Plate Boundaries
Place two books flat and side by side. Imagine each book represents a tectonic plate. A plate boundary exists where your books touch. How many different ways can you move the books with respect to each other? You can pull the books apart, you can push them together, and you can slide your books past one another. Earth’s tectonic plates move much in the same way. 

Tectonic plates are constantly moving and changing Earth. The interactions of tectonic plates shape Earth’s beautiful landscapes and cause natural disasters. Mountains, volcanic eruptions, volcanic islands, deep trenches, and earthquakes are all caused by the interactions of plate boundaries. There are three types of plate boundaries that describe the relative motion of adjacent plates.

Divergent Plate Boundaries
Have you ever shared a lava cake with a friend? The force of your forks tearing the cake apart in opposite directions releases hot lava-like chocolate. A divergent boundary acts kind of like this! When tectonic plates move apart, the crust at the divergent plate boundary is stretched thin and magma moves up to the surface. The molten rock eventually cools and creates new crust. 

A divergent plate boundary is the boundary between two plates that move away from each other. Divergent plate boundaries exist at the bottom of the ocean and on continents. The Mid-Atlantic Ridge is a divergent plate boundary that cuts down the middle of the Atlantic Ocean. Divergent plate boundaries within a continent pull continents apart and form continental rift valleys. The deepest freshwater lake in the world was formed by continental rifting. Lake Baikal is a rift valley formed from the Amur Plate and Eurasian Plate tearing apart and it contains about 20 percent of Earth's surface freshwater.

Mid-Ocean Ridges

When you think about volcanoes what do you imagine? Do you think about the ocean? It might surprise you to know that under the sea there is a span of volcanoes over 65,000 kilometers long! Mid-ocean ridges wrap around Earth like the seams of a softball and account for more than 70 percent of volcanic activity on Earth. Mid-ocean ridges exist in the ocean at divergent plate boundaries. When the seafloor spreads at a mid-ocean ridge, it makes room for magma to bubble up and erupt. When the molten rock cools, it forms new oceanic crust.

The divergent plates at mid-ocean ridges spread apart at different rates. When the plates move apart quickly it formed a gentle slope on the ocean floor. When the plates move apart slowly it formed steep ridges and valleys. Some parts of mid-ocean ridges exist above water, such as the one in Iceland. 

Continental Rifts

Think about pizza dough being stretched apart. As the dough is stretched it becomes thinner. Similarly, when continental lithosphere is stretched apart it thins and magma can more easily move up to the surface. However, continental crust is much more brittle than pizza dough, so it often crumbles and breaks when stretched. When large blocks of rocks break and fall it creates an alternating pattern of valleys and mountains. 

New Ocean Basin Formation
Continents are surrounded by oceans. Over geologic time continents have moved due to plate tectonics. They have collided together and rifted apart. When plates spread apart within a continent, the continental crust at the site of rifting thins. Magma made of basalt moves up and fills the space where continental crust used to be. Eventually if a continental rift spreads apart enough, oceanic crust forms and creates new ocean basins.

Ocean Basins

An ocean basin is the land surface under the ocean. The ocean floor is not flat like a big swimming pool. It has volcanoes, mountains, deep valleys, gentle slopes, and trenches. The shape of land on the ocean floor is influenced by plate tectonics.

Convergent Plate Boundaries
A convergent plate boundary forms where two plates collide. The denser plate sinks below the more buoyant plate in a process called subduction. The area where a denser plate descends into Earth along a convergent plate boundary is called a subduction

When two oceanic plates collide, the older and denser oceanic plate will subduct beneath the younger oceanic plate. This creates a deep ocean trench, and a line of volcanoes called an island arc, such as the Mariana trench and the Marianas Islands. 

Subduction also occurs when an oceanic plate and a continental plate collide. When an oceanic plate and continental plate collide, the oceanic plate sinks below the continental plate. The denser oceanic plate subducts under the edge of the continent. This creates a deep ocean trench and a line of volcanoes forms above the subducting plate near the edge of the continent. Over time, an oceanic plate can be completely subducted, dragging an attached continent behind it.

When two continents collide, neither plate's crust is subducted, and mountains such as the Himalayas in southern Asia form from uplifted rock.

Mountain Building
Mountains form when continental plates collide over millions of years. When continents collide, the collision causes Earth’s crust to fold and form mountains. Have you ever held a magazine with both hands and pushed your hands together? The middle of the magazine probably folded up, like a mountain. The forces of the plates coming toward each other act like your hands pushing the edges of the magazine together.

