Download Evidence for plate tectonics, part 1

MAR 110: Introductory
Oceanography
Ocean basins and plate tectonics
Geologic hazards, part 1
• Tectonic activity plays a major role in the evolution
of ocean basins.
• Tectonic activity lies behind most major geological
hazards: volcanic eruptions, earthquakes, and their
aftereffects.
• Three of the greatest natural disasters of the past 200
years were geological in nature.
– Tambora, 1815
– Krakatau, 1883
– Indian Ocean earthquake and tsunami, 2004
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Geologic hazards, part 2
• Tambora is a volcano on the northern shores of
Sumbawa.
– From 5 April to 11 April 1815, Tambora came alive with
several large eruptions – the largest in recorded history. The
series of eruptions finally ended in July of that year.
– The largest eruptions, from 10 April-11 April, began as
“three columns of fire rising to a great height,” and
culminated in the ejection of 150 cubic kilometers of
magma and ash (more than 150 times Mount St. Helens).
– 90,000 died as a result of the eruptions, associated
pyroclastic flows, and famines that followed.
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Geologic hazards, part 3
• Tambora (continued):
– The effects of Tambora were felt far from Southeast Asia.
Dust and aerosols from the eruption blocked sunlight,
triggering in the following year (1816) what became known
as the “year without a summer” in northeastern North
America and western Europe.
• Average global temperatures dropped about 0.3 °F, which was
enough to significantly disrupt agriculture in North America and
Europe.
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Geologic hazards, part 4
• Krakatau (Krakatoa) is a volcano in the Sunda Strait
west of Java (not east of Java as in the movie title).
– From 20 May to 27 August 1883, Krakatau went through a
series of increasingly violent eruptions, culminating in a
final series of cataclysmic explosions that began mid-day
on 26 August and ended on 27 August.
– Two-thirds of the original island disappeared.
– The sound of the final blast was heard more than 4,600 km
away, from Rodriguez Island and Sri Lanka to Australia.
– At least 36,000 died in the eruptions and related tsunami
(with waves up to 40 m high).
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Geological hazards, part 5
• On the morning of December 26, 2004, a massive
earthquake struck along a subduction zone in the
Indian Ocean off the coast of Sumatra and the
Andaman Islands.
– The earthquake was determined (by the U.S. Geological
Survey) to be 9.1 magnitude on the Richter scale, which
makes it the third largest earthquake in history.
– The massive slip on the ocean floor triggered a tsunami that
raced through the Indian Ocean, killing more than 200,000.
• One victim was reported from Kenya.
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John McPhee, in his book, Basin and Range, wrote that
if he had to restrict his writing on plate tectonics to one
sentence, it would be, “The summit of Mt. Everest is
marine limestone.”
His sentence deftly captures the importance of plate
tectonics theory today. Plate tectonics explains the
origin of the Earth’s major structures, ranging in size
from continents and ocean basins down to mountain
ranges, rift valleys and oceanic islands, and from th
highest peaks of the Himalayas to the deepest trenches
of the Pacific.
– From Upheaval from the Abyss
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The results of plate tectonic processes abound, from the
mighty folds of the ancient and well worn Appalachians,
the majestic escarpments of the East African Rift Valley
and the Palisades of the Hudson River, and the
volcanoes that imperil cities in Iceland, Indonesia,
Japan and Mexico. Even idyllic island resorts in Hawaii
and the Caribbean owe their existence to the movements
of great slabs of the Earth’s crust. The theory explains
all volcanism and most earthquake activity on Earth. Its
importance reaches far beyond the earth sciences, too.
– From Upheaval from the Abyss
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For instance, one cannot understand the diversity of
life on this planet without some knowledge of both
evolution and plate tectonics. In many parts of world,
billions of dollars are spent to prepare buildings, roads
and humans for the inevitable volcanic and earthquake
event.
– From Upheaval from the Abyss
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Distribution of the oceans
• Ocean basins and continents are unevenly distributed,
with land (29 percent of the Earth’s surface)
concentrated in the Northern Hemisphere and ocean
(71 percent of the Earth’s surface) dominating the
Southern Hemisphere.
