Week 29 - Model of Earth, Sudden Earth Events and Incremental Changes

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.