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Effects of Tectonic Plate Movements
Convergent Boundaries
The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being
created. Such destruction (recycling) of crust takes place along convergent boundaries where plates are moving
toward each other. At convergent boundaries, crust is destroyed as one plate dives under the other. Recall from
the interior of the earth project that there are two different types of crust; oceanic and continental.
CONVERGENT BOUNDARIES: MEETING OF CONTINENTAL AND OCEANIC CRUST
When two plates converge, or meet together, a subduction zone occurs. A subduction zone is where the one
tectonic plate slides underneath the other one. Trenches are the deepest parts of the ocean floor and are
created by subduction zones. When continental and oceanic crust converges together the oceanic crust
creates a trench (because the plates are being pushed down) however, when two continental plates are pushed
together the continental crusts gets bunched up, creating mountains. This is how the Andes Mountains, in
South America, were formed.
IN THE SPACE BELOW, SKETCH A CONVERGENT
BOUNDARY BETWEEN CONTINENTAL AND
OCEANIC CRUST. INCLUDE THE FOLLOWING
TERMS IN YOUR SKETCH: Oceanic crust,
continental crust, trench, subduction zone
Use arrows to indicate direction of tectonic plates!
IN THE SPACE BELOW, SKETCH A CONVERGENT
BOUNDARY BETWEEN CONTINENTAL AND
OCEANIC CRUST. INCLUDE THE FOLLOWING
TERMS IN YOUR SKETCH: Continental crust #1,
continental crust #2, mountains
Use arrows to indicate direction of tectonic plates!
CONVERGENT BOUNDARIES: MEETING OF TWO OCEANIC PLATES
As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the
other, and in the process a trench is formed. The Mariana Trench, for example, marks where the fast-moving
Pacific Plate converges against the slower moving Philippine Plate.
Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes called
seamounts. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until an
underwater volcano (seamount) rises above sea level to form an island volcano. Such volcanoes are typically strung
out in chains called island arcs similar to Alaska is Aleutian Islands.
IN THE SPACE BELOW, SKETCH A CONVERGENT BOUNDARY BETWEEN TWO OCEANIC TECTONIC PLATES.
INCLUDE THE FOLLOWING TERMS AND YOUR SKETCH: Oceanic crust, trench, subduction zone, seamount
Use arrows to indicate direction of tectonic plates!
Divergent Boundaries
Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by
magma pushing up from the mantle. Picture two giant conveyor belts, facing each other, but slowly moving in
opposite directions as they transport newly formed oceanic crust away from the ridge crest (center).
Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This is a submerged mountain range,
which extends from the Arctic Ocean to beyond the southern tip of Africa. The rate of spreading along the MidAtlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. This rate may seem
slow by human standards, but because this process has been going on for millions of years, it has resulted in plate
movement of thousands of kilometers. Seafloor spreading over the past 100 to 200 million years has caused the
Atlantic Ocean to grow from a tiny cove of water between the continents of Europe, Africa, and the Americas into
the vast ocean that exists today!
But when plates are continental a rift forms. The most famous rift is in Africa. This rift is a dropped zone where the
plates are pulling apart. As the crust widens and thins, valleys form in and around the area, as do volcanoes, which
may become increasingly active. Early in the rift formation, streams and rivers flow into the low valleys and long,
narrow lakes can be created. Eventually, the widening crust along the boundary may become thin enough that a
piece of the continent breaks off, forming a new tectonic plate. At this point, water from the ocean will rush in,
forming a new sea or ocean basin in the rift zone.
IN THE SPACE BELOW, SKETCH A DIVERGENT BOUNDARY BETWEEN TWO OCEANIC TECTONIC PLATES.
INCLUDE THE FOLLOWING TERMS AND YOUR SKETCH: Oceanic crust, seafloor spreading, mid Atlantic ridge,
magma
***Use arrows to indicate direction of tectonic plates!
IN THE SPACE BELOW, SKETCH A DIVERGENT BOUNDARY BETWEEN TWO CONTINENTAL TECTONIC PLATES.
INCLUDE THE FOLLOWING TERMS AND YOUR SKETCH: Continental crust, rift, magma
***Use arrows to indicate direction of tectonic plates!
But what happens when plates neither push nor pull against each other?!?!?! They slide
Transform Fault Boundaries
At transform boundaries, tectonic plates are not moving directly toward or directly away from each other. Instead,
two tectonic plates grind past each other in a horizontal direction. This kind of boundary results in a fault — a crack
or fracture in the earth's crust that is associated with this movement.
The best-studied fault is the San Andreas Fault in California. It is located at the boundary between the Pacific and
North American plates and runs roughly 800 miles (1,300 km) through Northern and Southern California. As the
two plates grind past each other — the Pacific Plate moving northwest and the North American Plate moving
southeast — the motion produces numerous earthquakes along the fault.
IN THE SPACE BELOW, SKETCH A TRANSFORM FAULT BOUNDARY BETWEEN TWO TECTONIC PLATES.
INCLUDE THE FOLLOWING TERMS AND YOUR SKETCH: Plate #1, plate #2, fault
***Use arrows to indicate direction of tectonic plates!
Analysis Questions
1. What is the difference between a convergent and divergent boundary?
2. How are Sea trenches formed? Which two types of plates interact?
3. How are Sea Mounts formed? Which two types of plates interact?
4. In your own words, explain what a subduction zone is.
5. Which type of boundary is responsible for creating new crust? How does this happen?
6. Why are transform- faults the most worrisome to human beings?