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3.2.2 - Plate Boundary Interactions
Background: A volcano is created when an opening, or rupture, in the Earth’s crust
allows hot magma, ash and gases (+50% H2O, 20-40% CO2, up to 15% SO2, and other
trace gasses), to escape from below the surface. The ash and lava (it is magma inside
the Earth’s crust, lava once it reaches the surface) pile up and can produce small
cones several hundred feet high or mountains that can soar to 22,000 feet in the air,
like the Andes of South America.
Volcanoes are generally found where tectonic plates are either pulling away or running
into one another, which makes them a great visible clue for inferring and locating the
plate boundaries, otherwise known as the point where two plates meet. But volcanoes
aren’t the only evidence; earthquakes, trenches, ridges, and mountains are all evidence of two plates interacting. Today we are
going to look at the 3 major ways that plates can interact: divergent boundary, convergent boundary, transform boundary.
What causes plates to move?
Earth’s crust and rigid upper mantle are broken into enormous sections called plates. The movement of the plates is driven by
the unequal distribution of heat within Earth that set up convection currents within the upper mantle. Hot material found deep in
the mantle moves slowly upward and serves as one part of Earth’s internal
convection system. Also cooler, denser sections of oceanic lithosphere descend into
the mantle, setting the outer crust into motion. Once the descending plate starts to
cool and fall, it becomes more dense, and essentially pulls the rest of the plate along
after it in a process called slab pull. Convection currents in the asthenosphere are
thus set in motion by the transfer of energy between Earth’s hot interior and the
cooler exterior.
Tectonic plates move in different directions and at different rates over Earth’s surface.
The plates are continually changing in shape and size because any volcanic activity
creates new crust, while any subduction (when one plate is pushed under another
plate) destroys crust.
Boundary Interactions
(1) Divergent boundary occurs when two tectonic plates move away (separating) from each
other. Most divergent boundaries are found on the sea floor and form ocean ridges. Beneath
the ridge, magma (molten rock) rises from the mantle. It oozes up into the gap and hardens
into solid rock, forming new crust on the torn edges of the plates. Magma from the mantle
solidifies into basalt, a dark, dense rock that underlies the ocean floor. Thus at divergent
boundaries, oceanic crust, made of basalt, is created. The formation of new crust occurs at
most divergent boundaries and accounts for volcanic eruptions and earthquakes. If a
divergent boundary is found on land, when continental crust begins to separate, the
stretched crust forms a long, narrow depression called a rift valley.
(2) A convergent boundary is a place where two plates are moving toward each. There are three different types, which are
classified by the type and density of crust involved. Earlier this unit, we discussed that oceanic crust is generally made of basalt
(density of 3.0 g/cm 3), a mafic rock high in magnesium and iron content. Continental crust however, varies a lot more.
Generally however, the minerals that make up continental crust are less dense than basalt, with the continental crust averaging
a density of 2.3 g/cm 3. The differences in these densities dictate what will happen when two different plates meet.
i.
Oceanic crust converging with oceanic crust – One of the two plates becomes denser due to cooling and descends
beneath the other in a process called subduction. The subducted plate descends into the mantle and melts, thus
recycling the crustal material. This forms deep oceanic trenches, like the Mariana
Trench, which bottoms out at 6.85 miles below sea level. When the subducted
plate is forced down due to the difference in density, it is dragged into the mantle
where it begins to melt because of an increase in temperature and pressure.
Magma rises into and through the other plate, creating a volcanic eruption. This
volcanic activity forms new crustal material, and over many eruptions, forms a
volcanic arc of islands. The Japanese Archipelago and the Philippines are both
examples of this process at work. Undersea mountain ranges, are also built this
way.
ii.
