Chapter 5 Lesson 3 Volcanoes Vocabulary 1. Volcano 2. Lava 3. Shield volcano 4. Cinder-cone volcano 5. Composite volcano 6. Island chain 7. Hot spot 8. Island arc NOTES Where are volcanoes found? Volcanoes form on land and on the ocean floor. Found on certain places on Earth’s surface. Most are found where plates meet. Ring of Fire-circle of volcanoes surrounding the Pacific Ocean. More likely to erupt at plate boundaries than anywhere else on Earth. An eruption is an outpouring of melted rock, ash, gases, or a combination of these. Scientists concluded that volcanoes tend to erupt where one plate is pushed under another plate. When rocks in the plate that is being pushed down reach the heat and pressure in the mantle, they melt. Magma forms and pools in a chamber underneath the crust. Magma may rest quietly for 100s or 1000s of years. A crack forms above the chamber or the pressure in the chamber grows too great to be held in by the rock above it. The magma rushes toward the surface. More than 80% of Earth’s volcanic activity occurs on the ocean floor. All volcanoes have at least one vent, or opening. Lava, ashes, and gases erupt through vents. A cup-shaped depression called a crater may form around the vent. o This is usually found at the top of a volcanic mountain. Sometimes the magma chamber beneath a volcano is emptied. The volcano may then collapse inside itself. The hole that forms is called a caldera. How do volcanoes build land? Sometimes magma cools and hardens before it reaches the surface. A dike forms when magma hardens in vertical or nearly vertical cracks. A sill forms when magma hardens between horizontal layers of rock. Sometimes the magma pushed into a sill does not spread horizontally, but pushes upward. o It forms a dome shape called a laccolith. o Laccolith may raise the rock layers above it when formed. The largest and deepest of all underground magma formations are batholiths. o Batholiths are huge and irregularly shaped. o Reaches deep into the crust. When lava comes out of a vent, it is liquid. Lava forms a solid layer of rock as it hardens. o Layers increase the height of a volcano and form a volcanic mountain. Active Volcano-currently erupting or has erupted. Dormant Volcano-doesn’t erupt for a long time. Extinct Volcano-volcano that stops erupting or is dead. Active volcanoes differ in how they form and in the shapes of the mountains they build. o Shield volcano, cinder-cone volcano, and composite volcano. (SEE VOCABULARY) How do volcanoes build islands? The Hawaiian Islands are an island chain. They rest on a slowly moving tectonic plate. As it moves, the plate passes over a stationary pool of magma call a hot spot. Lava erupting from the hot spot formed a mountain that eventually becomes a volcanic island. As the plate moved, that island moved away from the hot spot and a new island began to form. In areas where an ocean-floor plate is pushed under another ocean-floor plate an island arc is formed. o As the plate is pushed down it melts. o Magma forms, rises upward, and erupts through the ocean floor. o The eruptions form a series of volcanic islands along the plate boundary. o The Aleutian Islands in Alaska form an island arc.