It was around the year 1900, when a man by the name of Alfred Wegener (pictured below) was coming up with evidence to support a theory that would change the way we thought of the world. Alfred Wegener was a meteorologist with a fancy for geology. He was especially keen on recognizing patterns. Looking at land formations across the world, he couldn’t help but notice how the mountain ranges of Northern Europe and the mountain ranges of the East Coast of the United States seemed very similar.

There were other rock formations too. Rock strata in Africa seemed to match up with rock strata in South America. When he took a look at the fossils in those rocks, an amazing thing happened. The rock strata held the same species of plants and animals from the same time period. Today it is common knowledge that every continent has its own special variety of species. It was a common understanding in the 1900’s as well, so why would the same species of plants and animals be found fossilized in rock split by the ocean?

Take a look at the world map of today (pictured below). Wegener was looking at this same thing. Take a look at the shape of the continents. Do you see any matching patterns? Take a closer look at Africa and South America. It’s almost a perfect match. With all of his evidence, Wegener published his Theory of Continental Drift. Basing his theory on evidence of matching rock strata, same species of plant and animal fossils on different continents, and the shape of the continents, Wegener confidently stated that the continents had once been combined into a supercontinent he named Pangea.

Although Wegener didn’t have everything quite right, his theory did pave the way for the theory we hold to today. That theory is the Theory of Plate Tectonics.

Plate Tectonics

In the theory of Plate Tectonics, the outer layer of the Earth is not one continuous slab of rock, but instead it is made up of at least 15 different sections we call tectonic plates. Tectonic plates are made up of the cooler, breakable part of the upper mantle and the crust. Tectonic plates move, and this movement is caused by the internal heat that flows in convection currents through the mantle transferring heat energy from the core up to the crust. This theory is explained by the evidence and patterns we see around the world.

The Plates

The crust of the Earth is split into several large chunks called plates. These plates sit on top of the mantle and are able to move. Where two plates meet, it is called a plate boundary. There are three ways that plates can move so there are three types of plate boundaries. Plates move away from each other at a divergent plate boundary. At a convergent plate boundary they move toward each other. In a transform plate boundary they slide past each other.

Although the plates are moving very, very slowly, we can measure this using satellite data. Typical plate movement is only about 2-3 cm per year. The fastest moving plates in the world are the Nazca and Cocos plates located under the Pacific ocean off the west coast of Central and South America (seen in the tectonic plate map at the top of the page). These plates move at a rate of about 10 cm per year.

Plate boundaries determine what type of landforms are created; what types of new rocks are produced, if any; and what kind of earthquake and volcanic activity there is. Mount Everest is forming from a convergent plate boundary where two continental plates are pushing into one another. This range is not built up due to volcanic activity, but instead created due to rock folding actions.

On the other hand, Mount St. Helens, located in the Northwestern US, is being created by the Juan de Fuca plate as it subducts (moves under the neighboring North American Plate). Even though they are both convergent boundaries, very different landforms are formed at these locations.

At the boundary where the Juan de Fuca plate runs into the North American Plate, the Juan de Fuca plate subducts under the continent and its matter is recycled back into the Earth. This process causes melting of the mantle above this plate. This melting rises to the surface and creates the range of volcanic mountains (including Mount St. Helens) that make up the Cascade Range. These can be seen in the images generated by Google earth located below. You can see in the center image, dots of high peaks covered in snow. These are all volcanoes that follow a pattern in the Cascade range. What is not as obvious in the photo is that smaller volcanoes also dot the landscape between these larger ones in a nearly straight line.

By contrast, a transform fault will never have volcanoes. Volcanoes that form on plate boundaries form because plates move/push apart from one another or because subduction and melting is occuring in an area. A transform fault has neither of these conditions. The landform created here is a fault scarp (an area where land is pushed up slightly and appears as a raised scar on the land surface). The image to the right shows an example of the most famous fault scarp in the world, the San Andreas Fault, California.

Volcanoes and the Pacific Rim

Many volcanoes are found along the Pacific Rim (the edges of the Pacific Ocean). Because of this display of volcanic activity, this is often referred to as the Ring of Fire. This occurs due to many different subduction zones-areas where one tectonic plate moves under another at plate boundaries. These types of boundaries are convergent boundaries where the plates are pushing into one another, but not crumpling up into folded mountains.

There are a few cases where volcanoes exist outside of plate boundaries. These are not caused by the boundaries themselves, but an entirely different phenomenon called a hot spot. These volcanoes are caused by intense heat from the core that makes its way the way to the crust in a long stream of heat. Examples of these would be Hawaii, Yellowstone, and Iceland.

Earthquake Zones

Although earthquakes can happen anywhere in the world, nearly 95% of all earthquakes take place along one of the three types of plate boundaries. About 80% of all earthquakes strike around the Pacific Ocean basin because it is lined with plates moving toward or sliding past each other. About 15% take place in the Mediterranean Asiatic Belt, where the Indian Plate runs into the Eurasian Plate (The Himalayan Mountain Range). The remaining 5% are scattered around other plate boundaries or are intraplate (not at plate boundaries) earthquakes.

Fossil Evidence Supports Plate Tectonic Theory

Fossil evidence for plate movement includes the presence of similar or identical species on continents that are now great distances apart. For example, fossils of the therapsid Lystrosaurus have been found in South Africa, India, and Antarctica, alongside members of the Glossopteris flora, whose distribution would have ranged from the polar circle to the equator if the continents had been in their present position; similarly, the freshwater reptile Mesosaurus has been found only in localized regions of the coasts of Brazil and West Africa.

Additional evidence for plate movement is found in the similar geology of formerly adjacent continents, such as the eastern coast of South America and the western coast of Africa. The polar ice cap of the Carboniferous Period covered the southern end of Pangaea (when all the continents were one large landmass). Glacial deposits of the same age and structure are found on many separate continents which would have been together in the supercontinent of Pangaea.

Why does all of this evidence point to moving continents? Today, if you look at the world’s fauna and flora (animals and plants) you’ll be able to note that each continent has its own variety or species of these organisms. Not only that, specific regions and isolated islands have their own species. This is because as populations become isolated from other members in their species, they will change and adapt just slightly over time until they become different species. We’ll cover this in more detail in Chapter 27. When organisms are in contact with one another they mix their genes through reproduction and stay one species. If the same species from an ancient organism is found in one location and then in another thousands of miles apart, it is assumed that these land masses were once in contact.