Instruction 1-3

Structure of the Earth | Plate Tectonics | Earthquakes, Volcanoes, Mountain Building | Geology of California | Summary

Earthquakes, Volcanoes, Mountain Building 
CCSTD Grade 6 1.d, 1.e, 1.g

In our last two Instructions, we talked about how the Earth is made up of three layers. Those layers are the Core, the Mantle and the Crust. We also told you how the Earth's surface is made up of moving masses of rock called tectonic plates.
Currents of heat radiating up from the Core through the Mantle cause these plates to move. And it is this movement that causes major geologic events like earthquakes, volcanoes and mountain building.
For an interesting exploration of tectonic plates and our changing Earth, click: 

For NASA animations of various Earth processes, click:


Earthquakes can happen anywhere. But most occur either along plate boundaries or along fault lines. Faults are cracks in the middle of plates.

To see an interesting animation of how earthquakes happen, click: 

And for an in-depth exploration of the subject of earthquakes, click: 

Earthquakes along Plate Boundaries

Many earthquakes take place where oceanic and continental plates meet.

They happen when the plates run into each other or try to slide past one another. That's because when the plates rub up against each other, they don't slide smoothly, they stick a little. They keep pushing against each other, but they can't move -- they're stuck. After a while the pressure builds up and the rocky plates break. This releases a sudden burst of energy that makes the earth shake. This energy is called seismic waves.
The underground spot where the rock breaks is called the focus of the earthquake. The spot directly above the focus is called the epicenter.

Earthquakes along Fault Lines

Earthquakes also occur along fault lines far from the edges of plates. A fault is a crack in the earth where sections of a plate (or two plates) are moving in different directions. There are three major kinds of faults:

Normal faults -- in which one block of rock slides downward and away from the other.

Strike-slip faults -- in which two plates slide past each other, and

Reverse faults -- in which one plate pushes into another.

Measuring Earthquakes

Earthquakes are measured by an instrument called a seismograph. The
seismograph records the motion of the ground underneath it. By studying data from the seismograph, scientists can figure out exactly where and when the earthquake took place. They can also tell how big it was.

The magnitude (size) of an earthquake is measured on something called the Richter Scale. The Richter Scale goes from Magnitude 1, which means Very Minor, to Magnitude 8 and above, which indicates a Great Earthquake.

The biggest earthquake ever recorded took place in Chile on May 11,1960. It measured 9.5 on the Richter Scale and caused damage as far away as Japan. The world's second biggest earthquake, measuring 9.2 on the Richter Scale, took place on Good Friday 1964 in Prince William Sound, Alaska.


olcanoes are mountains -- but they are not like other mountains.

Other mountains are created by the folding and crumpling of the Earth where continental plates collide.

Volcanoes are created by debris from their own eruptions.

Most volcanoes occur where oceanic plates collide. However, some arise far from plate boundaries in "hot spots" where the Earth's Crust is thin.

A volcano begins with a crack (or vent) in the Earth's Crust. When pressure inside the Earth builds up, magma (hot molten rock from inside the Earth) and gasses shoot up through this vent. Geologists call magma "lava" once it comes out of the volcano.

Over time, material builds up around the vent and a volcano is born. About 80 percent of the Earth's surface (both above and below sea level) originally came from volcanoes.



To learn more about volcanoes, click: 

To see a dramatic animation of a volcanic eruption, click: 

There are three main types of volcanoes:

Cinder Cones. Cinder Cones are the simplest type of volcano. They are built up from blobs of lava ejected from a single vent. Cinder Cones rarely get to be more
than a thousand feet high.

Stratovolcanoes (also called Composite Volcanoes). Stratovolcanoes are built up of alternating layers of lava flow,  volcanic ash and cinders. Stratovolcanes usually have large, steep sides and symmetrical cones. They can get to be as high as 8,000 feet. Mt. Fuji in Japan and Mount St. Helens in the U.S. are Stratovolcanoes.

Shield Volcanoes. Shield Volcanoes are built up almost entirely of lava flow, which pours out in all directions. This results in a flat, conical shape that looks a little like a warrior's shield. Many of the world's largest volcanoes are Shield Volcanoes. The Hawaiian  Islands are a linear chain of them.

To learn more about volcanoes, click: 

To see a dramatic animation of a volcanic eruption, click: 
All volcanic eruptions are dangerous, especially the pyroclastic flow that rushes down the sides of the volcano. Pyroclastic flow is a ground-hugging avalanche of hot ash, pumice, rock fragments and volcanic gas. Nobody escapes it alive -- and nobody can outrun it.
There are over 500 active volcanoes in the world today. "Active" means that the volcano is currently erupting or showing signs of unrest . Some scientists also consider volcanoes active if they have erupted within recorded history.
Fifty of these volcanoes are in the United States -- in Hawaii, Alaska, Washington, Oregon and California.
Mountain Building

Like earthquakes and volcanoes, mountains are created because of the movement of Earth's tectonic plates.
When plates collide, the Earth's Crust folds up into mountains or mountain ranges. Many of the rocks inside these ranges were once buried deep beneath the Earth. Some of these rocks are over 3.5 billion years old. 
There are five ways in which mountains are formed:

  1. Volcanic activity (which we've already discussed)
  2. Folding
  3. Faulting
  4. Dome building
  5. Erosion

Folding takes place when the Earth's continental plates are pushed together. Folding bends many layers of rocks without breaking them. The Appalachian Mountains and The Rocky Mountains of the United States were formed in this way.

Other mountains are formed when pressure along fault lines causes layers of the Earth's Crust to move upward. The Sierra Nevada range in California is an example of Fault-Block mountain building.

Dome mountains are formed when the Earth's crust is heaved upward without folding or faulting. Typically, this results in low, rounded mountains. The Black Hills of South Dakota and Wyoming and the Adirondack Mountains of New York are good examples.

Erosion changes the contours of many mountains and creates others. Erosion is the wearing away of the Earth's surface -- primarily by water (rain, oceans, lakes and rivers) and wind.

Reading List:
From the California Dept. of Education 

Title, Author
Description from the California Dept. of Education
Earth's Fiery Fury (Downs, Sandra) 
Earthquakes (Maslin, Mark ) 
Volcanoes (Simon, Seymour)
Mountains (Simon, Seymour) 



for Students, Parents and Teachers

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