Transcript Chapter 14
Chapter 14 BFRB Pages 220-222 VOLCANIC ERUPTIONS • Rocks melt underground where TEMPERATURE & PRESSURE are great • Since the melted rock is slightly less dense then the surrounding rock, it begins to rise to the surface (it may travel through cracks in the rock, or melt its way upward) • When the magma reaches the surface, it may erupt out of the ground through an opening called a volcano Volcanoes take on the characteristic cone shape as lava and ash pile up around the opening in the ground over a long period of time FELSIC MAGMA • Contains a lot of silica • Lighter in color, slow moving and very thick • Moves slowly out of the volcano • This allows pressure from gases to build up (think of shaking up a bottle of soda), causing very violent explosions – very little lava flowing, mostly ash, dust and hot gases Mt. St. Helens was a FELSIC magma eruption VERY VIOLENT and VERY EXPLOSIVE! Animation of a FELSIC volcano erupting MAFIC MAGMA • contains very little silica • magma/lava darker in color, faster moving, and very thin • These volcanoes tend to have less violent eruptions, since the lava easily flows out of the volcano • The eruptions result mostly in fast moving lava flows Mt. Etna – Sicily, Italy Stromboli – see the lights of the city in the background of the “river of lava”? Kilauea – Pahoehoe Lava Flow “Toe” like lava “Rope” like lava Types of Eruptions I Rift Eruptions • occur at narrow cracks (fractures) in the Earth’s crust • can occur on land, or on the ocean floor (at spreading centers – ex. mid-Atlantic Ridge) • contain mafic magma that flows out smoothly and quickly • resulting rock (when the magma/lava cools) is usually the igneous rock basalt Icelandic Rift Eruption Underwater Rift Zones (Mid-Atlantic Ridge to be exact!) Ancient Rift Zone – note the BASALT on either side! II Subduction Boundary Eruptions • occur when magma is formed by the melting rock at subduction plate boundaries • magma tends to felsic, which makes it very thick – causing violent eruptions • Most of the world’s active volcanoes occur along subduction plate boundaries Chain of Subduction Boundary Volcanoes along the Andes Mountains in South America (as viewed from space – note that the white areas are snow peaks) The Cascade “Mountain” Range – in reality a range of “VOLCANOES”! III Hot Spot Eruptions • occur in the middle of tectonic plates where there is a concentration of heat • magma melts its way through the plate and pours out onto the surface • magma is usually mafic, so it easily flows out of the ground • the hot spot remains in one location, as the plate moves above it • results in a chain of volcanoes • the only active volcano will be the one above the hot spot • the direction of the chain indicates the direction the plate is moving • if the hot spot is on the ocean floor, a chain of volcanic islands will form (ex. Hawaiin Islands chain) The next Hawaiian island that will come into being - already named Loihi - is building on the seafloor southeast of Kilauea. It’s top is 1000 m (3000 feet) below the water surface, and it will break the surface in the next 10,000 to 100,000 years. The shoreline is the boundary between yellow and green and is labeled. Purples are the deepest water. Light reds are the highest peaks on the island. Direction of Plate movement as indicated by the Hawaiian Hot Spot Note that the elevation of “Hawaii” is greater because of the hot spot supplying new rock. The older islands are “sinking” into the ocean because the rate of erosion supercedes the rate of formation (which is zero because they are off the hot spot) Galapagos Islands off the coast of western South America are an another example of Hot Spot island formation. See the volcanic “mountain peaks”? CALDERAS • an erupting volcano empties a shallowlevel magma chamber & the edifice of the volcano collapses into the voided reservoir, thus forming a steep, bowlshaped depression called a caldera Ancient volcanic opening Crater Lake, Oregon is one of the USA’s most famous Calderas. It is part of the Cascade volcanic mountain range. With a water depth of 600 m, Crater Lake is the deepest fresh-water lake in North America. This large depression formed from the violent eruption and collapse of the ancestral volcano Mt. Mazama about 6850 years ago! Yellowstone Caldera • Yellowstone National Park sits on a high plateau supported by a hot mantle plume. This hot-spot has been the site of three caldera-forming eruptions in the last two million years. The last eruption was ~630,000 years ago and the tephra (ash)fell as far away as Louisiana! • This hot mantle plume is what fuels all of the hot springs and geysers in the park – including Old Faithful. Krakatoa,Indonesia (1883) • In 1883, after over 200 years of calm, the massive volcano under the island of Krakatoa exploded in one of the worst eruptions in modern history. • 2/3 of the island exploded and sank into the caldera formed • Over 36,000 people died and over 150 coastal cities were completed destroyed by the Tsunami that was created by the violent eruption Devil’s Tower Revisited The Legend of Devil's Tower Devil's Tower, Wyoming, is a volcanic plug or neck. In Kiowa Indian mythology it was said that: Once upon a time seven little girls were playing in the woods far from home and they came upon some bears that chased after them. They found refuge on a great rock, that rose into sky with them on it, making them into stars. The bears tried to pursue them, but all their efforts were in vain. You can still see the struggle and their claw marks in the rock that makes up Devil's Tower.