Earth’s Surface Features Water covers about three-fourths of the earth’s surface. It is mainly contained in the ocean, but also in groundwater, rivers, streams, and lakes, as well as glaciers and polar ice caps. Landforms cover about one-fourth of the earth’s surface. They include mountains, hills, plains, and plateaus.
Active Volcanoes <ul><li>Magma forms when heat and pressure melt rocks inside the earth. </li></ul><ul><li>Volcanoes form when heat and gases cause pressure to build up under the surface of the earth. When enough pressure builds up, the magma forces its way up through weak spots called vents. </li></ul>
Active Volcanoes <ul><li>Then the magma breaks through the surface of the earth. Magma that flows onto the earth’s surface is called lava. The lava cools and hardens forming a mountain called a volcano. </li></ul>
Violent Eruption <ul><li>Sometimes a volcano erupts with such force that part of the volcanic mountain may be blown to pieces. Rock, volcanic ash, and lava may spread over a wide area. </li></ul>
Dormant Volcano <ul><li>A dormant volcano is one that is “sleeping.” It hasn’t erupted in recent times. Inside, the mountain may be building up pressure that will some day be released. </li></ul>
Quick Quiz <ul><li>What is a volcano? </li></ul><ul><li>Does a volcano change the earth’s surface slowly or quickly? </li></ul><ul><li>What is the difference between an active volcano and a dormant volcano? </li></ul><ul><li>Do you think people should live near dormant volcanoes? Why or why not? </li></ul>
<ul><li>Cinder Cones, sometimes called Scoria cones, are the most common type of volcano. Cinder cones are the smallest type of volcano. They are circular or oval shaped and their crater is bowl shaped. These volcanoes grow very fast until they reach their maximum height. Cinder cone volcanoes form from small, jagged pieces of rock and ash, also known as cinders, thrown in the air during an explosion. An example of a cinder cone volcano is in Mexico, called the Paricutin. It started growing in 1943, and in just five days, the volcano had grown to 300 feet! </li></ul>
Shield cone volcanoes are huge; some of the largest volcanoes in the world are shield volcanoes. They are built mainly from lava flows. After an eruption the lava flows out of the vents and creates sloping sides. Their shape is similar to a warrior’s shield. When a shield volcano erupts it is not usually explosive. Sometimes instead of lava flowing from the vents of a shield cone, the lava pours out of long fissures, which is why they are so wide. The most famous shield cone volcanoes are the Hawaiian Islands. Mauna Loa is the world’s largest shield cone volcano and also the most active!
<ul><li>Composite cone volcanoes are sometimes called stratovolcanoes. These volcanoes form from layers of melted rock. They are steep-sided, and cone shaped. Some of the most beautiful mountains in the world are composite volcanoes. This type of volcano usually erupts in a very explosive way, from the viscous magma. The viscous magma gets stuck in the pipes and is trapped. This causes the pressure to increase, which causes an explosive eruption. The composite cone is the most deadly type of volcano. Mt. St. Helens is a very famous example of a composite volcano. </li></ul>
Earth’s Plates <ul><li>The crust, or top layer, of the earth is made up of large sections of rock called plates . These plates are always moving. Sometimes the movement of the plates cause cracks in the earth’s crust. </li></ul>