Instruction 3-3

The Internal Structure of the Earth | Features of the Ocean Floor | Plate Boundaries | Rocks and their Properties | How Earthquakes Happen and Where | How to Measure Earthquakes | VolcanoesSummary

Plate Boundaries 
CCSTD Earth Science 3.b.

As we told you in a recent Instruction, much of the Earth's lithosphere (its Crust and the top layer of its Outer Mantle) is made up of moving masses of rock called tectonic plates (tectonic means moving).
There are a dozen or so major plates and several minor ones. Most are both partly oceanic (under oceans) and partly continental (under land).
These plates float on the asthenosphere like an iceberg floats on the ocean. (The asthenosphere, as you remember, is the bottom layer of the Earth's Outer Mantle. It is made up of tough liquid rock.)
As we said, these plates are constantly moving -- but they move at different speeds. The Arctic Ridge moves the slowest -- at less than 2.5 centimeters a year. The East Pacific Rise (near Easter Island in the South Pacific) is the fastest. It moves at more than 15 centimeters a year.
To see a diagram of the Earth's major plates, click:
The boundaries between plates are sometimes hard to see here on Earth. But it's easy to map them from space with GEOSAT satellites.
There are four major types of plate boundaries:

Divergent boundaries
Convergent boundaries
Transform boundaries
Plate boundary zones

These boundaries are where events like earthquakes, tsunamis (massive tidal waves) and volcanic eruptions usually occur.
Divergent Boundaries
Divergent boundaries are places where the plates pull away from each other. When this happens, new crust is created as magna (molten rock) pushes up from the Earth's Mantle to fill the space the plates left behind.
The best-known divergent boundary is the Mid-Atlantic Ridge. This is a submerged mountain range that extends from the Arctic Ocean to below the southern tip of Africa.
The rate at which plates move apart here is 2.5 centimeters a year. This has been going on for the past 100-200 million years.
It's what caused the Atlantic Ocean to start growing (during the Mesozoic Era) from a tiny inlet between Europe, Africa and the Americas into the mighty ocean it is today.
This ridge also runs right through the country of Iceland, which is splitting apart between the North American and Eurasian Plates.
Volcanic activity is also frequent here, especially around Krafla Volcano in the northeastern part of Iceland.
Convergent Boundaries
Convergent boundaries are places where plates bump into each other, like bumper cars at a carnival.
When this happens, the Earth's crust is often destroyed as one plate is subducted (sinks) under another.
What type of convergence takes place depends on what type of plates are involved.
There are three types of convergences:


Oceanic-Continental Convergence

Under the Pacific Ocean, there are trenches that are thousands of kilometers long. These trenches cut into the ocean floor and were created by oceanic-continental convergence. They are the deepest parts of the ocean.

Off the coast of South America along the Peru-Chile Trench, the oceanic Nazca Plate is pushing into (and sinking under) the continental part of the South American Plate. This is causing the already huge Andes Mountains to rise even higher.

It is also causing earthquakes.

On June 9, 1994, a huge earthquake (magnitude 8.3) struck near La Paz, Bolivia. This was within the subduction zone between the Nazca and South American Plates.

Fortunately little damage was done since the earthquake occurred 636 km deep in the ocean.

Oceanic-Oceanic Convergence

When two oceanic plates collide, one is usually subducted under the other -- and an ocean trench is formed.

The Mariana Trench, near the Mariana Islands, marks where the Pacific Plate bumps up against the Philippine Plate. A portion of this trench, called the Challenger Deep, is thought to be over 11,000 meters deep.

Oceanic-oceanic convergence frequently results in the formation of volcanoes.

There are two primary types of volcanoes: shield volcanoes and strato volcanoes.

Shield volcanoes have broad gentle slopes and are built up by the eruption of fluid basalt lava. Strato volcanoes are tall and conical and are made from hardened lava and volcanic ash.

In either case, debris from volcanoes piles up over time on the ocean floor. After millions of years, the debris grows tall enough to rise out of the sea.

