The Four Layers
The Earth is composed of four different layers. Many geologists believe that as the Earth cooled the heavier, denser materials sank to the center and the lighter materials rose to the top. Because of this, the crust is made of the lightest materials (rock- basalts and granites) and the core consists of heavy metals (nickel and iron).
The crust is the layer that you live on, and it is the most widely studied and understood. The mantle is much hotter and has the ability to flow. The Outer and Inner Cores are hotter still with pressures so great that you would be squeezed into a ball smaller than a marble if you were able to go to the center of the Earth!!!!!!
The Earth's Crust is like the skin of an apple. It is very thin in comparison to the other three layers. The crust is only about 3-5 miles (8 kilometers) thick under the oceans(oceanic crust) and about 25 miles (32 kilometers) thick under the continents (continental crust). The temperatures of the crust vary from air temperature on top to about 1600 degrees Fahrenheit (870 degrees Celcius) in the deepest parts of the crust. You can bake a loaf of bread in your oven at 350 degrees Fahrenheit , at 1600 degrees F. rocks begin to melt.
The crust of the Earth is broken into many pieces called plates. The plates "float" on the soft, plastic mantle which is located below the crust. These plates usually move along smoothly but sometimes they stick and build up pressure. The pressure builds and the rock bends until it snaps. When this occurs an Earthquake is the result!
Notice how thin the crust of the Earth is in comparison to the other layers. The seven continents and ocean plates basically float across the mantle which is composed of much hotter and denser material.
The crust is composed of two basic rock types granite and basalt. The continental crust is composed mostly of granite. The oceanic crust consists of a volcanic lava rock called basalt.
Basaltic rocks of the ocean plates are much denser and heavier than the granitic rock of the continental plates. Because of this the continents ride on the denser oceanic plates. The crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere. The layer below the rigid lithosphere is a zone of asphalt-like consistancy called the Asthenosphere. The asthenosphere is the part of the mantle that flows and moves the plates of the Earth.
The mantle is the layer located directly under the sima. It is the largest layer of the Earth, 1800 miles thick. The mantle is composed of very hot, dense rock. This layer of rock even flows like asphalt under a heavy weight. This flow is due to great temperature differences from the bottom to the top of the mantle. The movement of the mantle is the reason that the plates of the Earth move! The temperature of the mantle varies from 1600 degrees Fahrenheit at the top to about 4000 degrees Fahrenheit near the bottom!
The mantle is made of much denser, thicker material, because of this the plates "float" on it like oil floats on water.
Many geologists believe that the mantle "flows" because of convection currents. Convection currents are caused by the very hot material at the deepest part of the mantle rising, then cooling, sinking again and then heating, rising and repeating the cycle over and over. The next time you heat anything like soup or pudding in a pan you can watch the convection currents move in the liquid. When the convection currents flow in the mantle they also move the crust. The crust gets a free ride with these currents. A conveyor belt in a factory moves boxes like the convection currents in the mantle moves the plates of the Earth.
The core of the Earth is like a ball of very hot metals. (4000 degrees F. to 9000 degrees F.) The outer core is so hot that the metals in it are all in the liquid state. The outer core is located about 1800 milesbeneath the crust and is about 1400 miles thick. The outer core is composed of the melted metals nickel and iron.
The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place as a solid. The inner core begins about 4000 miles beneath the crust and is about 800 miles thick. The temperatures may reach 9000 dgrees F. and the pressures are 45,000,000 pounds per square inch. This is 3,000,000 times the air pressure on you at sea level!!!
Answer the following questions on a sheet of paper with your partner. If you need to look back to find the answers use the page titles located directly under the questions to help you. When you finish the questions click on the Earth icon to return the program to the beginning.
1. Name the four layers of the Earth in order from the outside to the center of the Earth.
2. What causes the mantle to "flow"?
3. What are the two main metals that make up the outer and inner core?
4. Describe in your own words how the Earth's layers were formed. "The Four Layers" will help you.
The Earth is a dynamic or constantly changing planet. The thin, fragile plates slide very slowly on the mantle's upper layer. This sliding of the plates is caused by the mantle's convection currents slowly turning over and over. This overturn is like a conveyor belt that moves the plates of the crust.
These plates are in constant motion causing earthquakes, mountain building, volcanism, the production of "new" crust and the destruction of "old" crust. The following cards will teach you more about the Earth's plates.
The Earth's crust is broken into many pieces. These pieces are called plates. There are twelve main plates on the Earth's surface. The red lines on this map of the world represent the largest plate boundaries. A plate boundary occurs where two plates come together. There are three kinds of plate boundaries:
1. Convergent boundary -where two plates collide to form mountains or a subduction zone.
2. Divergent boundary -where two plates are moving in opposite directions as in a mid-ocean ridge.
3. Transform boundary -where two plates are sliding past each other as in the San Andreas fault of California.
The Earth's plates are in constant, but very, very slow motion. They move at only 1/2 to 4 inches (1.3 to 10 centimeters) per year!! This does not seem like much, but over millions of years it adds up to great distances of movement.
