What is Plate Tectonics?

The theory of plate tectonics

Three major plate boundaries
The Theory of Plate Tectonics explains how and why crustal plates drift around on the surface of the Earth. Tectonic plates have three major boundaries. Divergent boundaries are areas on the Earth’s surface where two plates are moving apart. Convergent boundaries are areas where tectonics plates collide. Transform boundaries are places where two tectonic plates move horizontally past each other without increasing or decreasing in size.

San Juan Bautista Mission on the San Andreas Fault line

My husband is looking across the San Andreas Fault  at the North American Plate. The San Juan Bautista Mission sits on the Pacific Plate and has been damaged by earthquakes several times.  

What is a Divergent boundary?

Where divergent boundaries form
Divergent boundaries usually form deep in the ocean where two ocean plates are moving apart. In the upper mantle molten rock collects in magma chambers at the edges of ocean plates.

Formation of pillow basalt
Pressure from expanding gases in the molten rock (magma) force it upward toward a crack on the seafloor. The molten lava flows onto the seafloor forming pillow basalt. When the pillow basalt cools it forms new seafloor between the divergent plates. This explains what is plate tectonics theory of divergent boundaries.

Atlantic Ocean is a divergent boundary
The continents on each side of the Atlantic Ocean have been drifting apart forming new seafloor in the Atlantic Ocean basin for millions of years. The rate the African Plate and the South American Plate are moving apart is 2.3 cm each year.

Our Earthquakes textbook contains sixteen chapter.  Each chapter includes a lesson, quiz and activity about how and where earthquakes occur. 
Chapters include information about  P waves, S waves,  three major earthquake scales, and different types of faults.                            Myrna

Click for More Information and to Order

Ascension Island in the Atlantic Ocean

Green turtles nesting beach
Ascension Island has been a nesting area for green turtles for thousands of years. Green turtles lay their eggs on the same beach where they were born. Every 4 or 5 years the female turtles return to the island to lay their eggs.

Distance turtles nesting areas is increasing
When the turtles started laying their eggs on Ascension Island the trip to the island was much shorter. Each year the trip increases 2.3 cm. Recently a female turtle that laid her eggs on Ascension Island was tracked returning to her feeding grounds on the coast of Brazil. Her return trip was 2983 km long. It takes between five and six weeks for the turtles to swim that distance. This supports the question, what is plate tectonics theory of divergent boundaries.

Subduction zones are convergent boundaries

Pacific Ring of Fire
Subduction zones form a horseshoe shaped ring around the Pacific Ocean. Each subduction zone is a convergent boundary. When a continental plate converges with an ocean plate the ocean plate always subducts beneath the continental plate.

Basalt rocks
Basalt rocks contain iron-rich minerals that are denser and heavier than continent rocks. The difference in weight causes the ocean plate to subduct beneath the continental plate. The ocean plate melts in the upper mantle and it is recycled into new rocks. This clarifies what is plate tectonics theory of convergent boundaries.

What is a transform boundary?

Plate movement is horizontal
Transform faults are areas where two plates slip past each other. Because we live on a round planet you will see on maps where the lines of divergent boundaries zig-zag on the ocean floor. The short distance between the spreading ridges are transform faults.

San Andreas Fault
The San Andreas Fault runs through California. It is more than 1200 km long. The fault separates the North American Plate and the Pacific Plate. My husband is standing on the Pacific Plate and looking across the San Andreas Fault to the North American Plate. The picture was taken at San Juan Batista Mission. The Mission has been damaged several times by earthquakes. Recent research indicates the epicenter of the 1906 San Francisco earthquake was near the town of San Juan Batista.

Right-lateral strike-slip fault
The Pacific Plate moves in a northeasterly direction relative to the North American Plate. The San Andreas fault is also called a right-lateral strike slip fault. If you look across the fault it looks like the land has moved to the right.This describes what is plate tectonics theory of right lateral strike-slip faults.

More Plate Tectonic Links

The Theory of Plate Tectonics

Tracking Plate Movement Worldwide

Nazca Plate Creates Deadly Earthquakes

What are Thrust Faults?

What is a Divergent Boundary?

Crustal Plates Cover the Surface of Earth

Global Mid-Ocean Ridge System

Where is the Epicenter of an Earthquake?

The Caribbean Plate is Surrounded 

What is an Earthquake?

What is a Convergent Boundary?

What is a Transform Boundary?

The Pacific Plate is an Oceanic Plate

Ocean Trenches Separate Tectonic Plates

S Waves are Seismic Waves


KIDS FUN Science Bookstore

Check out Myrna Martin's award winning textbooks, e-books, videos and rock sets. The Kids Fun Science Bookstore covers a wide range of earth science topics.  Click here to browse.