Hydrosphere

Millions and trillions of gallons of water! There is so much water that it covers all of the low spots in Earth's lithosphere. We call most of that water the ocean. Here we see the ocean in different colors that show the different temperatures you find on the ocean's surface. Water near the equator is warm (light blues). Water temperatures along the Gulf Coast of the United States reach 90 degrees in the summertime--about like a warm bath. Water near the poles are cold (dark purples)--cold enough to freeze into icebergs.
Water in the oceans flows in huge currents around the edges of the continents, changing the climate on the land. You can see the cold water flowing along the west coast of the United States, keeping that part of the nation cool. The warm Gulf Stream current flows up the east side of the nation and across the Atlantic Ocean to Europe, keeping the climate warm there. The hydrosphere also includes the water underground, in lakes, rivers, and oceans. This water is usually fresh and is where we get the water we drink and use in our kitchens and bathrooms.

Lithosphere

See how uneven the surface of the lithosphere is. There are high mountains ranges like the Rockies and Andes (in reds), huge plains like those in Texas, Iowa and Brazil (in greens), and the deep valleys of the ocean floor (in blues). We walk and climb on this part of the lithosphere.
But the surface of Earth is only the very top of the lithosphere. If you look at the cut-away globe above, you can see the rest of the lithosphere with its many layers like an onion. The very thin crust on top, the thick mantle underneath, and the huge core of liquid iron at the center.
You must be careful with the word "lithosphere," though. It is tricky! "Lithosphere" has different meanings. As we are using it here, it means both the rocky surface and the whole inside of Earth. But the deep interior of Earth is very hot, and even though the rocks there are mostly solid, they can flow over long periods of geologic time like soft butter. This flow on the inside causes the cold, hard surface layer of Earth to break into pieces and move around. This movement causes earthquakes and mountain ranges and is called "plate tectonics." When talking about plate tectonics, geologists use the word "lithosphere" to mean only the cold, hard part at the surface, and not the whole inside of the Earth.

Transform Boundary

Places where plates slide past each other are called transform boundaries. Since the plates on either side of a transform boundary are merely sliding past each other and not tearing or crunching each other, transform boundaries lack the spectacular features found at convergent and divergent boundaries. Instead, transform boundaries are marked in some places by linear valleys along the boundary where rock has been ground up by the sliding. In other places, transform boundaries are marked by features like stream beds that have been split in half and the two halves have moved in opposite directions.
Perhaps the most famous transform boundary in the world is the San Andreas fault, shown in the drawing above. The slice of California to the west of the fault is slowly moving north relative to the rest of California. Since motion along the fault is sideways and not vertical, Los Angeles will not crack off and fall into the ocean as popularly thought, but it will simply creep towards San Francisco at about 6 centimeters per year. In about ten million years, the two cities will be side by side!

Convergent Boundaries

Places where plates crash or crunch together are called convergent boundaries. Plates only move a few centimeters each year, so collisions are very slow and last millions of years. Even though plate collisions take a long time, lots of interesting things happen. For example, in the drawing above, an oceanic plate has crashed into a continental plate. Looking at this drawing of two plates colliding is like looking at a single frame in a slow-motion movie of two cars crashing into each other. Just as the front ends of cars fold and bend in a collision, so do the "front ends" of colliding plates. The edge of the continental plate in the drawing has folded into a huge mountain range, while the edge of the oceanic plate has bent downward and dug deep into the Earth. A trench has formed at the bend. All that folding and bending makes rock in both plates break and slip, causing earthquakes. As the edge of the oceanic plate digs into Earth's hot interior, some of the rock in it melts. The melted rock rises up through the continental plate, causing more earthquakes on its way up, and forming volcanic eruptions where it finally reaches the surface. An example of this type of collision is found on the west coast of South America where the oceanic Nazca Plate is crashing into the continent of South America. The crash formed the Andes Mountains, the long string of volcanoes along the mountain crest, and the deep trench off the coast in the Pacific Ocean.

divergent boundaries

Places where plates are coming apart are called divergent boundaries. As shown in the drawing above, when Earth's brittle surface layer (the lithosphere) is pulled apart, it typically breaks along parallel faults that tilt slightly outward from each other. As the plates separate along the boundary, the block between the faults cracks and drops down into the soft, plastic interior (the asthenosphere). The sinking of the block forms a central valley called a rift. Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface.

The Cause of Earthquakes

The plates consist of an outer layer of the Earth, the lithosphere, which is cool enough to behave as a more or less rigid shell. Occasionally the hot asthenosphere of the Earth finds a weak place in the lithosphere to rise buoyantly as a plume, or hotspot. The satellite image below shows the volcanic islands of the Galapagos hotspot. Only lithosphere has the strength and the brittle behavior to fracture in an earthquake.

The Story of Plate Tectonics

The story of Plate Tectonics is a fascinating story of continents drifting majestically from place to place breaking apart, colliding, and grinding against each other; of terrestrial mountain ranges rising up like rumples in rugs being pushed together; of oceans opening and closing and undersea mountain chains girdling the planet like seams on a baseball; of violent earthquakes and fiery volcanoes. Plate Tectonics describes the intricate design of a complex, living planet in a state of dynamic flux.