Plate Tectonics

The Earth’s layers The layers of the Earth differ in thickness, composition, and temperature. The layers are the crust, the mantle, and the inner and outer cores. The crust and the top portion of the upper mantle (which is solid rock) make up the lithosphere. The rest of the upper mantle is made of partly melted rock. This layer is called the asthenosphere. The lower mantle is solid rock. All of the Earth’s liquid and solid water – oceans, lakes, rivers, glaciers, underground water – make up the hydrosphere.

The crust The first layer of the Earth is called the crust. It is a thin layer of solid rock that makes up the outermost part of our planet. The thickness of the crust varies from place to place, but it is generally very thin.

The Earth’s crust makes up the surface of the Earth. The surface includes all the continents and the ocean floor. Geological features are physical structures on the Earth’s surface. These include hills, rivers, lakes, deserts, plateaus, mountains, caves and valleys.

Mantle The next layer of the Earth is called the mantle. The mantle is made of a thick, solid, rocky substance that represents about 85% of the total weight and mass of the Earth.

The middle of the Earth is called the core. It is made up of 2 parts: the outer core and the inner core. The outer core is made of liquid metals. The core

*The inner core is at the very center of the Earth. *It is made up of made up of solid metals.

German geologist Alfred Wegener noticed that the Earth’s continents looked like pieces of a jigsaw puzzle. Wegener thought that millions of years ago the Earth’s continents were joined together. As time passed, some force pulled the pieces apart. The continents slowly moved to the positions they are in today. Scientists have found evidence that supports Wegener’s theory. The Theory of Continental Drift

Mountains on the east coast of South America had the same types of rocks as the mountains on the west coast of Africa. The rocks were also the same age. Plant and animal fossils provided additional evidence. A fossil of a freshwater reptile called Mesosaurus has been found in very old rocks in South America and in Africa. Mesosaurus could not have swum the long distance of the Atlantic Ocean’s salt water.

Scientists have concluded that South America and Africa were once part of a supercontinent called Pangea. Over millions of years the continents drifted apart and water filled the spaces between them.

Why is the ocean floor moving? The ocean floor between South America and Africa is spreading at a rate of about 4 centimeters every year. In the 1950s, scientists mapping the ocean floor discovered that there was an underwater mountain chain in the middle of the Atlantic Ocean. On both sides of the mountain chain, the ocean floor was moving.

Scientists have developed the theory of plate tectonics to explain how forces deep within Earth can cause ocean floors to spread and continents to move. This theory describes the lithosphere as being made of huge plates of solid rock. The Earth’s continents rest on these plates. The almost-melted rock of the asthenospere acts as a slippery surface on which the plates can move. In the middle of the ocean where the plates are moving apart, magma, is pushed up from the mantle toward the surface. Magma is hot, melted rock.

The upward movement of magma causes tension on the plates. This moves the ocean floor apart and separates the plates. As the ocean floor grows wider, the continents move farther apart. This is called sea floor spreading.

Sea-floor spreading is a type of divergent boundary. A divergent boundary is a boundary where plates are moving away from each other.

Divergent boundaries can also occur on land. Tension makes the crust longer and thinner. When the force exceeds the strength of the crust, a fault occurs. A fault is a break or crack in the rock of the lithosphere along which movements take place. The Great Rift Valley in Africa is a type of divergent boundary.

The San Andreas Fault between the North American and Pacific Plates is the most noticeable land transform boundary. A transform boundary occurs when plates slide past each other. Many powerful earthquakes occur along this fault, as well as many that are not felt at all.

The forces that move continents can also change the continents’ shapes. A force called shearing works like a pair of scissors, causing rocks to break along transform boundaries.

Transform Boundaries Eventually this rock may pile up and form narrow ridges and valleys. Earthquakes occur along these boundaries as strain on the rock build up and energy is released. Most transform boundaries are located on the ocean floor

Convergent Boundaries occur at locations where plates collide. Depending on what types of plates collide at these boundaries, different things occur.

If both colliding plates are continental plates, the pressure lifts and crumples the plates, forming mountains.

The Himalayan Mountains were formed as a result of continental convergent boundaries.

Continental plates are light and buoyant. Oceanic plates are more dense. When these two types of plates collide, the oceanic plate gives way to the continental plate, causing a subduction zone.

Volcanoes tend to erupt at subduction zones. The Ring of Fire is a circle of volcanoes that surrounds the Pacific Ocean. The Ring of Fire follows the boundaries of the plates that meet around the Pacific Ocean.

If both colliding plates are oceanic plates, the older of the ocean plates will give way to the newer plate. This causes a trench in the ocean floor.

Faults Forces cause different kinds of faults. Shearing forms strike-slip faults. Tension produces normal faults. Compression produces reverse faults.

Strike-Slip Fault The San Andreas Fault is an example of a strike-slip fault. This type of fault is produced due to shearing along transform boundaries. The plates slide past each other without moving up or down.

Strike – Slip Fault

Reverse Fault A reverse fault is produced due to compression. This type of fault often occurs at convergent boundaries. As the plates push together, rock above the fault surface moves upward. The Himalayas in central Asia are forming at a reverse fault.

Reverse Fault The Himalayan Mountains

Reverse Fault

Normal Fault A normal fault is produced due to tension and often occurs at divergent boundaries. As the plates pull apart, rock above the fault surface moves down. The Sierra Nevada Mountain Range in California was formed at a normal fault.

Normal Fault Sierra Nevada Mountains

Normal Fault

Mountains Mountains form where plates converge. Folded Mountains are made up of rock layers that are folded together by compression. Great Smoky Mountains

Folded Mountains Millions of years ago, the eastern edge of the North American Plate collided with the African Plate. Over millions of years, the Great Smoky Mountains formed as horizontal layers of the rocks were bent or folded.

Folded Mountains The Himalayas are another example of folded mountains. They began forming millions of years ago when the Indian and Asian Plates collided. As the plates continue to push into each other, the Himalayas grow about 5 millimeters taller every year.

Fault-Block Mountains At other times, when plates push against each other, the rocks break and forms faults. Mountains made by huge, tilted blocks of rock are called fault-block mountains. Sierra Nevada Mountains

Fault-Block Mountains The Sierra Nevada Mountains and the Teton Range are examples of fault-block mountains. Teton Range