– NH: 39.3 percent land, 60.7 percent water
– SH: 19.1 percent land, 80.9 percent water
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Ocean basins
• While the ocean basins are interconnected, four are
largely separated by continents.
–
–
–
–
Pacific Ocean
Atlantic Ocean
Indian Ocean
Arctic Ocean
• Another ocean was recognized by the International
Hydrographic Organization in 2000.
– The Southern Ocean was defined as waters between 60
degrees S and Antarctica.
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Polar asymmetry
• In the Northern Hemisphere, the Arctic Ocean is an
ocean surrounded by continents.
• In the Southern Hemisphere, Antarctica is a continent
surrounded by oceans.
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Arbitrary boundaries
• Longitude 150 degrees E between Australia and
Antarctica separates the Pacific and Indian oceans.
• Longitude 70 degrees W between Cape Horn and
Antarctica separates the Atlantic and Pacific oceans.
• Longitude 20 degrees E between the Cape of Good
Hope and Antarctica separates the Indian and Atlantic
oceans.
• The Bering Strait separates the Arctic and Pacific
oceans.
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The crust, part 1
• The crust is the solid poriton of the lithosphere; it is
the geosphere’s interface with the atmosphere,
biosphere, and hydrosphere.
• The crust is made of rocks, which in turn are made of
one or more minerals.
– Minerals are naturally occurring inorganic solids
characterized by an orderly internal arrangement of atoms
and with fixed physical and chemical properties.
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The crust, part 2
• Rocks are classified as one of three types:
– Igneous
– Sedimentary
– Metamorphic
• Igneous rocks form from the cooling and
crystallization of molten magma.
– Magma that cools slowly within the crust forms coarsegrained rocks such as granite.
– Magma that reaches the surface becomes lava; lava cools
quickly and forms fine-grained rock such as basalt or even
glassy rocks such as obsidian.
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The crust, part 3
• Sedimentary rocks are composed of compacted and
cemented fragments of rock and mineral grains, of
partially decomposed remains of organisms, or of
minerals precipitated from the water.
– Sediments form from rocks that undergo physical and
chemical weathering at or near the Earth’s surface.
– Sediments are transported via water, wind, or ice, and
typically settle in low-lying areas. In time, the sediments
are buried beneath other sediments and compacted into
layers of solid rock.
– Examples include sandstone, shale, limestone, and salt.
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The crust, part 4
• Metamorphic rocks are derived from other rocks that
are subjected to high pressures, high temperatures,
and/or chemically active fluids.
– Metamorphic rocks are crystalline like igneous rocks.
– Marble is metamorphozed limestone.
– Quartzite is metamorphozed sandstone.
• Bedrock is typically igneous, with metamorphic rock
locally.
– Sedimentary rocks overlie igneous or metamorphic rocks.
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The crust, part 5
• The crust and the ridgid upper portion of the mantle
comprise the lithosphere proper.
• Crust comes in two main types:
– Continental, composed mostly of granite and rich in silica
and aluminum (sial).
• Continental crust is thick (20 to 90 km), and is less dense than
oceanic crust.
• Continental lithosphere ranges from 100 to 150 km in thickness.
– Oceanic, composed mostly of fine-grained rock such as
basalt and rich in iron and magnesium (sima).
• Oceanic crust is thin (5 to 10 km).
• Oceanic lithosphere is no more than 100 km in thickness.
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The crust, part 6
• The lithosphere floats on the aesthenosphere, a
deformable region of the upper mantle that exhibits
plastic-like behavior.
– Because continental crust is less dense than oceanic crust, it
is more buoyant, thus floats higher than oceanic crust; this
is known as isostasy.
• Where ocean depths are shallow (usually less than
1,000 m), the bottom is typically of continental crust;
in deeper waters (more than 4,000 m), the bottom is
typically of oceanic crust.
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The rock cycle
• External and internal geological processes transpofrm
rock from one type to another in the rock cycle.
– Rocks and their component minerals are constantly
recycled.
– The rock cycle is extremely slow.
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Ocean bottom profile, part 1
• The average ocean depth is 3,800 m.
– Large areas are less than 200 m deep
– Other areas are as deep as 11,000 m
• Bottom features range from extremely flat to the most
rugged mountains in the world.
