago the gap between
the zones had been consumed. Proto-North America
was more buoyant. Gondwanda was pulled toward the
subduction zone (and, inevitably, toward
Laurentia).
| Circa 375 Million
Years Ago - With Complex Mountains Lining
the Coast, Collision Looms with Gondwana |
 |
| Proto-
North America (Coast Near Present
Location) |
Gondwana
(Africa plus S. America) |
| Please
visit our Alleghenian
Orogeny page for
more information on what was happening at
this time. |
Once collision
began, about 350 million years ago, the thick
continental crust of Gonwanda rode up and onto
the edge of Proto-North America. In New England,
Avalonian Terrane was thrust under Iapetus
Terrane and the remnants of older mountains were
pushed, crushed and buried.
| Circa 350 Million
Years Ago - CRUNCH TIME! - Collision of
North America With Africa (Gondwana) |
 |
| Proto- North
America |
Gondwana
(Africa, S. America and more) |
| Please
visit our Alleghenian
Orogeny page for
more information on what was happening at
this time. |
| |
 |
|
Along the impact zone between
Proto-North America and
Gondwana, a
vast mountain range arose
extending from eastern Mexico (which collided
with northwestern South America) along
the east coast of North America into
Canada. Near-simultaneous collisions
occurring between parts of Europe and
Northeastern Canada and Greenland,
extended the mountain range into
Northern Europe.
Collision energy deformed and
metamorphosed rocks that had already
been altered by earlier mountain
building episodes. Effects
penetrated far beyond the
Acadian and Taconic
mountain ranges,
to most of the
area underlain by the remnants of the
ancient Grenvilles. |
| Compression
and folding produced a
valley and ridge province
far inland (most
pronounced in
Pennsylvania and other
states located south of
New York.) |
| |
|
|
| In the Narragansett Bay area
and part of Massachusetts, folded rocks
formed a large depression (or basin)
during the Alleghenian Orogeny. Sediments
accumulated in the Narragansett Basin
forming shale, which makes up the upper
layers of bedrock beneath much of the
area. Continuing collisions
led to creation of the
"supercontinent" Pangaea
(meaning "all lands"
in Greek) about 306 million years
ago. (See color-coded sketch at
right.) The present east coast of North
America was far from the sea.
|
 |
About 220 Million Years Ago:
Rifting Creates the Atlantic Ocean
By the
time North America began to pull away
from Africa, about 220 million years ago,
weathering had greatly reduced the size
of the huge mountains that had been
created in the Alleghenian Orogeny. Land
that had been compressed and
metamorphosed by the energy of collision
was now stretched, cracked and rearranged
as the land masses separated.
|
Circa
220 Million Years Ago - Breakup of
Pangaea - Atlantic Ocean Forms (then
continues to widen)
|
 |
| North
American Coast Re-forming Offshore of
Present Location |
Gondwana
(Africa, S. America and
other lands) |
| Please
visit our Atlantic
Formation page for
more on what was happening at this time. |
The
(Relatively) Recent Past:
Glaciations and Rising Seas Reshape the Coast
| The most
recent changes to the Appalachian
mountains, and all of New England, have been driven by erosion and glaciations, rather than orogeny.
|
From about 1.8 million to about 11,000 years ago, a period
known as the Pleistocene, much of the northern portion of the world was
alternately covered with glacial ice and uncovered again
during interglacial periods. The last two glaciations reached the New England coasts
about 55,000 and 18,000 years ago.
Everything was covered, with slowly
moving ice; even the tops of tall
mountains. The map at right
depicts the maximum reach of the last
great glaciation, about 18,000 years ago.
As the ice sheets
advanced they scoured rocks, uncovered
Grenville formations and carved out
Narragansett Bay. Plants and animals were
driven far south. As the ice retreated it
left debris - sometimes isolated rocks
and sometimes huge accumulations of rock,
sand and silt. Many well known
geographical landmarks like Long Island,
Long Island Sound, Block Island,
Nantucket and Cape Cod were created by
glacial action.
|
 |
| The Northern
Appalachians Today - After 200 Million
Additional Years of Erosion, Deposition
and Glaciation |
 |
| The
Present New England Coast |
Continental
Shelf and
Expanding Atlantic Ocean |
| Please
visit our Holocene
and Reference
pages for more information on what is
going on NOW. |
As
temperatures have risen since the last ice age,
so have ocean levels - by as much as 400 feet -
moving the shoreline inland many miles from the edge of the
continental shelf. As the glacial ice began to melt, Long Island and
Block Island Sounds filled with fresh water. As the ocean level
rose, fed by melted ice and snow, salt water eventually flowed
over portions of the moraines that had contained these large
lakes. As the sea continued to rise, salt water eventually
entered and filled Narragansett Bay as well.
The
only significant mountain building event occurring in the
Northern Appalachians the continuing rise of the ancient Grenvilles in the
Adirondack region. Vertical uplift has been going on for
10 to 20 million years.
For most of the region, erosion
is the dominant transforming force.
Color
Code for the event sketches above:
- Ocean crust is shown in red
- Pre-Grenville crust is mustard
yellow
- Crust created during the
Grenville orogenies (and
predecessor sediment deposits) is
purplish-brown
- Sediment laid down after the
Grenville orogonies, but prior to
the Taconic Orogeny, is pale
yellow-green
- Taconic arc materials are mustard
brown
- Sediment laid down between the
Taconic and Acadian Orogenies is
light yellow - green
- Avalonian arc materials are olive
green
- African crust is light dusty rose
We are indebted to
Roy
W. Schlische, Department
of Geological Sciences, Rutgers
University, for posting
a
set of cross-section sketches for New
Jersey (parts of which have been
incorporated into this set) and to Andrew Birrell for making a set of relief maps
available on the internet, from which the
map at the top of the page has been
excerpted.
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