The
Coast Mountains are a major mountain range of western North America, extending from southwestern
Yukon
through the Alaska Panhandle
and virtually all of the Coast of British Columbia.
They are so-named because of their proximity to the sea coast, and
are often referred to as the
Coast Range. It
includes volcanic and non-volcanic mountains and the huge icefields
of the
Pacific and
Boundary Ranges, and the northern end of the
notable
volcanic system known as the
Cascade Volcanoes.
The Coast Mountains
are part of a larger mountain system called the Pacific Coast Ranges or the Pacific
Mountain System, which includes the Cascade Range, the Insular Mountains, the Oregon and California Coast Ranges and the
Saint
Elias
and Chugach
Mountains.
The Coast Mountains are approximately long and average in width.
Its
southern and southeastern boundaries are surrounded by the Fraser River and the Interior
Plateau
while its far northwestern edge is delimited by the
Kelsall
and Tatshenshini
Rivers
at the north end of the Alaska Panhandle, beyond
which are the Saint Elias Mountains, and by Champagne
Pass
in the Yukon Territory
. Covered in dense
temperate rainforest on its western
exposures, the range rises to heavily
glaciated peaks, including the largest
temperate-latitude icefields in the world.
It then tapers to the
dry Interior Plateau on its eastern flanks, or to the subarctic
boreal forest of the Skeena
Mountains and Stikine
Plateau
.
The Coast
Mountains are part of the Pacific
Ring of Fire, the ring of volcanoes and associated mountains
around the Pacific
Ocean
. It contains some of British Columbia's
highest mountains.
Mount Waddington
, northeast of the head of Knight Inlet
with an elevation of , is the highest mountain of
the Coast Mountains and the highest that lies entirely within
British Columbia.
Geography
The Coast
Mountains consists of three subdivisions known as the Pacific Ranges, the Kitimat Ranges
, and the Boundary
Ranges. The Pacific Ranges are the southernmost
subdivision of the Coast Mountains, extending from the lower
stretches of the Fraser River to
Bella
Coola
. Included in this subdivision is four of the
five major coastal icecaps in the southern Coast Mountains. These
are the largest temperate-latitude icecaps in the world and fuel a
number of major
rivers.
Other than logging and
a large ski resort at the resort town of
Whistler
, most of the land in the range is completely
undeveloped. Mount Waddington, the highest mountain of the Coast Mountains, lies
in the Waddington Range of the
Pacific Ranges.
Just north of the Pacific Ranges lies the central subdivision known
as the Kitimat Ranges.
This subdivision extends from the Bella Coola
River
and Burke Channel in
the south to the Nass River in the
north.
The third
and northernmost subdivision of the Coast Mountains is the Boundary Ranges, extending from the Nass
River in the south to the Kelsall River in the north. It is also the
largest subdivision of the Coast Mountains, spanning the British
Columbia-Alaska border and northwards into Yukon flanking the
west side of the Yukon
River
drainage as far as Champagne Pass, north of which being the
Yukon Ranges. The Boundary Ranges
include several large icefields, including the Juneau
Icefield
between
Juneau,
Alaska
and Atlin
Lake
in British Columbia, and the Stikine
Icecap
, which lies between the lower Stikine River
and the Whiting River
.
Because
the Coast Mountains are just east of the Pacific Ocean, they have a profound effect on British Columbia's
climate by forcing moisture-laden air off the Pacific Ocean to
rise, dropping heavy rainfalls on the western slopes where lush
forests exist. This precipitation is among the heaviest in
North America. The eastern slopes are relatively dry and less steep
and protect the
British
Columbia Interior from the Pacific weather systems, resulting
in dry warm summers and dry cold winters.
Beyond
the eastern slopes is a plateau occupying the southern and central
portions of British Columbia called the Interior Plateau. Included within the Interior Plateau is a
coalescing series of layered
flood
basalt lava flows. These sequences of fluid
volcanic rock cover about of the Interior
Plateau and have a volume of about , forming a large
volcanic plateau constructed atop of the
Interior Plateau.
North of the Interior Plateau on the range's
northeastern slopes lies a huge mountainous area known by
geographers as the Interior
Mountains, which includes the neighboring Skeena, Cassiar
and Hazelton Mountains
.
Geology
Origins and growth
The Coast Mountains consists of deformed
igneous and
metamorphosed structurally complex
pre-
Tertiary rocks. These originated in
diverse locations around the globe: the area is built of several
different
terranes of different ages with a
broad range of tectonic origins.
In addition, oceanic crust under the Pacific Ocean is being subducted at the
southern portion of the range to form a north-south line of
volcanoes called the Garibaldi
Volcanic Belt, a northern extension of the Cascade Volcanoes in the northwestern
United
States
, and contains the most explosive young volcanoes in
Canada. Further north the northwesterly structural trend of
the Coast Mountains lies partly in a large continental
rift responsible for the creation of several volcanoes.
These volcanoes form part of the
Northern Cordilleran
Volcanic Province, the most volcanically active area in
Canada.