Continental crust is very thick and made of buoyant material, which is less dense than basaltic oceanic crust. Because continental plates are thick but light, when two continents collide, neither crust will subduct. Think of an ice cube in a glass of water, a larger cube of ice will take up more space above and below the surrounding water. The thick, folded continental crust at mountain ranges acts like a giant block of ice floating on top of the asthenosphere.

Volcanoes
You have read about volcanoes that form at the bottom of the ocean along divergent boundaries and volcanoes that form on land when continental plates and oceanic plates collide. A volcano is a vent in Earth’s crust through which molten rock flows. Molten rock flows up to fill in the space between spreading tectonic plates. How can a vent of molten rock form where plates collide? 

‘When an oceanic plate and continental plate collide, the oceanic plate sinks below the continental plate. This happens because oceanic crust is much denser than continental crust. 

When an oceanic plate subducts, some water also slides below the other plate. This addition of water to the mantle lowers the melting temperature of the rocks, which causes magma to form. The magma rises and forms volcanoes along the convergent boundary.

Subduction can also occur when two oceanic plates collide. As oceanic crust cools over time, it becomes denser. This means the older, colder oceanic plate will sink beneath the younger, warmer oceanic plate as they converge. The volcanoes that form from this process are also formed by water in the subducting plate causing surrounding mantle rock to melt at a lower temperature and form magma. Magma is more buoyant than the surrounding rocks, so it rises and forms volcanoes.

Hot Spots
The majority of volcanoes form at plate boundaries. However, some volcanoes are not formed by plate boundary interactions. Volcanoes like this are called hot spot volcanoes. A hot spot is a column of mantle that is hotter than the surrounding material. Because the magma is 5o hot and buoyant, it rises to the surface and forms a volcano.

The position of a hot spot does not tend to move or change locations for millions of years. Tectonic plates slowly move over a hot spot and form chains of volcanoes. The Hawaiian Islands were formed by the Pacific Plate moving slowly over a hot spot in the mantle over millions of years.

Supervolcanoes
Did you know that a supervolcano is not a specific type of volcano? It is just any volcano that has had at least one big eruption! Specifically, a supervolcano is a volcano that has had an explosive eruption of more than 1000 cubic kilometers of material. Supervolcanoes are formed by plate motion or hot spots, just like any other volcano.

Did you notice that there were different names for Yellowstone eruptions in the graph? Because it was formed by a hot spot that the North American Plate has slowly moved over, there are multiple Yellowstone volcanoes.

Volcanoes can create extreme environments that make life difficult or impossible for many species. But life finds a way! The lesser flamingo has adapted to live in extreme heat and very alkaline waters. Lake Natron, in Tanzania, Africa, is a volcanic lake that reaches temperatures up to 40°G regularly, and it is also the breeding ground for the lesser flamingo. Their skin is tough enough to withstand the hot spring fed waters.

Ocean Trenches
Ocean trenches are some of the deepest natural features on Earth's surface. These trenches form at convergent boundaries where subduction occurs. When two oceanic plates converge, the older, denser plate slides under the less dense plate and forms an ocean trench. 

Ocean trenches are very extreme environments. Think about being 10,000 meters below water—it is very dark, cold, and you are under so much pressure! You might think that nothing could live in such extreme conditions as an ocean trench but there are creatures without bones or lungs that can withstand the extreme pressures of living in the trenches.

Transform Plate Boundaries
The famous San Andreas Fault in California is an example of a transform plate boundary. A transform plate boundary is a location where two plates slide past each other. As they move past each other, the crust gets stuck and stops sliding along the boundary. Stress builds up where the sliding stops. Eventually, the stress is too great and the rocks rapidly slide past each other. This results in a rapid release of energy known as an earthquake.

Earthquakes
Tectonic plates are always slowly moving. Brittle tectonic plates do not easily slide under or past each other. Earthquakes result from the buildup and rapid release of stress along active plate boundaries. Earthquakes are the vibrations in the ground that result from the movement along breaks in Earth’s lithosphere. An earthquake can change Earth’s surface rapidly and dramatically.

During an earthquake, a rapid release of energy produces seismic waves that radiate in all directions. These waves ripple through rocks like the waves in water when a person jumps into a pool. The energy transferred by seismic waves moves and shakes the ground.