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Where can we find plate boundaries
on the planet? Well the best way to see plate boundaries is to look for the major earthquakes.
Earthquakes form when building stresses within the Earth’s solid crust are released in single
instants of motion. These stresses build where plates are being stretched apart, pushed together,
or slid beside each other. Think of the friction that must exist in these zones. The plates will
stick together until the stresses build high enough to overcome the friction. The stress is
released all at once as the plates jump. We call this catastrophic stress release and ground motion
an earthquake! This image of a decade’s worth of major earthquakes on the planet shows that most
of these earthquakes happen at plate boundaries and, in fact, mark the edges of these plates.
You’ll also notice earthquakes happening in areas where old plate boundaries have fused together,
such as in fracture zones or old suture zones in the center of continents conjoined in the
past. Minor earthquakes happen as magmas move through the crust above hotspots,
and where isostatic adjustment is happening especially along coastlines.
Let’s look more closely at these earthquakes and color code them by depth. The shallowest
earthquakes dominate at divergent plate boundaries and transform plate boundaries. At some plate
boundaries, such as this one off of South America, we get three different depth earthquakes in
parallel stripes. Why? As the ocean plate subducts under South America, it descends to deeper and
deeper depths. The shallowest frictional sticking and consequent release will happen at the top
of the subduction zone. Further down in the subduction zone, you’d expect deeper sticking
and deeper quakes. So these color-coded earthquake maps act like an x-ray, giving us an image of the
depth of the subducting plate at various distances from the subduction zone itself. We can SEE the
plate subducting under South America.
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Now that we know where the plate boundaries are, how can we determine what types of
boundaries they are? We can measure their movement, and we can study the
surrounding topography, bathymetry, and geology. For example, what do we see in South America that
shows us subduction is going on? The dark-blue arc-shaped deep trench on the seafloor forms
as the plate subducts. What else? The volcanic arc – the chain of active volcanoes known as the
Andes Mountains. And the coastal mountains here? The uplift? That’s associated with the terrane
accretion. In fact, this satellite image shows that there is a chain of underwater seamounts
colliding with the South American coast, and over millions of years, they pile up, are scraped off,
and become part of the coastal mountain system. South America is growing through volcanism and
terrane accretion along its western margin. What about its eastern margin? That side is part of the
same plate as the nearby ocean – the entire unit is being pushed away from the seafloor spreading
center that marks the center of the Atlantic Ocean. In fact, that’s why the subduction
zone on the western margin even exists – the seafloor spreading in the Atlantic is causing
the collision along on the western margin. Now let’s look at a few more world examples
and review the evidence of spreading in the Atlantic Ocean.
First, let’s review the entire Atlantic Ocean. The dark blue-black arc-shaped features are trenches.
Do you see any in the Atlantic? No. There’s one in the Caribbean Sea, but otherwise the edges
of the Atlantic Ocean are free of trenches. What does that mean? We are looking here at
two plates – both spreading away from the center of the Atlantic Ocean. These two plates each have
ocean crust fused to continental crust. At the center of the plate boundary, we see an ocean
ridge with a central rift valley. We also see the ridge is broken into segments, each offset from
the one next to it by a transform boundary. Scars, which we referred to earlier as fracture zones
extend outward from these transform boundaries. What would happen if we moved the plates backward
and undid the spreading? The coasts of North and South America would come back together and merge
with those of Europe and Africa. The perfect puzzle-piece fit and the snail-trail marks of
the fracture zones confirm the story of seafloor spreading. Other evidence that connects these two
halves together include rocks and mountain ranges that exist on one side of the Atlantic and then
continue and are found identically on the other side. The Atlantic Ocean has been spreading for
the past 200 million years and is spreading today at a rate of about 1-2 cm per year a little slower
than your fingernails grow. In fact, as this image shows, it is the slowest spreading center
in the world. If we look only at the apricot color age range – we see that during that time period,
the Atlantic spread only this far, but the southeastern Pacific spread much further. In fact,
the East Pacific Spreading Center spreads at a rate of about 18 cm/year. Why the difference?
Let’s look more closely at what happens to new seafloor forming in the Southeast Pacific
Rift – on the west it moves toward the Marianas Trench and other trenches that line the western
edge of the Pacific Ocean – a long distance away – almost 1/3 the circumference of the planet. On
the east, it enters a nearby subduction zone under South America. In fact, the entire Pacific Ocean,
except for a short stretch along the California Coast consists of trenches, subduction,
and related volcanoes, hence the name the ring of fire. Does it seem likely that seafloor
spreading accompanied by subduction would move faster than seafloor spreading that has to push
away continents and has no subduction to help it? Also, notice that spreading is faster on the east
of the ridge. Why? We can see that the subduction zone is much closer on this side. Does that make
a difference? These are all good questions to consider as we study global plate tectonics.
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Let’s go to the Indian Ocean – what do we see
here? Three ocean ridge spreading centers meeting at one point in the center of the ocean. Are there
any trenches? Not along the east coast of Africa. Not along the southern coast of India or the west
coast of Australia. There is one, however, along the islands of Indonesia. This was the subduction
zone that caused the large tsunami in 2004 that killed ¼ of a million people in the Indian Ocean.
Let’s go back to India. This satellite image shows quite clearly the borders of India are marked by
large mountains -- the Himalayas – and a high plateau that extends into Tibet and Nepal. About
45 million years ago, India collided with Asia in a continent-continent convergent plate boundary.
That collision is still going on today as the Himalayas continue to grow and India continues
to accordion Asia as it pushes northward. Now let’s look at satellite images of Eastern
Africa. Notice the rift valley atop a chain of active volcanoes that includes Mount Kilimanjaro?
If we follow this chain north, it joins with a seafloor spreading ridge that runs across
the center of the Red Sea and another that runs along the center of the Gulf of Aden. A triple
junction where three seafloor spreading centers come together – just like we saw in the center
of the Indian Ocean. This cartoon shows what a divergent plate boundary first looks like
as it breaks through continental crust. Notice the square shape to the valley and the jagged
down-dropped edges of the continent? Now compare that to this photograph. Notice the square shape
to the valley and the down-dropped edges of the continent? Eastern Africa is part of a new rift
zone and one of the only places to see this type of plate boundary at this stage on land.
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Let’s move back to California.
What’s happening here? There’s a divergent seafloor spreading center off of Washington,
Oregon, and Northern California, and one that runs up the Sea of Cortez (or Gulf of California).
What happens to it between there? Notice that it’s offset with transform boundaries north and
south. Imagine that the North American continent is moving over the Pacific Ocean and causing the
eastern edge of that ocean to subduct, including the subduction of a seafloor spreading center.
The zone between the two active spreading centers is just one big transform boundary that connects
the two – what used to be between them has long-since subducted under the continent. That
transform boundary is known to us more commonly as the San Andreas Fault System. And north of us,
in Oregon and Washington and Northern California, what’s happening along the coast? New seafloor
forms offshore and then spreads and collides with North America, where it subducts underneath
and causes the Cascade Volcanoes to form, the southernmost extension of which include both Mt.
Shasta and Mt. Lassen in northern California. If we continue around the rim of fire,
we also see a number of ocean-ocean convergent plate boundaries in the western Pacific,
most notably in Japan, the Philippines, and New Zealand. These areas are rocked by large
earthquakes and explosive volcanic eruptions, making them very dynamic places to live.
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For more information and more detail, continue on to the next video in this series.
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