The surface of the Earth is constantly changing. That may sound like a strange statement to make. I'm sure that throughout your life you have looked out on a landscape that hasn't seemed to change at all. Most of the time, the changes that occur are very subtle. For example, Heavy rains and strong winds can move particles of soil to other areas without your notice. These subtle changes go on all the time. Over the course of centuries and millennia the land changes shape. Geologists consider these to be slow changes. Other major changes happen within seconds or minutes. Earthquakes, volcanoes, avalanches and floods are examples of fast changes. In the next two lessons we will examine fast and slow changes to the Earth's crust.

When you think about quick changes on the Earth's crust, earthquakes must surely come to mind. The Earth's crust is broken into large sections called tectonic plates. The lighter rock, which makes up the continents, rides on top of the heavier rock below. Earthquakes and volcanoes occur along the edges of these crustal plates. You will learn more about these crustal boundaries in a later lesson.

Tectonic plates are always in motion. Their movement is very slow so it is generally unseen and unfelt by those living on the surface. But sometimes, the plates stick together and the pressure builds up until it snaps. This rapid motion at the plate boundary is called an earthquake . The longer the pressure builds up the larger the earthquake when the plates release. Scientists use the Richter scale to measure the power released during an earthquake. The Richter scale measures the magnitude, or strength of an earthquake. Earthquakes, greater than 6 on the Richter scale, can cause significant damage and loss of life. It all depends where the earthquake strikes. If the quake is close to a populated area then the results can be catastrophic.

More about earthquakes

There is another scale that is used to measure the damage caused by earthquakes. The Mercalli scale measures the intensity of an earthquake by the amount of damage that is done and how much movement was felt. This scale helps us look at the recorded information about earthquakes before the Richter scale was invented. 

Volcanoes are caused by cracks or hot spots in the earth's crust. Through these regions, material from the mantle rises to the surface. Magma and lava are commonly used terms when discussing volcanoes.

There are two basic types of lava. Lava which flows freely and has little gas is found in the Islands of the Pacific. Aside from burning your house down, there is little personal danger from this type of volcano. A second type of lava is much more viscous, and contains a great deal of gas and water vapour. This type of material plugs up the vent of a volcano until it literally blows itself apart. Mount St. Helens was this type of volcano.

Although there is danger to property, few people are ever injured by this form of eruption in Hawaii.

Many of the volcanoes on the west coast of North America are explosive and capable of killing thousands of people if they erupt.

Volcano Structure

There are three basic types of volcanoes.

The first is called a shield cone, and is made almost entirely of layers of lava. Shield cones have gently sloping sides. The Hawaiian islands are shield cone volcanoes. Interesting fact (Mauna Loa is the world's highest mountain. Yes, it is even higher than Mt. Everest, if you count the total height of the mountian. Mauna Loa starts far below the surface of the ocean. So from the base of the volcano, instead of from sea level, it is even higher than Mt. Everest.)

A second form of volcano is called a cinder cone. As the name says, it is made from small bits of rock that has been thrown high up into the air and settles around the vent. These volcanoes have very steep sides. Cinder cone volcanoes are the smallest and most common form of volcano.

The third type is a combination of the other two. It is called a composite cone volcano. Sometimes during an eruption, lava flows out and forms another layer on the volcano. At other times, cinders and ash explode from the volcano and form other layers. The sides on these volcanoes are not as steep as the cinder cones, but steeper than shield cones.

It is important to note that the material being broken down is moved from place to place but is never lost to the global system. In the next module you learn about the 'rock cycle' and that even though one type of rock is weathered away, a new rock will eventually be made from the deposited material.

Mechanical Weathering

There are a number of ways that material can be broken down. We use the term mechanical weathering when rock is broken down by physical forces.

Rocks that undergo changes in temperature expand and contract. As they get warmer, they expand and contract as they cool. But this warm and cooling, expanding and contracting does not occur evenly throughout the rock. This uneven expansion creates stresses in the rock that eventually cause it to crack. It is at this point that a second type of mechanical weathering begins. Water falling as rain or from melting snow seeps into the newly formed cracks in the rock. If that rock is in a climate where the temperature falls below freezing then the water in the crack will freeze. In an earlier lesson you learned that water expands as it freezes. This expansion is so powerful that it enlarges the crack in the rock and, eventually the rock will fall part. This process is called frost wedging.

Activity: The Power of Freezing Water

Parental permission and assistance is required to do this lab.

Materials:

1 small glass jar with a tight fitting lid
(this jar will be destroyed so don't use one you want to keep!)
water
freezer
3 plastic bags large enough to fit around the jar.
1 twist tie

Procedure:

Valley Glaciers

Valley glaciers can still be found in mountainous regions around the world. Even after they melt, there is still evidence of their weathering. Normal V-shaped valleys have been ground down into a U- shape, and striations (scratches on the rocks) tell us the direction they were travelling.

Continental Glaciers

In the past the Earth has gone through many periods of long cooling. These are called ice ages. The last ice age ended and the glaciers that covered all of Canada and much of the United States began to retreat northwards. Like the valley glaciers, the continental ground down all it came across. The rock, gravel and mud that they eroded became captured in the glaciers.

When the glaciers retreated, these materials were deposited as funny shaped geography that go by names such as: drumlins, eskers, and kettle lakes. These deposits can be found throughout Alberta and the rest of the continent as well. Sometimes huge boulders are carried hundreds of miles before being deposited by melting glaciers. These huge stones are called erratics.

Watch this animation of the receding Laurentide ice sheet that covered much of North America.