• Geological processes range from violent volcanism to
a gentle rain of sediment and organic debris.
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Ocean bottom profile, part 2
• Continental margins: There are three distinct zones as
one moves seaward from the shore.
– The continental shelf is a gentle slope that extends from the
shore out to a depth of about 130 m.
– The continental slope continues on from there, dropping
rapidly in depth until about 3,000 m.
– The continental rise is a narrow transition zone between the
base of the continental slope and either a flat ocean basin or
a deep sea trench beyond.
• Continents end at the base of the continental rise.
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Ocean bottom profile, part 3
• Continental shelves are characterized by a gentle
slope, generally about 1 degree (about 2 m/km)
seaward.
– They are generally wider along tectonically passive
continental margins and narrower along tectonically active
continental margins.
• The dropoff along a continental slope ranges from 1
to 25 degrees seaward, the average is about 50 m/km.
– Along passive margins, the continental slope may merge
with the continental rise.
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Ocean bottom profile, part 4
• Continental rises have somewhat steeper slopes than
the ocean floor beyond.
– Along passive margins, sediments may spread out from the
continental rise to form vast abyssal plains.
– Along active margins, the continental rise may descend into
deep-sea trenches.
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Ocean bottom profile, part 5
• Submarine canyons may cut into the continental
slopes and shelves.
– Many are carved by turbidity currents, a type of submarine
mass movement akin to an avalanche.
• Turbidity currents may reach speeds of up to 100 km/hr.
– Sediments carried by turbidity currents are deposited onto
submarine fans, akin to alluvial fans.
– Examples of submarine canyons include Hudson Canyon
and Monterrey Canyon.
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Ocean bottom profile, part 6
• Ocean basins include topography ranging from flat
plains to trenches, seamounts, and submarine
mountain ranges.
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Evidence for plate tectonics, part 1
• Francis Bacon only noticed the general similarity in
shape between Africa and South America – he wrote
nothing about a supposed fit between the two.
“The very configuration of the world itself in its greater
parts presents conformable instances which are not to be
neglected. Take for example Africa and the region of Peru
with the continent stretching to the Straits of Magellan, in
each of which tracts there are similar isthmuses and similar
promontories, which hardly can be by accident.”
Francis Bacon, Novum Organum
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Evidence for plate tectonics, part 3
• Abraham Ortelius did write in 1596 that the
coastlines of the Americas appeared to have been
joined in the past with Europe and Africa.
“But the vestiges of the rupture reveal themselves, if
someone brings forward a map of the world and considers
carefully the coasts of [Europe, Africa and the Americas],
where they face each other – I mean the projecting parts of
Europe and Africa, of course, along with the recesses of
America.”
Abraham Ortelius, Thesaurus Geographicus
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. . . the great ocean basins are permanent features of
the earth’s surface and they have existed, where they are
now, with moderate changes of outline, since the waters
first gathered.
– Bailey Willis, Principles of paleogeography
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Evidence for plate tectonics, part 5
• Eduard Suess proposed a southern supercontinent,
Gondwanaland, early in the twentieth century.
• Frank Bursley Taylor did propose a theory of
continental drift in 1908, but it was limited to an
explanation of the origin of equatorial mountain belts
in the drift of continents away from the poles – hardly
a comprehensive theory of continental drift as
proposed by Alfred Lothar Wegener, thus I don’t
regard Taylor as anything near Wegener’s equal.
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Evidence for plate tectonics, part 5
• Alfred Wegener noticed the similarity in shapes of the
coastlines of the Americas, Africa, and Europe.
“Please, look at a map of the world! Does not the east coast
of South America fit exactly with the west coast of Africa as
if they had formerly been joined? The correspondence is
still better if one compares not the present coasts but the
lines of descent to the deep sea.”
Alfred Wegener in a 1910 letter to his fiancée, Else Köppen
• Wegener presented his ideas publicly in 1912.
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Wegener’s argument was simple: About 300 million
years ago, all large land masses were united to form
one supercontinent, Pangaea. Beginning at about 150
million years ago, Pangaea began breaking up and the
fragments drifted apart, and, in some cases, collided
again, eventually becoming the continents we recognize
today. (Wegener also postulated that the process of
division and collision had been active before Pangaea
came into existence, but found it difficult to reconstruct
pre-Pangaea geography.)
