Plate Interactions
Earth's crustal plates interact in several ways as described
on the previous page as meeting, parting ways, or passing in the night. In a
few short words, this means that two plates can move away from each other and
leave a crack between; plate could collide with each other and either one moves
under the other or both meet and push each other up into the sky; or slide side-by-side
and make bridge-building a real challenge.
Spreading Centers
The earth is repeatedly shifting its plates, and there are some really exciting
things that happen when the plates interact with each other. As mentioned above,
the plates can separate, collide, or slide next to each other. Each interaction
typically results in an earthquake and shuddering of the earth. While local
residents may mind the effects of these quakes on their china cabinets, seismologists
love them for each quake reveals more information about the Earth's interior.
I hope now to take you to some of the nifty places on the Earth where plates
are interacting. Below left is the spreading center between South American and
Africa. As convection from deep in the mantle continues to rise and move westward
or eastward, the moving rock drags the overlying crust in opposite directions,
splitting the ocean bottom. Molten magma rises from the depths and fills the
widening crack, only to be reopened by the continual convection. Of great interest
to geologists is an opportunity to collect fresh, molten rock from the spreading
centers and see what the mantle is made of in terms of minerals and gases. While
most of the Atlantic Ocean covers this gigantic spreading center, the country
of Iceland lies directly on top of the fissure. This island nation is growing
each year with the addition of new lava, and widening as the crack spreads.
Now I want to to look at the image above and consider too the image depicting
the Ocean Crust Ages. Notice how
the rock has aged as it has moved from its initial spreading center site farther
and farther to the west and east relative to North America and Europe. You learned
about the geology of ocean and continental rock in the Structure
page of this unit, and now know that ocean rock increases in density as it cools.
The older the rock becomes, the heavier it will be. While the image at the end
of the Pangea sequence on the previous page depicts the Earth 100 million years
from now, the image might not be entirely true. The Ocean Crust in the image
above shows blue rock as being ancient, with red rock as being younger. Everywhere
there is blue rock in the image, a process of subduction is occurring. You will
learn about subduction below and in the Volcano
page. The blue rock is so old and dense that it simply breaks from the continental
rock against which it is pushing and subducts under the adjoining plate. Apparently,
oceanic crust that reaches the 100 million year age simply becomes so dense
that it sinks back into the mantle and is drawn all the way down to the core
where it is re-melted. This sinking happens everywhere! Everywhere, that is,
except along the east coast of America and the west coast of Europe. Subduction
is not happening along those boundaries because the ocean plate is stuck!
Oceanographers were mapping the bottom of the oceanic crust of the Atlantic
Ocean when they noticed the tiny evidence of magnetic fields switching as the
mapping ship move from east to west. As magma wells up from deep chambers, iron
filings in a rock mineral known as hematite become oriented along the magnetic
field of the Earth. When the ocean lava cools and solidifies, these iron filings
remain permanently oriented along the north-south pole. Geologists noticed that
the iron filings were not uniformly oriented north and south, but showed reversals
occuring periodically as one moved away from the spreading center in the mid-Atlantic
ridge. Geologists learned two things from this discovery: 1) The poles of the
Earth shift every 20,000-40,000 years (far more slowly than the Sun's 11 year
periodic change); and 2) These "magnetic anomalies" provide geologists
with a means of measuring the rate of spreading over the crust's entire history.
Geologists believe that America split from Europe almost 180 million years ago.
This means that the rocks off the coast of New York, Hilton Head, and Florida's
Gold Coast are MUCH older than they ought to be, according to models offered
by the Geology community, and that the magnetic field of the Earth does not
stay constant with the positive pole being north.
Imagine waking up one morning while hiking in the BWCA with your compass.
You would be totally disoriented trying to use your compass and map to hike
and canoe in the region. Worse still is the fate awaiting migratory birds. Some
species use the stars to navigate, others follow inland waterways like rivers.
But many navigate along magnetic field lines of the Earth. They have small iron
filings in their brain which sense the Earth's magnetic field. If the poles
were to suddenly shift, these poor birds would migrate north for the winter
and die. Birds which "stupidly" migrate south would live, be favored
by pressures of Natural Selection, and leave their foolish navigating skill
genes to future generations, thus preserving the species.
Long ago, that ocean bottom ought to have begun subducting under the North
American plate. This break off will probably occur long before the Atlantic
widens and convection pushes America into Asia. Instead of the future scenario
above, more likely, the ocean plate will subduct, and the following happen.
Huge earthquakes will level every city along the east coast. Subduction will
produce huge explosive volcanoes that will obliterate the fallen cities. Then
the North American plate will likely collide again with Europe and crush anything
else that remains after the earthquake and volcanism. If you are college-bound,
and hoping to attend a prestigious Ivy League School, you never know when the
ocean plate break-off will begin, thus ending your education. While I may be
talking doom and gloom in a somewhat humorous manner, and while there is little
immediate danger compared to geologic time, these events will occur! Pangea
will reform and we will start the break-up all over again.
