Earth’s Internal Processes

Plate Tectonics

Earthquakes

Volcanoes

Review

• Recall that the Earth is composed of several layers.

• Inner Core- Solid iron/nickel

• Outer Core- Molten iron/nickel

• Mantel- “Plastic” Molten Rock

• Crust- Solid Rock

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History

• Throughout most of history people believed that the continents were permanent and unmoving.

• Alfred Wegener was one of the first to suggest the continents used to be joined together in one Super-continent he called Pangea, meaning “all land”

Continental Drift

• Wegener proposed the hypothesis of continental drift which suggested that the continents were like icebergs of granite floating in a sea of more dense basalt.

• While now accepted as the truth this idea was laughed at until sufficient evidence was found.

Evidence

• One of the first pieces of evidence to support continental drift was the discovery of fossils.

• An ancient freshwater reptile, Mesosaurus, was found in both Africa and South America.

• Tropical plant fossils were found in places that they would not normally be able to grow.

• Conclusion… the continents are moving.

Evidence (cont)

• Evidence of glaciers were found in places were they could not have developed.

• People also noticed that similar rocks could be found on opposite sides of the Atlantic.

• The rocks near the mid-ocean ridge are much newer than the rocks that are farther away.

• It was concluded that the seafloor was spreading along the mid-oceanic ridge.

High Tech Evidence

• Later studies showed that the rocks in the seafloor had alternating bands of magnetism that correlated with reversals in Earth’s Magnetic Field.

• Today very accurate lasers have determined that most continents are drifting at a rate of about 1inch per year.

Plate Tectonics

• The Theory of Plate Tectonics states that the Earth’s crust is divided into sections called plates.

• These plates are moved around by convection currents in the mantle.

Boundary Issues

• If all the plates are moving in different directions how do their boundaries interact.

• There are three types of plate boundaries– Divergent– Convergent– Transform

Divergent Boundaries

• Divergent boundaries exist where two plates are moving apart from one another.

• Divergent boundaries in the ocean form mid ocean rifts and ridges.

• On continents they form rift valleys like Africa’s Great Rift Valley.

Convergent Boundary

• When two plates are moving toward each other one of three convergent boundaries will form.

• This depends on the density of each of the plates involved

Subduction Zone

• Where a seafloor plate impacts a less dense continental plate and sinks below it.

• The sinking plate brings water with it helping to create volcanoes.

• A similar type of convergent boundary occurs when two oceanic plates collide.

• Both tend to create volcanoes, earthquakes, and trenches.

Going up?

• When two continental plates impact, they tend to move upward.

• This process created some of the biggest mountains in the world.

• The Himalayas were created as the Indian Plate smashed into the Asian Plate.

• They are still pushing up at about 0.5cm/year.

Transform Fault Boundary

• Transform fault boundaries occur where plates are sliding past or along side one another.

• These boundaries are known for many powerful earthquakes that release energy that builds up as the plates snag on one another.

• The San Andreas Fault is a famous transform fault boundary located under California.

Earthquakes

• Earthquakes are caused by the motion of Earth’s Crust.

• We can measure the seismic waves caused by an earthquake.

• We can also see where the waves originated and find the focus or point of origin for an earthquake.

• Most foci occur along plate boundaries, also called faults

Seismic Waves

• Two types of seismic waves radiate out from the focus of an earthquake

• Primary waves move particles back and forth like sound waves.

• Secondary waves move particles up and down like ocean waves

• On the surface these two waves join to give the ground an elliptical motion.

• Earthquakes cause massive damage to man made structures.

Seismic Waves (cont)

• Seismic waves don’t just locate the focus or epicenter.

• They can also be used to map Earth’s interior because only primary waves are able to travel through molten rock.

• They can show us the location of magma chambers beneath volcanoes.

Measuring Earthquakes

• Earthquakes can be measured using a seismometer, which traces the affect of the seismic waves on a rotating drum.

• These tracings can be used to measure the energy or strength of the earthquake which is described with the Richter Scale.

Richter Scale

• Is a log base scale.• A 2.00 earthquake is 10 times stronger than a 1.00

earthquake.• Under 4.00 is considered to be minor and over

6.00 is considered to be strong.• Weaker earthquakes are very common while

strong earthquakes are very rare.• The strongest recorded earthquake was a 9.5 in

Chile in 1960.

Tsunami

• When an earthquake strikes under or near water the waves it releases can travel through the water generating an enormous wave.

• These waves can cause lots of damage when they hit shore.

• Dec 26, 2004 an earthquake of the coast of Sumatra generated a tsunami that had a death toll of 230,000 people and traveled across the entire Indian Ocean

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Volcanoes

• Volcanoes are mountains that form as lava and ash from repeated eruptions from a vent build up layer by layer.

• The lava comes from magma chambers below the Earth’s surface.

• The opening at the top of the volcanoes vent is called the crater or cauldera.

Where?

• Divergent Boundaries - lava erupts through the rifts and cools. Mid-oceanic Ridge, Iceland.

• Convergent Boundaries. Occur as water mixes with lava increasing pressure. Mt. St. Helens, Mt Redoubt.

• Hot Spots - Some volcanoes occur as hotspots in the mantle heat portions of the crust enough to melt them and build up pressure. Hawaii, Yellowstone.– Hot spots stay in place as plates move above them

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Types of Eruptions

• Eruptions can either be explosive and violent or lava can quietly seep out of a vent.

• Magma that has lots of gasses trapped inside tend to erupt more violently than magma with little gas.

• Basaltic magma tends to trap less gas than granitic magma because it is more liquid.

Pyroclastic Flows

• Translated roughly to fire rock these are the result of explosive granitic eruptions.

• Mixture of hot gas and rock (tephra) that moves away from the volcano at speeds of up to 450mph and temperatures of near 2000ºF

• Often the most deadly aspect of a volcano, a pyroclastic flow was responsible for the creation of the ruins of Pompeii

Types of Volcanoes

• Shield Volcanoes form when quiet eruptions build up shallow layers of igneous rock.

• Cinder Cone Volcanoes form as tephra builds up to form a steep sided mountain.

• Composite or Strato-volcanoes form when a volcano regularly switches between quiet and violent eruptions.