– From Upheaval from the Abyss
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Wegener produced maps of how he thought the
continents had been assembled or disassembled at
various stages in time. He presented the geological,
geographical, climatological and paleontological
evidence in support of his theory, and he explained why
competing theories did not work .
– From Upheaval from the Abyss
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Evidence for plate tectonics, part 6
• British geologist Arthur Holmes, though not a fan of
Wegener, proposed in 1930 that convection currents
in the mantle could drive continental drift.
• Marie Tharp discovered a rift valley in the middle of
the Mid-Atlantic Ridge in 1952.
• Tharp and Bruze Heezen used several types of data –
including location of earthquake epicenters – to trace
a seismic belt running through mid-ocean ridges
throughout the world.
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As Tharp plotted the more northern profiles, she
noticed a large valley at the center of the Mid-Atlantic
Ridge. Although the valley wasn’t as prominent in the
three southern profiles, it was still there, typically 1000
fathoms deep and nine to 30 miles wide – as deep as the
Grand Canyon, but much wider. In the six weeks it took
her to prepare the profiles, Tharp became convinced
that she was looking at a rift valley and told Heezen so.
– From Upheaval from the Abyss
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Heezen did not want to hear the news. A rift valley
indicated that the Earth’s crust was spreading apart,
and that might mean the continents on either side of the
Atlantic were getting farther and farther away from
each other. If the continents were drifting apart, one
would have to conclude that there was something to
Alfred Wegener’s crackpot theory. Speaking out in favor
of continental drift would be an act of professional
suicide.
– From Upheaval from the Abyss
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Heezen looked at Tharp’s profiles. No matter how hard
he tried to make the rift valley disappear, it would not.
He groaned, and said, “It can’t be. It looks too much
like continental drift.
– From Upheaval from the Abyss
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The Atlantic belt of earthquake epicenters follows the
crest of the Mid-Atlantic Ridge and its prolongations
into the Arctic and Indian Oceans with a precision
which becomes more apparent with the improvement of
our knowledge of the topography and of epicenter
locations. These are all shallow shocks. Their apparent
departure from the narrow crest of the ridge seldom
exceeds the probable error of location.
– Maurice Ewing, Bruze Heezen, and Marie Tharp
The Mid-Atlantic Ridge Seismic Belt
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The crest is 30 to 60 miles wide, very rough, and on a
typical section shows several peaks at depths of about
800 to 1100 fathoms. There is usually also a
conspicuous median depression reaching depths of
about 2300 fathoms. This is interpreted as an active
oceanic rift zone which continues through the African
rift valleys.
– Maurice Ewing, Bruze Heezen, and Marie Tharp
The Mid-Atlantic Ridge Seismic Belt
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Evidence for plate tectonics, part 7
• On March 26, 1957, Bruce Heezen gave a talk on the
mid-ocean ridge seismic belt at Princeton University.
Harry Hammond Hess, a leading geologist of the time
stood up after the talk and said “Young man, you
have shaken the foundation of geology.”
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Evidence for plate tectonics, part 8
• Hess was inspired to develop a theory of sea-floor
spreading, based on part on Holmes’ ideas, that said
that the crust, driven by convection currents in the
mantle, was created and split apart in rift zones. A
comparable amount of crust was destroyed by
subduction into trenches, thus ensuring that the Earth
did not expand.
– Robert Sinclair Dietz proposed similar ideas at about the
same time, but later said that Hess should receive priority.
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Evidence for plate tectonics, part 9
• Frederick John Vine and Drummond Hoyle Matthews
proposed a test of the sea-floor spreading hypothesis,
using patterns of magnetic reversals recorded in rocks
along the ocean floor.
– If sea-floor spreading occurred, the magnetic patterns
would look roughly the same on either side of a mid-ocean
ridge.
– Their proposal was published in the journal Nature.
