Transcript volcanoes
VOLCANOES CHAPTER 7 7.1 VOLCANOES AND PLATE TECTONICS Section Objectives: • Describe the formation and movement of magma • Define volcanism • List three locations where volcanism occurs I. VOLCANISM 1. Volcanism is any activity that includes the movement of magma toward or onto the surface of the earth. 2. Asthenosphere is very hot but stays mostly solid because of the great pressure of the surrounding rock. 3. Magma forms where decreased pressure allows the rock to melt 4. Lower density of magma compared to the surrounding rock and allows it to rise toward the surface. I. VOLCANISM • Lava: Magma that reaches Earth’s surface • Vent: The opening through which the molten rock flows onto the surface • Volcano: The structure that is formed by the vent and volcanic material that builds up on Earth’s surface around the vent volcano lava vent cone II. MAJOR VOLCANIC ZONES Three major locations where volcanism occurs: 1. Subduction Zone 2. Mid-ocean Ridge 3. Hot Spot Subduction Zone Mid Ocean Ridge Hot Spot VOLCANIC ERUPTIONS Section Objectives: • Summarize the relationship between lava types and the force of volcanic eruptions. • Describe the major types of pyroclastic material. • Identify the three main types of volcanic cones. • Summarize the events that may signal a volcanic eruption. VOLCANIC ERUPTIONS Type of Lava Color: Rich in: Commonly found in: mafic dark iron and magnesium oceanic crust felsic light silica continental crust VOLCANIC ERUPTIONS Mafic lava textures: pahoehoe: “ropy”; forms as lava first forms a crust aa: jagged chunks; forms when crust is too thick pillow lava: lava that cools very quickly, usually underwater. Pahoehoe lava Aa lava Pillow lava KINDS OF ERUPTIONS Type of Eruption Lava Characteristics Where Found Type of Lava "quiet" hot, thin, flows easily; gases escape easily mid-ocean ridges; hotspots mafic cooler, thicker, lots subduction zones of trapped gases felsic explosive KINDS OF ERUPTIONS FOR EACH TYPE OF LAVA “Quiet” eruptions: Mafic lavas flow more easily gases escape easily lava flows out onto the surface Explosive eruptions: Felsic lava is thicker gasses cannot escape easily gas pressure builds up and creates a more violent eruption pyroclastic material is thrown into the air. VOLCANIC ROCK FRAGMENTS Pyroclastic Material • Associated mainly with felsic eruptions because they are explosive. • Dust: less than .25 mm in diameter; can travel completely around the Earth in the upper atmosphere • Ash: less than 2 mm diameter • Lapilli: less than 64 mm diameter • Blocks: larger than 64 mm; some as big as a house • Bombs: formed when lava is thrown into the air and solidifies while in the air VOLCANIC CONES NAME SIZE LAVA TYPE MADE UP OF SLOPE TYPE OF ERUPTION LOCATION shield largest mafic lava unsteepest "quiet" hot spot smallest mafic and felsic pyroclastic fragments steepest explosive hot spot, subduction zone, midocean ridge felsic lava and pyroclastic fragments medium explosive subduction zone cinder cone composite middle Shield Volcano Here are 4 of the volcanoes that make up the big island of Hawai'i. They are Mauna Kea (MK), Mauna Loa (ML), Hualalai (H), and Kohala (K). The photo was taken from near the summit of East Maui volcano (EM). These are the largest volcanoes on Earth Cinder Cone Composite Cone Popocatepetl Mt. Merapi, Indonesia Fujiyama, Japan Mt. Mt. St St.Mt. Helens, Helens, Shasta, Washington Washington California Mt. Shasta, California VOLCANIC FEATURES craters and calderas • crater = the funnel shaped pit at the top of a volcanic vents • caldera = when the magma chamber below a volcano is emptied, the volcanic cone may collapse leaving, a large basin shaped depression VOLCANIC FEATURES crater VOLCANIC FEATURES caldera Caldera Formation What controls eruptions? The amount of trapped gas When more gas is trapped, pressure builds up MORE PRESSURE MAKES A MORE VIOLENT ERUPTION The amount of silica in the magma Silica makes the magma thicker Thicker magma does not flow easily so can get stuck underground, allowing pressure to build up THICKER MAGMA MAKES A MORE VIOLENT ERUPTION Predicting Volcanic Eruptions • Increase in the number and strength of small earthquakes (magma working its way upward) • Slight bulging of the surface of a volcano (upward movement of magma just below the surface) • Information from previous eruptions 7.3 EXTRATERRESTRIAL VOLCANISM • Mars – volcanic cones on Mars – most famous is Olympus Mons – Mars seems to be seismically active • Io – one of Jupiter’s moons – has active volcanoes Fig. 22.9 Fig. 22.10c Fig. 22.10a Fig. 22.11b Fig. 22.1_4 Fig. 22.17 Olympus Mons Io Io Fig. 22.1_2 Fig. 22.14b FAMOUS ERUPTIONS • • • • • • ELDFELL MOUNT ST. HELENS KILAUEA VESUVIUS MOUNT PINATUBO CRATER LAKE