• Some questions we will answer today:

– What forces inside the earth create and change landforms on the surface?

– What is the theory of plate tectonics and how does it work?

– 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.

• Natural forces interact with and affect the earth’s crust, creating the landforms, or natural features, found on the surface of the earth.

Before we start to look at the forces that contribute to landforms,lets 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. 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.

Crust

Mantle

Outer Core

Inner Core

Plate Tectonics

• Most of these changes in the earth’s surface takes place so slowly that they are not immediately noticeable to the human eye.

• 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 TheoryAbout 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. 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.

So, if my contintents fit together, why does the earth look like it

does today?

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 single “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.

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, stretching more than 64 thousand kilometers (40 thousand miles).

• The theory of seafloor spreading suggests that molten rock (think of a melted chocolate bar that has been left in your pocket for too long)... This hot substance (lava) from the mantle rises under the underwater ridge and breaks through a split at the top of the ridge (the crust... Remember, 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. 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?

• When a geologist or a geographer looks at a piece of land they often ask, ”What forces shaped the mountains, plains, and other landforms that are here?”

So....

• What is their answer?

Plate Tectonics

But this doesn’t actually tell me how the mountains or volcanoes were formed or

how earthquakes happen, does it?

YES!

• As mentioned earlier, those tectonic plates are always moving. They are always moving:– pulling away from each other – crashing head-on – or sliding past each other.

Depending on which way these plates are moving will decide what is happening on the earth you and I are standing on.

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.

– Therefore, 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. A temperature hot enough to melt si about a thousand degrees!) This process is called subduction. Molten material produced in a subduction zone can rise to the earth’s surface and cause volcanic building, 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.

• When both are oceanic plates, one slides under the other. Often an island group forms at this boundary.

Converging... They crash!

And they’re both ocean plates!

Converging...They Crash! And they’re both Continental Plates• When both are continental plates, the plates

push against each other, creating mountain ranges.

They 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!

• Sometimes, instead of pulling away from each other or colliding with eac hother,

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. This fault frequently plagues California with earthquakes.

– What forces inside the earth create and change landforms on the surface?

– What happens when the plates crash together, pull apart, and slide against each other?

• Earthquake Severity Richter Earthquake Magnitudes • Effects Less than 3.5 Generally not felt, but recorded.• 3.5-5.4 Often felt, but rarely causes damage. • Under 6.0 At most slight damage to well-designed

buildings. Can cause major damage to poorly constructed buildings over small regions.

• 6.1-6.9 Can be destructive in areas up to about 100 kilometers across where people live.

• 7.0-7.9 Major earthquake. Can cause serious damage over larger areas. 8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers

across.

• I. People do not feel any Earth movement. • II. A few people might notice movement if they are at rest and/or on the upper

floors of tall buildings. • III. Many people indoors feel movement. Hanging objects swing back and forth.

People outdoors might not realize that an earthquake is occurring. • IV. Most people indoors feel movement. Hanging objects swing. Dishes,

windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.

• V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.

• VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.

•

• VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings.

• VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change.

• IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.

• X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly.

• XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.

• XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

• All graphics were taken from Google Images, enchanted learning, boom zone, and other educational sites.

• All written information was taken from Prentice Hall, World Geography, PBS.org, and other educational websites.

• A good website for a deeper understanding is www.observe.arc.nasa.gov/