The Continental Drift Theory states that the continents have moved and are still moving today. In 1912 Alfred Wegener introduced this theory, but he did not fully understand what caused the plates to move. A theory is an explanation of a scientific process that has been successfully tested by many different methods.
The motion of the Earth's plates help scientists to understand why earthquakes, volcanoes, and mountain building occur.
You will learn more about why the plates are moving in the next lesson, "How Plates Move".
Scientists believe these plates have been moving for millions of years. In fact, 250 millions years ago the Earth's seven continents were all grouped together into a supercontinent called Pangea.
Just before the days of the dinosaurs the Earth's continents were all connected into one huge landmass called Pangaea . This huge supercontinent was surrounded by one gigantic ocean called Panthalassa.
Notice the postion of the continents of Antarctica (Far north of its current position), Australia (flipped sideways and far west of its current position) and the subcontinent of India (Hundreds of miles from Asia).
Scientists believe that the North American continent was located much farther south and east of it's position today. In fact, much of North America was in or near the tropics!! How do scientists know this?? They have found fossils from this period of time. These fossils are of tropical plants and animals. The fossils have been found in cold regions like North Dakota and Greenland!!!
180 Million Years Ago
About 180 million years ago the supercontinent Pangea began to break up. Scientists believe that Pangea broke apart for the same reason that the plates are moving today. The movement is caused by the convection currents that roll over in the upper zone of the mantle. This movement in the mantle causes the plates to move slowly across the surface of the Earth. About 200 million years ago Pangaea broke into two new continents Laurasia and Gondwanaland. Laurasia was made of the present day continents of North America (Greenland), Europe, and Asia. Gondwanaland was made of the present day continents of Antarctica, Australia, South America. The subcontinent of India was also part of Gondwanaland. Notice that at this time India was not connected to Asia. The huge ocean of Panthalassa remained but the Atlantic Ocean was going to be born soon with the splitting of North America from the Eurasian Plate.
How do we know that South America was attached to Africa and not to North America 180 million years ago?
Scientists today can read the history of the rock record by studying the age and mineral content of the rocks in a certain area.
The Triple Junction was formed because of a three-way split in the crust allowing massive lava flows. The split was caused by an upwelling of magma that broke the crust in three directions and poured out lava over hundreds of square miles of Africa and South America.
The rocks of the triple junction, which today is the west central portion of Africa and the east central portion of South America, are identical matches for age and mineral make up. In other words the rocks in these areas of the two continents were produced at the same time and in the same place. This tells us that South America and Africa were connected at one time!
Today these two continents are separated by the Atlantic Ocean which is over 2000 miles wide!
135 Million Years Ago
About 135 million years ago Laurasia was still moving, and as it moved it broke up into the continents of North America, Europe and Asia (Eurasian plate). Gondwanaland also continued to spread apart and it broke up into the continents of Africa, Antarctica, Australia, South America, and the subcontinent of India. Arabia started to separate from Africa as the Red Sea opened up.
The red arrows indicate the direction of the continental movements. Notice how far the Indian subcontinent has to move to get to its present postion connected to Asia.
The Atlantic, Indian, Arctic, and Pacific Oceans are all beginning to take shape as the continents move toward their present positions.
The plates are still moving today making the Atlantic Ocean larger and the Pacific Ocean smaller. The yellow arrows on the world map indicate the direction of plates movements today.
Notice the position of the Indian Subcontinent today. It moved hundreds of miles in 135 million years at a great speed (4 inches per year!!!) The Indian plate crashed into the Eurasian plate with such speed and force that it created the tallest mountain range on Earth, the Himalayas! What do you predict the world will look like in 100 million or 200 million years? What new mountain ranges will form? Where will new volcanoes erupt?
The Atlantic Ocean will be much larger 50 million years from now and the Pacific Ocean will be much smaller. North and South America will have moved farther west (California moving north) while Greenland will be located farther west but also farther north. The western part of Africa will rotate clockwise and crash into Europe causing great mountain building, while the far eastern region of Africa will rotate eastward toward the Arabian peninsula. Australia will move farther north into the tropics, while New Zealand will move to the south of Australia.
All of these predictions are just that, predictions. These movements of the continents may happen if the plates continue to move in the same direction and with the same speed as they are moving today. Scientists are not certain of the movement today, let alone 50 million years into the future.
What do you think the world will look like in 50 million years???
Write the answers for the questions on a sheet of paper. When you finish the lesson click on the "Earth" icon so that the next pair of students will be at the start of the lesson.