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Evidence for plate tectonics, part 10
• Another researcher, Lawrence Whitaker Morley,
proposed a similar test, but his very short paper on
the topic was rejected at the time, first for an alleged
lack of space in Nature, and then by a Journal of
Geophysical Research editor who wrote:
“. . . such speculation makes interesting talk at cocktail
parties, but it is not the sort of thing that ought to be
published under serious scientific aegis.”
– This was after the publication of Vine and Matthews!
• Ocean research, largely conducted by LamontDoherty, eventually proved sea-floor spreading.
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Plate boundaries
• John Tuzo Wilson, became a convert to plate
tectonics and made many contributions to the
development of the theory, including the
determination that tectonic plates are separated by
three types of boundaries.
– Divergent plate boundaries
– Convergent plate boundaries
– Transform faults
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Divergent boundaries, part 1
• Rifts, whether in the depths of the ocean or an land,
mark divergent plate boundaries, where convection
currents moving in opposite directions pull plates
apart. Magma wells up through the split to become
lava and cools to form new, essentially oceanic, crust.
– Mid-Atlantic Ridge and Iceland
– Red Sea and East African Rift Valley
– Connecticut lower Hudson river valleys are remnants of
former rifts
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Divergent boundaries, part 2
• Lava that emerges along a deep-sea rift forms tubelike and pillow-like structures.
• Shallow earthquakes often accompany volcanic
eruptions.
• New oceanic crust is warm and is less dense, thus
rides higher on the mantle – leading to the ridges on
either side of the rift. As the rock cools and
condenses, it sinks.
– Ocean depths increase with distance from the rift.
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Convergent boundaries, part 1
• Convergent plate boundaries form where tectonic
plates collide head on. The type of feature that is
formed depends on the type of plates involved.
– Where an oceanic plate, which is more dense, collides with
a continental plate, which is less dense, the oceanic plate is
forced below the continental plate, forming a subduction
zone, which features offshore trench. Subduction zones are
often accompanied by a mountain range and/or volcanic arc
on the continental side of the trench.
• Cascadia Subduction Zone and Cascade Range
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Convergent boundaries, part 2
• Features of convergent boundaries (continued):
– Where two oceanic plates collide, trenches typically form
on the seaward size while island arcs form in shallow seas
on the continental side.
• Sunda Trench and Sunda Arc in Indonesia
• Islands of Japan
• Aleutian Islands
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Convergent boundaries, part 2
• Features of convergent boundaries (continued):
– Where two continental plates collide, great mountain belts
form.
•
•
•
•
Appalachians in North America
Alps in Europe
Atlas in Africa
Himalayas in Asia
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Transform faults
• Transform faults occur where two tectonic plates
slide horizontally past one another.
• Once called fracture zones where noticed on the
ocean floor.
– San Andreas fault is arguably the most famous transform
fault.
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Hot spots
• Hot spots are sites of long-term upwelling of hot
magma.
• They may form chains of mountains or islands where
tectonic plates drift over the site of the hot spot.
– Hawaiian Islands and Emperor Seamount Chain
– Yellowstone
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Atolls
• Atolls are ring-shaped islands surrounding a seawater
lagoon.
• They form from subsidence of seamounts coupled
with growth of fringing reefs of coral.
– Coral growth keeps up with subsidence.
– Charles Darwin proposed the accepted theory of atoll
formation after observations of atolls on the HMS Beagle
expedition.
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Hydrothermal vents
• Hydrothermal vents form in divergent margins along
fractures in the sea floor.
• Waters as high as 400 °C discharge through the vents.
– These waters are often rich in minerals, giving the water a
black color; chimneys formed from such vents are called
black smokers.
• Hydrothermal vents are home to unique communities
of organisms that do not depend on light from the sun
for existence; rather than photosynthesis, the
communities are based on chemosynthesis.
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Wilson cycles
• Wilson cycles, named for John Tuzo Wilson who first
described them, are cycles of repeated closing and
opening of ocean basins.
• They have six stages:
–
–
–
–
–
–
Embryonic: formation of a rift valley
Juvenile: rift valley widens and connects to an ocean
Mature: ocean basin lined with passive margins
Declining: subduction widespread along rim of ocean basin
Terminal: narrowing of the sea as continents close together
Suturing: collision of continents complete.
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