Insular and Omineca Arc eruptive periods
The Bridge River Ocean between North
America and the Insular Islands
The first event began 130 million years ago when a group of active
volcanic islands approached a pre-existing
continental margin and coastline of North
America. These volcanic islands, known as the
Insular Islands by geoscientists, were
formed on a pre-existing
tectonic
plate called the
Insular Plate by
subduction of the former
Farallon Plate to the west during the early
Paleozoic period. This subduction zone
records another subduction zone to the east under an ancient
ocean basin between the Insular Islands
and the former continential margin of North America called the
Bridge River Ocean. This
arrangement of two parallel subduction zones is unusual in that
very few twin subduction zones exist on Earth; the
Philippine Mobile Belt off the
southeastern coast of
Asia is an example of a
modern twin subduction zone. As the Insular Plate drew closer to
the pre-existing continential margin by ongoing subduction under
the Bridge River Ocean, the Insular Islands drew closer to the
former continential margin and coastline of western North America,
supporting a pre-existing volcanic arc on the former continential
margin of North America called the
Omineca
Arc. As the
North American
Plate drifted west and the Insular Plate drifted east to the
old continential margin of western North America, the Bridge River
Ocean eventually closed by ongoing subduction under the Bridge
River Ocean. This subduction zone eventually jammed and shut down
completely 115 million years ago, ending the Omineca Arc and the
Insular Islands collided, forming the
Insular Belt. Compression resulting from this
collision crushed, fractured and
folded rocks along the old continental
margin. The Insular Belt then welded onto the pre-existing
continental margin by magma that eventually cooled to create a
large mass of
igneous rock, creating a
new continental margin. This large mass of igneous rock is the
largest
granite outcropping in North
America.
Plate tectonics of the Coast Range Arc
100 million years ago.
Coast Range Arc eruptive period
The final event began when the
Farallon
Plate continued to subduct under the new continental margin
after the Insular Plate and Insular Islands collided with the old
continental margin, supporting a new continental volcanic arc
called the
Coast Range Arc about 100
million years ago during the
Late
Cretaceous period. Magma ascending from the Farallon Plate
under the new continential margin burned their way upward through
the the newly accreted Insular Belt, injecting huge quantities of
granite into older igneous rocks of the Insular Belt. At the
surface, new volcanoes were built along the continental margin.
Named after the Coast Mountains, the basement of this arc was
likely Early Cretaceous and
Late
Jurassic intrusions from the Insular Islands.
Plate tectonics of the Coast Range Arc
about 75 million years ago
One of the major aspects that changed early during the Coast Range
Arc was the status of the northern end of the Farallon Plate, a
portion now known as the
Kula Plate.
About 85 million years ago, the Kula Plate broke off from the
Farallon Plate to form a
mid-ocean
ridge known to geoscientists as the
Kula-Farallon Ridge. This change
apparently had some important ramifications for regional geologic
evolution. When this change was completed, Coast Range Arc
volcanism returned and sections of the arc were uplifted
"""considerably""" in latest Cretaceous time. """This started a
period of mountain building that affected much of western North
America called the
Laramide
orogeny.""" In particular a large area of dextral transpression
and southwest-directed thrust """faulting""" was active from 75 to
65 """million years ago.""" Much of the record of this deformation
has been """overridden""" by
Tertiary
"""age""" structures """and""" the zone of Cretaceous dextral
thrust faulting appears to have been widespread. It was also during
this period when massive amounts of molten granite intruded highly
deformed ocean rocks and assorted fragments from pre-existing
island arcs, largely remnants of the Bridge River Ocean. This
molten granite burned the old oceanic sediments into a glittering
medium-grade
metamorphic rock
called
schist. The older intrusions of the
Coast Range Arc were then deformed under the heat and pressure of
later intrusions, turning them into layered metamorphic rock known
as
gneiss. In some places, mixtures of older
intrusive rocks and the original oceanic rocks have been distorted
and warped under intense heat, weight and stress to create unusual
swirled patters known as
migmatite,
appearing to have been nearly melted in the procedure.
Volcanism began to decline along the length of the arc about 60
million years ago during the
Albian and
Aptian faunal
stages of the Cretaceous period. This resulted from the
changing geometry of the Kula Plate, which progressively developed
a more northerly movement along the
Pacific Northwest. Instead of subducting
beneath the Pacific Northwest, the Kula Plate began subducting
underneath southwestern Yukon and Alaska and during the early
Eocene period. Volcanism along the entire
length of the Coast Range Arc shut down about 50 million years ago
and many of the volcanoes have disappeared from erosion. What
remains of the Coast Range Arc to this day are outcrops of granite
when magma intruded and cooled at depth beneath the volcanoes,
forming the present Coast Mountains. During construction of
intrusions 70 and 57 million years ago, the northern motion of the
Kula Plate might have been between and per year. However, other
geologic studies determined the Kula Plate moved at a rate as fast
as per year.
High-prominence peaks
The North Shore Mountains near
Vancouver.
View of the Juneau Icefield.
Mountain ranges
See also
References
- [1]
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