When this happens, a volcanic island, or chain of islands, is formed -- like the Mariana Islands or the Aleutians.

Not all volcanoes come about when plates run into each another. Some emerge in the middle of plates over hot spots (places where rising plumes of molten Mantel escape to the Earth's crust). We'll tell you more about these is an upcoming Instruction.

But back to oceanic plates -- sometimes one plate sinks beneath another and disappears completely. This is happening right now off the coast of Oregon and Washington.

The small Juan de Fuca Plate, a remnant of the larger Farallon Plate, will one day be totally consumed as it continues to sink beneath the North American Plate.

Continental-Continental Convergence

When two continents meet head on, neither is subducted. That is because
continental rocks are relatively light and resist downward motion.

Instead, the crust of the Earth buckles and gets pushed upward or sideways.

About 50 million years ago, the continent of India collided with Asia. This caused the Eurasian Plate to crumple and override the Indian Plate. After the collision, continental-continental convergence pushed the Himalayas and the Tibetan Plateau up to their present astonishing heights.

Transform Boundaries

Transform boundaries neither create nor consume the Earth's crust.

Instead, two plates move back and forth --or up and down -- against each other.
This builds up tension that is eventually released in a sudden, violent jerk.

This sudden jerk is an earthquake.

The world's most famous transform boundary is The San Andreas Fault in the Western United States.

To the west of the San Andreas Fault lies the Pacific Plate, which is moving northwest. To the east is the North American Plate, which is moving southeast.

Earthquakes occur here frequently.

Land movement occurs daily.

Los Angeles, located on the Pacific Plate, is now 340 miles south of San Francisco, which is located on the North American Plate.

In 16 million years, the plates will have moved so much that Los Angeles will be north of San Francisco.

To see what happens daily at the San Andreas Fault line, click:
Plate Boundary Zones

Some boundaries between plates are not clear-cut or simple.
In some places, boundaries are not well defined because deformation (structural change) takes place over a very broad belt called a plate boundary zone.
The Mediterranean-Alpine region between and Eurasian and African Plates is one such zone. A number of smaller fragments of plates (microplates) have been discovered here.
These zones have complicated geological patterns and earthquake structures.

YouTube Video Instruction
*Availability of You Tube video links may vary. eTAP has no control of these materials.


Experiments for Home and Classroom

Wycombe High School in Frogmore, England, has an excellent Earth Sciences program, including an interactive web site that allows students to explore the movement of tectonic plates at various plate boundaries. Visit this web site and click as appropriate:

Discovering Plate Boundaries is a 3-day activity that its author uses to teach classes from elementary grades through freshman year in college. Although the needed materials and maps are available for free download from this site, a complete Discovering Plate Boundaries kit is also available to order. Teachers can begin with the Teacher's Guide at:
or go directly to Quick Start at:

There are four major types of plate boundaries: Divergent, Convergent, Transform and Plate Boundary Zones. The world's most famous Transform Boundary is the San Andreas Fault in the Western United States. A popular United States Geological Survey exhibit in San Francisco featured a model showing activity all along the San Andreas Fault. Students can construct the model for themselves and explore a wide range of different earthquake scenarios. Click: http://quake.wr.usgs.gov/research/deformation/modeling/eqmodel.html 


Reading List
from the California Department of Education
Fradkin, Philip L.: Magnitude 8: Earthquakes and Life Along the San Andreas Fault
Levy, Matthys & Salvadori, Mario: Why the Earth Quakes: The Story of Earthquakes and Volcanoes
Sieh, Kerry & LeVay, Simon: The Earth in Turmoil: Earthquakes, Volcanoes, and Their Impact on Humankind

for Students, Parents and Teachers

Now let's do Practice Exercise 3-3 (top). Choose printer friendly or online exercises. Printer friendly version requires the Adobe Acrobat Reader 5. Click HERE to obtain a free copy.


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