Plate Tectonics:Earth's Plates and Continental Drift

• Some questions we will answer:

– 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 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

The Crust• This is where we live!

• The Earth’s crust is made of:

Continental Crust

- thick (10-70km)- buoyant (less dense than oceanic crust) - mostly old

Oceanic Crust

- thin (~7 km)- dense (sinks under continental crust)- young

What are tectonic plates made of?

• Plates are made of rigid lithosphere.

The lithosphere is made up of the crust and the upper part of the mantle.

What lies beneath the tectonic plates?

• Below the lithosphere (which makes up the tectonic plates) is the asthenosphere.

Plate Movement

• “Plates” of lithosphere are moved around by the underlying hot mantle convection cells

• The large landmasses in the oceans are called continents.

List the continents in your notes.

Landforms are commonly classified according to differences in relief. The relief is the difference in elevation between the highest and lowest points. Another important characteristic is whether they rise gradually or steeply.

•The major types of landforms are mountains, hills, plateaus, and plains.

Land

• Most people know that Earth is moving around the Sun and that it is constantly spinning.

• But did YOU know that 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

• 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.

–How is the earth always changing?

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

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 found perserved remains and evidence from ancient animals and plants from South America, Africa, India, and Australia that were almost identical.

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.

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.

• Sea Floor Spreading

• 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.

Mid ocean ridge

• Iceland has a divergent plate boundary running through its middle

Iceland: An example of continental rifting

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 ends get to a depth which is hot enough. A temperature hot enough to melt it is 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.

• Oceanic lithosphere subducts underneath the continental lithosphere

• Oceanic lithosphere heats and dehydrates as it subsides

• The melt rises forming volcanism

• E.g. The Andes

Subduction

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 Oceanic Crust

• When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.

• The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench.

• The worlds deepest parts of the ocean are found along trenches. – E.g. The Mariana Trench is 11 km deep!

Ocean-Ocean Plate Collision

Converging Continental Plates

• When both are continental plates, the plates push against each other, creating mountain ranges.

Converging 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!

Himalayas

When plates slide past each other

Transform Boundary

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

plates slip or grind past each other along faults.

• 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 wit hearthquakes.

Three Types of Boundaries

Sea-Floor Spreading and Plate Boundaries

Converging Margins: India-Asia Collision

Transform Faults and Seafloor Spreading

…what’s the connection?

Volcanoes and Plate Tectonics…

Volcanism is mostly focused at plate margins

Pacific Ring of Fire

- Subduction - Rifting - Hotspots

Volcanoes are formed by:

Pacific Ring of Fire

Hotspot volcanoes

• Hot mantle plumes breaching the surface in the middle of a tectonic plate

What are Hotspot Volcanoes?

Photo: Tom Pfeiffer / www.volcanodiscovery.com

The Hawaiian island chain are examples of hotspot volcanoes.

The tectonic plate moves over a fixed hotspot forming a chain of volcanoes.

The volcanoes get younger from one end to the other.

Plate Tectonics Summary• The Earth is made up of 3 main layers (core,

mantle, crust)• On the surface of the Earth are tectonic plates that

slowly move around the globe• Plates are made of crust and upper mantle

(lithosphere)• There are 2 types of plate (Continental & Oceanic)• There are 3 types of plate boundaries (Convergent,

Divergent, Transform)• Volcanoes and Earthquakes are closely linked to

the margins of the tectonic plates

…what’s the connection?

Earthquakes and Plate Tectonics…

• As with volcanoes, earthquakes are not randomly distributed over the globe

• At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur.

Figure showing the distribution of earthquakes around the globe

Where do earthquakes form?

Figure showing the tectonic setting of earthquakes