Some questions we will answer today: How is the earth always changing? What forces inside the earth create and change landforms on the surface? What is the theory of plate tectonics? What two theories help make up the theory of plate tectonics? What is continental drift and sea floor spreading? What happens when the plates crash together, pull apart, and slide against each other?

The Earth’s Layers The Earth is made of many different and distinct layers. The deeper layers are composed of heavier materials; they are hotter, denser and under much greater pressure than the outer layers.

Let’s look at the different layers of the earth that play a vital role in the formation of our continents, mountains, volcanoes, etc.

crust - the rigid, rocky outer surface of the Earth, composed mostly of basalt and granite. The crust is thinner under the oceans. mantle - a rocky layer located under the crust - it is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium. Convection (heat) currents carry heat from the hot inner mantle to the cooler outer mantle.

Remember the crust and the upper part of the mantle make up the lithosphere the rest of the mantle is the asthenosphere outer core - the molten iron-nickel layer that surrounds the inner core. inner core - the solid iron-nickel center of the Earth that is very hot and under great pressure.

The continents and oceans are moving across the surface of the planet? Volcanoes and earthquakes as well as mountain ranges and islands all are results of this movement. Why is this?

Plate Tectonics

The idea that the earth’s landmasses have broken apart, rejoined, and moved to other parts of the globe forms part of the plate tectonic theory.

Plate Tectonic Theory About forty years ago, scientists exploring the seafloor found that it is full of tall mountains and deep trenches, a single seafloor mountain chain circles Earth and contains some of Earth’s tallest mountains.

Along this mountain chain is a deep crack in the top layers of earth Along this mountain chain is a deep crack in the top layers of earth. Here the seafloor is pulling apart and the two parts are moving in opposite directions, carrying along the continents and oceans that rest on top of them. These pieces of Earth’s top layer are called tectonic plates. They are moving very slowly, but constantly. (Most plates are moving about as fast as your fingernails are growing -- not very fast!) Currently Earth’s surface layers are divided into nine very large plates and several smaller ones.

According to the theory of plate tectonics, the earth’s outer shell is not one solid piece of rock. Instead the earth’s crust is broken into a number of moving plates. The plates vary in size and thickness.

The North American Plate stretches from the mid-Atlantic Ocean to the northern top of Japan. The Cocos Plate covers a small area in the Pacific Ocean just west of Central America. These plates are not anchored in place but slide over a hot and bendable layer of the mantle.

To really understand how the earth became to look as it does today, and the theory of plate tectonics, you also need to become familiar with two other ideas: Continental Drift and Seafloor Spreading.

Less than 100 years ago, many scientists thought the continents always had been the same shape and in the same place. A few scientists noted that the eastern coastline of South America and the western coastline of Africa looked as if they could fit together. Some also noted that, with a little imagination, all the continents could be joined together like giant puzzle pieces to create one large continent surrounded by one huge ocean.

Continental Drift Theory When the tectonic plates under the continents and oceans move, they carry the continents and oceans with them. In the early 1900s a German explorer and scientist proposed the continental drift theory. He proposed that there was once a ingle “supercontinent” called Pangaea.

Wegner’s theory was that about 180 million years ago, Pangaea began to break up into separate continents. To back this theory up, he perserved remains and evidence from ancient animals and plants from South America, Africa, India, and Australia that were almost identical. Rock sequences and glaciation also was evidence.

Seafloor Spreading The other theory theory supporting plate tectonics emerged from the study of the ocean floor. Scientists were suprised to find that rocks taken from the ocean floor were much younger than those found on the continents. The youngest rocks were those nearest the underwater ridge system which is a series of mountains that extend around the world.

The theory of seafloor spreading suggests that molten rock, the hot substance (lava) from the mantle rises under the underwater ridge and breaks through a split at the top of the ridge (the plate). The split is called a rift valley. The rock then spreads out in both directions from the ridge as if it were on two huge conveyor belts. As the seafloor moves away from the ridge, it carries older rocks away.

Evidence to support sea-floor spreading is magnetic mapping Evidence to support sea-floor spreading is magnetic mapping. When the rock is magma/lava it allows the atoms of the metals to align with the magnetic poles of the earth. These magnetized metals cool and form rocks and then can be mapped using magnetic equipment. This along with radiometric dating proves sea-floor spreading.

Seafloor spreading, along with the continental drift theory, became part of the theory of plate tectonics.

Plate motions also can be looked at into the future, and we can have a stab at what the geography of the planet will be like. Perhaps in 250 million years time there will be a new supercontinent.

What two theories help make up the theory of plate tectonics? What is continental drift and sea floor spreading?

As mentioned earlier, tectonic plates are always moving. They are always moving: pulling away from each other (Divergent plate boundary) crashing head-on (Convergent plate boundary) or sliding past each other. (Transform plate boundary)

They’re Pulling Apart! When plates pull away from one another they form a diverging plate boundary, or spreading zone. Thingvellir, the spreading zone in Iceland between the North American (left side) and Eurasian (right side) tectonic plates. January 2003.

The Crash! What happens when plates crash into each other depends on the types of plates involved. Because continental crust is lighter than oceanic crust, continental plates ”float” higher. When an oceanic plate meets a continetnal plate, it slides under the lighter plate and down into the mantle. The slab of oceanic rock melts when the endges get to a depth which is hot enough. This process is called subduction. Molten material produced in a subduction zone can rise to the earth’s surface and cause mountains, and islands.

When they Crash When two plates of the same type meet, the result is a process called converging. Depending on what type of plates these are, depends on what occurs.

Converging...They Crash! And they’re both Continental Plates Converging...They Crash! And they’re both Continental Plates When both are continental plates, the plates push against each other, creating mountain ranges.

A Crash and are both continental plates! Earth’s highest mountain range, the Himalayas, was formed millions of years ago when the Indo-Australian Plate crashed into the Eurasian Plate. Even today, the Indo-Australian Plate continues to push against the Eurasian Plate at a rate of about 5 cm a year!

They meet and slide past each other! Transform plate boundary They meet and slide past each other! Transform plate boundary Sometimes, instead of pulling away from each other or colliding with each other, plates slip or grind past each other along faults. This process is known as faulting.

These areas are likely to have a rift valley, earthquake, and volcanic action. For example: Here, the San Andreas Fault lies on the boundary between two tectonic plates, the north American Plate and the Pacific Plate. The two plates are sliding past each other at a rate of 5 to 6 centimeters each year.

What evidence supports continental drift. similar rock sequences. Similar glaciation patterns. Similar fossils of plant and animals. All of the above. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

What evidence supports sea floor spreading. Magnetic mapping. Igneous intrusion. Similar glaciation patterns. All of the above. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

When two plates slide past each other what type of plate boundary is it?. Convergent . Divergent. Transform. Subduction. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

When two plates go away from each other what type of plate boundary is it?. Convergent . Divergent. Transform. Subduction. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32