Transcript Volcanoes
Volcanoes The Global Distribution of Volcanoes Mid-Ocean Ridges • mid ocean ridges • The following video talks about how midocean ridges form volcanoes, and “repave” the Earth’s surface. I. Volcanoes Beneath the Sea • A. Evidence of volcanic rocks on the oceans floor indicate there are more volcanoes under water than on land. • B. Mid-Ocean Ridge – A continuous mountain range extending through the North and South Atlantic Oceans, the Indian Ocean, and the South Pacific Ocean. I. Volcanoes Beneath the Sea (cont’d) • C. Rift Valley – The deep central cleft in the mid-oceanic ridge. • D. Magma – Naturally occurring molten rock material generated within the Earth. • i. The magma rises and forms submarine (underwater) volcanoes. • ii. Some places along the mid-ocean ridges have so much volcanic activity, they can form islands. II. Volcanoes on Land • A. The Cascades mountain range along the western U.S. are formed by volcanoes. • B. The “Ring of Fire” is a notable belt of volcanoes that is spread throughout the Pacific Ocean. • i. The Pacific plate is sliding beneath continental plates and that forms the “Ring of Fire.” These plates are called convergent plates. II. Volcanoes on Land (cont’d) • ii. When a plate travels downward, it creates heat. Water escapes the melting rock, and eventually magma is formed. The magma rises because it is less dense than the existing rock. • iii. 4/5’s of volcanoes on land are formed when one plate slides under another. III. Volcanoes Formed by Rifting on the Continents • A. Some volcanoes (like the ones in Eastern Africa) are because the plates are pulling away from each other. This is called a divergent plate. • i. The plate is stretched and broken, and it opens up a rift valley. This rift valley allows magma to escape. III. Volcanoes Formed by Rifting on the Continents (cont’d) • B. There is evidence of continental rifting along the Eastern U.S. too. IV. Volcanoes at Hot Spots A. Hot spot – A fixed source of abundant rising magma that forms a volcanic center that has persisted for tens of millions of years. B. The Hawaiian Islands are evidence of hot spot volcanoes. Convergent Plate Boundaries Convergent Plate Boundaries Divergent Plate Boundaries • Most are at mid-ocean ridges • Mantle melts as pressure is reduced – Melt is basaltic in composition • Cooling basalt forms new oceanic crust Activity 2 Volcanic Landforms • 1. Topography of Volcanic Regions • i. Contour lines - A line on a map that connects points of equal elevation of the land surface. • a. Contour lines never cross. • b. The closer together the contour lines, the steeper the slope. • Details and pictures of contour lines Topographic Maps • Topographic map - A map showing the topographic features of the land surface. Volcanoes • National Geographic volcanoes II. Magma Composition • A. Magma is a mixture of liquid, melted rock, and dissolved gases. • B. The most abundant chemical elements in magma are silicon and oxygen. • C. Silicon and oxygen are the building blocks of most of the common minerals. • Lava Flows and Pyroclasts II. Magma Composition (cont’d) • D. Silica - the combination of the most common elements, oxygen and silicon, that are found in a volcano. The “oxide” is SiO2. • a. More silica means more viscous, or more of a violent explosion. • b. High silica will not form as much lava as low silica, but it probably will cause the pyroclastic flow to be huge. III. Types of Volcanoes • A. Composition Cone - A volcano that is tall and has steep slopes because the lava does not flow easily. • a. A volcano that is constructed of alternating layers of lava and pyroclastic deposits. • B. Shield Volcano - The are generally the largest in the world. They are not that tall as the lava slowly moves out of them. III. Types of Volcanoes (Cont’d) • A. Shield volcanoes are not explosive.