Inside Earth – Plate Tectonics NOTES
The center of the Earth is 6,000 kilometers deep. How do scientists figure out what the center of the Earth is like?
1. rock samples
2. seismic evidence – a seismic wave is a vibration that travels through the Earth carrying energy released during an earthquake. Using data from seismic waves, scientists have learned that the interior of the Earth is made up of several layers.
3 main layers of the Earth:
As you move down below the Earth’s surface, through the crust
the temperature gets hotter and the pressure rises.
Why do you suppose this happens? The heat is coming from the Earth’s core and the pressure comes from the weight of the rock above. The deeper into the Earth you go, the greater the weight of the rock above, and therefore, the greater the pressure. What happens to the water pressure as you dive deeper into the deep end of a swimming pool?
Fill in the blanks: As you go deeper into the Earth, the temperature
goes ______________, and the pressure goes ________________.
Let’s take a closer look at all the layers of the Earth….
The crust is the layer of rock that forms the Earth’s outer covering.
The crust includes dry land and ocean floor, usually between 5 and 40 kilometers thick. It is the thinnest of the Earth’s 3 layers. Where on the Earth do you think the crust is the thickest?
Continental (dry land) crust is mostly made up of granite rocks, while oceanic crust is mostly made up of rocks called basalt.
The mantle is the layer of hot, solid material between the Earth’s crust and core. The mantle is about 3,000 kilometers thick and it is made mostly of silicon, oxygen, magnesium, and iron. The mantle is divided into three layers:
1. lithosphere – the uppermost part of the mantle PLUS the crust. This rigid layer is about 100 kilometers thick.
2. asthenosphere – this middle layer of the mantle is soft and tar-like (it can bend like plastic). The asthenosphere is hotter and under more pressure than the lithosphere.
3. lower mantle – the lower mantle is solid and goes all the way down to the Earth’s core. What do you think the heat and pressure are like in the lower mantle?
VOCAB – Moho Discontinuity - is a layer within the lithosphere that marks the boundary between the crust and the mantle. At the Moho Discontinuity, seismic waves speed up as they pass into a denser layer of the lithosphere.
The core is the center of the Earth, made up of mostly iron and nickel. The core has two parts, the solid inner core, and the liquid outer core. The Earth’s core is believed to be responsible for the magnetic field that exists around the Earth. Why and how does a compass work?
Heat energy always moves from the warmer substance to the cooler substance. When you put a warm bottle of soda into the refrigerator, why does it get cold?
The heat energy from the warm soda leaves and goes toward the cool refrigerator
There are three types of heat transfer:
radiation, conduction, and convection
1. Radiation – the transfer of energy through space. The sun’s energy travels through space to the Earth by radiation.
3. Conduction – the transfer of heat energy within or between materials that are touching. When you touch a hot pot or pan, you burn your finger. The heat transfers to your finger by conduction.
4. Convection – the transfer of heat energy by the movement of currents within a fluid (fluids are defined as liquids or gases). The glitter lamp and lava lamps work by convection.
Convection current (as seen in the blue glitter lamp) is the flow that transfers heat within a fluid. As a fluid heats up, it becomes less dense and rises upward. As the fluid cools down (loses energy), it becomes more dense and moves downward.
Identify the following examples of heat transfer as conduction, convection, or radiation.
1. __________________ hot air and smoke rise up and out of the chimney when there is a fire in your fireplace.
2. __________________ you make ice cream by placing a contained mixture of milk/cream and sugar in a mixture of ice and salt.
3. __________________ while sitting on your living room couch, you feel the warmth from your wood stove heating your face.
What do convection currents have to do with the layers of the Earth?
Scientists believe that differences in density cause the hot, plasticlike rock in the asthenosphere (the soft, middle layer of the mantle) to be forced upwards towards the Earth’s surface, where it sometimes moves horizontally, then cools off and sinks slowly down again, taking some of the Earth’s crust with it. This cycle of heating and rising, followed by cooling and sinking has been going on for more than 4 billion years!!
Conduction activity: making ice cream
Take a good look at a globe or a world map that shows the continents and oceans (pages 20-21). What do you notice about the edges of some of the continents? It appears as if the coasts of some of the continents would fit together like a jigsaw puzzle.
In the early 1900s, a scientist named Alfred Wegener believed that the continents were once all one land mass, but then they slowly drifted apart. Wegener’s theory became known as continental drift and he called the single large land mass PANGEA. During his lifetime, Wegener’s theory was not fully accepted, but evidence eventually convinced the scientific world that the theory was correct.
What evidence do you think would indicate or show that the continents had once been a single land mass?
1. land feature such as coasts and mountain ranges
2. similar fossil types
3. climate (tropical fossils found in polar continents show that polar continents must have been in a warmer location at one time)
Wegener tried to convince other scientists that continents could move and collide. It wasn’t until the 1950s when scientists began examining the ocean floors that Wegener’s theory gained support. Although most of the deep ocean floor was cold, dark, and almost lifeless, some areas of the sea floor were found to be warm and full of organisms. This was due to the fact that the ocean water was entering cracks in the ocean floor and getting warmed by hot layers in the mantle. The heated water was then vented back out into the ocean. Scientists also found that the ocean floor had mountains and valleys. These mountain valleys were called mid ocean ridges. A geologist named Hess proposed that mid ocean ridges were places where the sea floor was spreading apart. As the sea floor spreads apart, magma (hot liquid rock) from below moves upward to fill the gap created. The magma cools and becomes new rock. As this process continues, the ocean floor moves and takes the continents with it. This theory became known as sea floor spreading and it fully supports Wegener’s idea of continental drift.
What proof exists to support the theory of sea floor spreading?
1. evidence of molten rock along ocean ridges
2. age of rocks - “younger” rocks are found along the ridge and rocks get progressively older as you get further from the ridge.
3. magnetic clues – rock that contained iron was lined up along magnetic poles of the time (the magnetic poles of the Earth have reversed over time)
The sea floor doesn’t only spread apart, it also drops down into deep ocean trenches in a process called SUBDUCTION. The ocean floor is like a giant conveyor belt…hot rock comes up through the ridge, cools down and becomes dense, drops into a trench and becomes hot again and the cycle continues.
Sea floor spreading activity
The Earth’s 7 continents (quick…can you name all seven??) have cracks in them. Scientists observed these cracks and developed the theory of plate tectonics. This theory states that the Earth’s crust and upper mantle (the lithosphere) are broken into sections. These sections, called plates, move around on the mantle.
What causes the plates to move?? The answer is convection currents (do you remember what convection currents are? Hot stuff rises, cold stuff sinks). Plates move VERY slowly (1-24 centimeters a year). As plates move, they could collide, pull apart, or slide past one another. These three interactions can cause volcanoes, mountain ranges, deep ocean trenches, and earthquakes.
Plate boundaries are the places where the edges of the Earth’s plates meet. There are three kinds of plate boundaries:
1. DIVERGENT BOUNDARIES – locations where plates are moving apart (diverging). Sea floor spreading occurs at divergent boundaries. On land, divergent boundaries are called rift valleys
2. CONVERGENT BOUNDARIES – locations where plates are coming together (converging). The plate that is more dense goes beneath the less dense plate. Oceanic crust is usually more dense than continental crust, so when these two collide, the oceanic crust goes under the continental crust. When two oceanic crusts collide, deep sea trenches or volcanic islands can be formed. When two plates carrying continental crusts collide, either is more dense and the plates are pushed up into mountain ranges such as the Himalayas.
3. TRANSFORM BOUNDARIES – locations where two plates are sliding past one another, often causing earthquakes. The San Andreas fault in CA is a transform boundary.