Oceanic crust converging with less dense continental crust – In this case, the oceanic plate always subducts because
it is more dense (and thinner) than the continental plate. As the oceanic plate descends it is forced into higher
temperature environments. At a depth of about 100 miles materials in the subducting plate begin to partially melting
because of the high temperatures. This partial melting produces magma chambers at the bottom of the continental
plate above the subducting oceanic plate. These magma chambers are less
dense than the surrounding mantle materials causing them to slowly rise through
the overlying materials, melting and fracturing their way upward. If a magma
chamber rises to the surface without solidifying the magma will break through in
the form of a volcanic eruption, with the lava creating new continental crust. The
Cascade Mountains in Western North America, the Andes in Western South
America, and the Aleutian Island chain in Alaska are all examples of this
converging.
iii.
Continental plates collide – Both continental plates are too buoyant to be subducted, so the colliding edges of the
continents are crumpled and uplifted to form a mountain range. The intense compressional forces can also cause
extensive folding and faulting of rocks within the two colliding
plates, with these deformations extending up to hundreds of
miles into the plate interior. The Himalaya Mountain Range
is the best example of this, as the Indian continental plate is
slamming into the Eurasian continental plate. This is also the
same method that formed the Appalachian Mountain Range.
Shallow earthquake activity is also present when this
collision occurs.
This density difference is the reason that continental crust has been found and dated to be billions of years old in certain cases,
while no oceanic crust has been found that is older than 200 million years old. Every time two plates interact, the denser
oceanic one is going to subduct. That means oceanic crust is continually getting recycled through subduction, increasing
pressure and temperature, and transitioning into metamorphic or igneous rock. For comparison, the oldest continental rock is
found at the heart of North America, with some highly metamophosed gneiss in Minnesota being dated at 3.6 billion years old.
(3) The third type of plate interaction is when two plates sliding past each other, otherwise known as transform plate boundary.
Friction between the two plates holds them in place until the buildup of pressure
along the boundary becomes too great. Once this happens, the fault may quickly
move resulting in an earthquake. Natural or human-made structures that cross a
transform boundary are offset—split into pieces and carried in opposite
directions. Rocks that line the boundary are pulverized as the plates grind along,
creating a linear fault valley or undersea canyon. As the plates alternately jam
and jump against each other, earthquakes rattle through a wide boundary zone.
In contrast to convergent and divergent boundaries, no magma is formed. Thus,
crust is cracked and broken at transform margins, but is not created or
destroyed. The San Andreas fault in California is the most famous example of
this.
Ridges, Trenches, and Rifts
These are three easily confused words, so let’s recap. A ridge occurs when
two oceanic plates are being pulled apart. Upwelling of magma here creates
new oceanic crust. A trench is when oceanic crustal plate gets subducted
under a less dense continental (or less dense oceanic) plate. These are both
seen in the image to the left. Rifts are the exact same concept as a ridge,
except they happen on land, when continental plates are pulling apart from
each other.
One More Last Note…
It is important to remember how big a time scale we are talking about with these interactions too. Lithospheric plates move at a
rate of a few millimeters to 10cm a year. So a lot of these processes take tens of thousands to millions of years to actually
occur. This idea that the Earth’s crust is made of plates and that they move has been coined continental drift and was
developed by Alfred Wegener in 1912. Once the mechanism of convection currents was developed in the 1960s, the Theory of
Plate Tectonics became one of the major overlying concepts in all of geology.
Review Questions
1. What are the major gasses that are released by volcanic eruptions and what is the percentage of
each gas released?
2. What is the difference between magma and lava?
3. What causes tectonic plates to move?
4. When is new crust formed?
5. When is crust destroyed?
6. List, and briefly explain, the 3 different types of plate boundaries.
7. Where do divergent boundaries usually occur?
8. What is the difference between a ridge and a rift?
9. List, and briefly explain, the 3 different types of convergent boundaries.
10. For each of the 3 different convergent boundaries, list the major geologic feature present.
11. How old is the oldest oceanic crust, and how old is the oldest continental crust?
12. Why is continental crust so much older than any oceanic crust that we have found (think density)?
13. How fast do plates move?