1. What caused Pangea to break up?
2. What is the Continental Drift Theory?
3. What happened at the Triple Junction? Where is it located today?
Geologists came to the conclusion in the 1960's that the Earth's rigid outer layer (crust and outer, rigid layer of the mantle) was not a single piece, but was broken up into about 12 large pieces called plates. The red lines on the map of the world above indicate
1. Convergent boundaries - two plates collide to form mountains or a subduction zone.
2. Divergent boundary - two plates are moving in opposite directions as in a mid-ocean ridge.
3. Transform boundary - two plates are sliding past each other as in the San Andreas fault of California. A transform boundary is like a tear in the Earth's crust. These plates move very slowly across the surface of the Earth as though they were on a conveyor belt. The convection currents in the much hotter mantle continually move the plates about 1/2 to 4 inches per year.
When the plates move they collide or spread apart allowing the very hot molten material called lava to escape from the mantle. When collisions occur they produce mountains, deep underwater valleys called trenches, and volcanoes. As mountains and valleys are being formed natural disasters such as earthquakes and volcanic activity can occur which has affected humans for thousands of years.
The Earth is producing "new" crust where two plates are diverging or spreading apart. This occurs in the middle of our great oceans. The mid-ocean ridges are the longest continually running mountain range in the world. These ridges are connected and are about 40,000 miles long!!
One of these mid-ocean ridges, the Mid-Atlantic ridge , is spreading apart making the Atlantic Ocean wider. As the two plates move the mantle melts, making magma and lava fill the void with newly formed rock. The bottom of the Atlantic Ocean is filled with some of the "youngest" crust on Earth. The island of Iceland, located in the North Atlantic, is still being formed at this Mid-Atlantic ridge.
The Atlantic Ocean is getting larger as the Western Hemisphere moves away from Europe and Asia. The Pacific Ocean, on the other hand, is becoming smaller and smaller. This is occurring because the North American and South American plates are moving westward toward Asia and Australia.
The North and South American plates are crashing into the thinner and denser oceanic plates of the Pacific. This drives the oceanic plates deep into the mantle destroying the oceanic plates. This boundary in which an oceanic plate is driven down and destroyed by a continental plate is called a subduction zone.
This Pacific Ocean region has more earthquakes and volcanic activity than any other area of the world. Because of all the volcanoes this region has been given the nickname of "The Ring of Fire" .
When the less dense, lighter continental plate overrides the oceanic plate a subduction zone forms. Because the oceanic plate is bent and driven down, a deep trench forms at this collison point. These trenches are the lowest points on the Earth's crust. One trench is a mile deeper than Mount Everest is tall!
As the oceanic plate descends into the mantle some of it melts. This material moves into the mantle above the plate and causes the mantle to melt. This liquid rock, called magma, rises to the surface because it is less dense then the surrounding rock. If the magma reaches the surface of the Earth, a volcano forms.
As the mantle rocks melt they form magma. The magma collects in a magma pool. Because the magma is less dense than the surrounding mantle material it will rise. Pressure in the magma cracks the overlying rocks. Then the magma injects into the crack. This process repeats thousands of times, bring the magma towards the surface.
A volcano will form if the magma reaches the surface. When magma does reach the surface it is then called lava.
You will learn more about volcanoes in the following lessons.
As the volcano erupts it may build a mountain. The lava along with ash and other pyroclastic material will continue to build the mountain higher with each eruption.
The Cascade mountain range in the Western United States and the Andes Mountains in South America were formed in this way!!
This is a cross section of the Earth in the Southern Hemisphere. The map shows a subduction zone that has created the Peru-Chile Trench at the western edge of South America. This subduction zone has produced the Andes Mountains which run along the entire west coast of South America. It also shows you the Mid-Atlantic Ridge which is spreading the Atlantic Ocean making it wider and wider. The cross section shows two processes at work;
1. "Old Crust" being destroyed at a subduction zone and
2. "New Crust" being produced at the Mid-Atlantic Ridge.
The pink lines on this map of the Pacific Ocean represent deep ocean trenches. These trenches are some of the lowest points on the crust of the Earth. Marianas Trench north of New Guinea is the deepest point on the Earth's surface at 36,201 feet below sea level. Marianas Trench is 7,173 feet deeper than Mount Everest is high!!!!
Trenches surround almost all of the Pacific Ocean. Some of the other trenches of the Pacific are the Aleutian, Peru-Chile, Kuril, and the Japan trench.
There are trenches wherever continental plates and oceanic plates collide. The Java Trench in the Indian Ocean is the deepest point of that ocean at 24,442 feet below sea level.
Write the answers to the following questions in complete sentences on a piece of paper. Use the page titles located directly under the questions to move your way through the lesson to locate the answers. When you finish the questions click on the Earth icon to return the program to the beginning.
1. In your own words explain what happens at a subduction zone.
2. In your own words explain what happens at a mid-ocean ridge.
3. At a subduction zone what causes magma to rise?