Seafloor
Spreading Lab - This is a pretty simply, but effective way to understand
the concept of magnetic anomalies, seafloor spreading, and the aging of rocks
all while using a few sheets of paper.
Subduction Zones
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The image above and right shows what happens when an ocean plate meets a continental
plate. Since the ocean plate is made of more dense rock, it slides underneath
the less dense continental plate in a process called Subduction. Here, the Pacific
Plate is subducting the Arctic Plate. The image below and left shows what happens
deep inside the Earth. As the old ocean crust slides under the continental crust,
friction between the two slabs heats the rock, melting it. The molten rock rises
through fissures in the crust. Since this molten rock is composed of part basalt,
part granite, and seawater, the result is a very viscous magma called andesite.
When the andesite emerges from depths, a volcano is born. These volcanoes are
quite spectacular in appearance ... rising like tall spires. Their beautiful
appearance is deceiving, for the high viscosity of the rock causes the volcanoes
to explode instead of erupting gently. Mt. St. Helens exploded in 1980. These
explosions are the reasons geologists refer to andesitic volcanoes as pyroclastic
mountains. They explode during eruptions in spectacular displays ... injecting
huge amounts of debris into the atmosphere. Indeed, the entire chain of Aleutian
Islands are andesitic volcanoes as seen in the image above and right, as too
are the Cascade Mountains in Washington, USA, and the Andes Mountains of South
America. Anyone living near one of these subduction-caused volcanoes is always
as risk of an unexpected eruption with the loss of their home.
If
a plate of oceanic crust splits, both slabs move in opposite directions and
create a gap. Large-Scale Subduction is shown to your left, on the right side
of the image, but the spreading center between South America and Africa (as
seen in the image on the top and left) as well as in the center of the image
to your immediate left. What is on the right side of the image is a depiction
of an ocean plate sliding under a continental slab ... producing perhaps the
Cascade Mountains. On the left side of the image is what happens when two ocean
plates collide, and one slides under the other ... as what occurs in Japan as
well as the South Pacific. The same melting and volcano production results.
As long as hot mantle convects upward and outward, oceanic plates will continue
to separate, forcing plates somewhere to collide and subduct.
Earth's plates can also push into each other = Collision Zones
In my dream location on the planet's surface, the entire country
of India is pushing its way deeper into Asia creating the Himalaya Mountains.
Long ago, India broke off from Antarctica and began sliding north, eventually
butting against Asia. Here, two pieces of continental crust met head-to-head
where neither subducts each other. With no where to go, and relentless pressure
to move north, India pushed rock at its leading edge as well as the rock of
Asia up into the sky creating the Himalayas. Rock that used to be at sea-level
as now 9000 meters above sea-level. Imagine Tenzing Norgay and Sir Edmund Hillary
ascending to the top of Mt Everest and digging a little under the snow pack
to find fossilized sea shells! A closer look that the topographical map of the
world will reveal the crumpling from many years of the collision process (image
below and to your right), and a closer look at the bottom of the Indian Ocean
will reveal the "scratch marks" from India's passage. (No ... India
did not slide over the ocean crust, but the evidence of its movement exists
there, in the global topographical image below, and right).
What is apparent from these pages is that the tectonic plates
interact with each other and continually reshape the surface of the planet.
Mountains are build, contineunts are ripped apart from each other in some places
and collide in other places. All of these interactions are directly the result
of the manner in which Earth releases the heat from within its core. Beyond
these tectonic activities that build mountains and split continents, Earth plates
can also slide alongside each other. The resulting motions are result in features
called transform faults, and Earthquakes are commonly the result of Transform
Fault Movements.
Plates slide by each other: Earthquakes
By the time you read this, you have returned from the Earthquake
page and discovered that nowhere in America is a safe place to live ... in term
sof Geology:) Maybe we all should move north into central Canada where the original
craton has survived undisturbed for billions of years, eh?
Mountains are built by a Volcanism or Collisions in a process called Orogeny,
and crumbled by a process called Mass Wasting
To better understand the process of volcanism, and to see what
active volcanoes can do, go to the Volcano
Page of this Unit and surf through those pages. I think the Hawaiian volcanoes
are a very interesting study, and I count it among my top three life experiences
to have stood directly above a lake of lava, and to have watched lava pour into
the ocean at night from a distance of three feet. Volcanoes and collision centers
build mountains in a process called Orogeny. Wind, rain, and other weather processes
erode the mountains down to sea level in a process called Mass Wasting. The
Black Hills of South Dakota represent almost 40,000 feet or vertical uplift,
with the top 32,000 feet of mountains having already been eroded and washed
to the east. Continual rain washout from the Black Hills then cut into this
sedimentary rock to the east creating today's Badlands. If you ever visit the
Badlands, watch out for snakes and dehydration. If you get thirsty, stop by
Wall Drug where the water is still free.
