Chapter 13 Volcanoes Section 1 Volcanoes and Plate

Chapter 13 - Volcanoes Section 1: Volcanoes and Plate Tectonics

Section 1 Objectives � Describe three condition under which magma can form. � Explain what volcanism is. � Identify three tectonic settings where volcanoes form. � Describe how magma can form plutons.

What causes volcanoes? � The cause of many volcanic eruptions is the movement of tectonic plates › The movement of these plates is driven by Earth’s internal heat

Formation of Magma � Mantle is mostly solid due to high pressure, even though high temps. › Solid mantle that melts and becomes liquid rock is known as magma. � Can form under 3 conditions �Temp of rock rises above melting pt. �Pressure is reduce and melting pt. lowers �Addition of fluids may decrease melting pt. of some minerals in the rock, causing rock to melt �Fig 1, p. 319

Volcanism � Any activity that includes the movement of magma toward or onto Earth’s surface � Bodies of magma rise to the surface in 2 ways › Magma is hot enough to melts surrounding rock › Magma rises, is forced into cracks, and causes large blocks to break off and melt. This adds to the magma body

Volcanism � Lava – magma that flows onto Earth’s surface; the rock that forms when lava cools and solidifies › May flow out of a vent, cool, and build up a cone of material that may become a mountain. � Volcano – a vent or fissure in Earth’s surface through which magma and gases are expelled

Major Volcanic zones � Fig 2 – map of active volcano locations � Most occur at or near both convergent and divergent plate boundaries � Pacific Ocean – Ring of Fire › Also a major earthquake zone

Subduction Zones � Area where one tectonic plate moves under another › Oceanic plates are more dense than continental plates are usually subducted › A deep trench forms on ocean floor along continent boundary › Continental plate forms Mtns as it is folded and pushed back › Subducted plate is melted to form magma › May form volcano chain › 2 oceanic plates – island arc may form

Mid-Ocean Ridges � Divergent plate boundaries – plates are pulling away › area where largest amount of magma rises to surface › Pillow lava › Fissures – cracks through which lava flows to Earth’s surface http: //images. google. com/imgres? imgurl=http: //oceanservice. noaa. gov/education/yos/multimedia/oceanexplorer. noaa. gov/oce anexplorer. noaa. gov/explorations/04 fire/background/volcanism/pillow_lava_220. jpg&imgrefurl=http: //oceanservice. noaa. gov/e ducation/yos/multimedia/oceanexplorer. noaa. gov/explorations/06 fire/background/volcanism/volcanis m. html&usg=__h. XQKTNkm. Pc. CI 1 nil. HA 5 o. K 0 I 0 i. Ps=&h=165&w=220&sz=8&hl=en&start=5&tbnid=LW 1 Sb 8 d 9 j. Rn. KM: &tbnh=80&tbnw=107&prev=/images%3 Fq%3 Dpillow%2 Blava%26 gbv%3 D 2%26 hl%3 Den

Hot Spots �A volcanically active area of Earth’s surface, commonly far from a tectonic plate boundary � Mantle plumes – columns of hot, solid material rise and reach the lithospere › Appear to remain stationary, even though lithospheric plates drifs above the plumes. � Theory that states hot spots result from cracks in Earth’s crust.

Intrusive Activity � Magma is not dense, therefore it rises through or intrudes the overlaying layer of rock. › Magma may fracture or melt the surrounding rock. › Rock that is melted may cool again into igneous rock. � Magma may cool and solidify within the crust, resulting in many large plutons › Dikes – small plutons that are tabular in shape › Batholiths – large plutons that cover a lot of area

Chapter 13 - Volcanoes Section 2: Volcanic Eruptions

Section 2 Objectives � Explain how the composition of magma affects volcanic eruptions and lava flow. � Describe the five major types of pyroclastic material. � Identify the three main types of volcanic cones. � Describe how a caldera forms. � List three events that may signal a volcanic eruptions.

Windows to Earth’s Interior � 2 › general types of lava Mafic: magma or rock rich in Mg and Fe; commonly dark in color �Commonly makes up oceanic crust �May also make up continental crust › Felsic: magma or rock that is rich in lightcolored silicate materials �Makes up continental crust

Types of Eruptions � Viscosity – the resistance of a liquid to flow; determined by magma’s composition �This can affect the force with which a volcano may erupt �Mafic magmas are runny with low viscosity and typically cause QUIET eruptions (typically oceanic volcanoes – Hawaii) �Felsic magmas are very sticky and therefore highly viscous. �They can trap large amounts of dissolved gases, which leads to more EXPLOSIVE eruptions.

Lava Flows � Pahoehoe – mafic flows that cool & form a crust, continues to flow causing wrinkles in crust; “ropy” lava flows � Aa – forms if crust deforms rapidly or grows too thick to wrinkle, and surface breaks into jagged chunks. › Same composition but different textures due to gas content, rate and slope of flow � Blocky – Higher silica and is more viscous, forming large blocks of volcanic rock when cooled

Explosive Eruptions � Felsic lavas of continental volcanoes such as Mount St. Helens tend to be heavier and stickier (more viscous) � Contain large amounts of trapped gases, such as carbon dioxide and water vapor � During an eruption, gases escape and molten and solid particles are thrown into the air › Pyroclastic material: fragments of rock that form during a volcanic eruption

Types of Pyroclastic Material � Particles are classified according to size �Volcanic ash: particles < 2 mm in diameter �Volcanic dust : particles < 0. 25 mm in diameter �Lapilli: “little stones” < 64 mm in diameter �Generally fall near the vent �Volcanic bombs: clumps of red-hot lava that spin and cool in the air; round or spindle �Volcanic blocks: solid rock blasted from the vent; may be as large as a small house

Types of Volcanoes � Table 1, page 328 � Lava &/or pyroclastic material ejected during both quiet &/or explosive eruptions build up around the vent � Forms 3 Types of Volcanic Cones › Shield volcanoes: Broad at base with gentle sloping sides; quiet eruptions; mafic lava › Cinder cones: Very steep slope; explosive eruptions with cones of pyroclastic material › Composite volcanoes: alt. layers of hardened lava flows and pyroclastic material; a. k. a stratovolcanoes; lg. volcanic mtns.

Calderas � Large circular or basin-shaped depressions in the ground › Form when the magma chamber below a volcanic cone collapses › Figure 4, page 329 › Eruptions discharge large amounts of magma can cause a magma chamber to collapse › Krakatau: volcanic island in Indonesia exploded in 1883 forming a caldera with diameter of 6 km › May fill with water to form lakes

Predicting Volcanic Eruptions � Earthquake activity may signal a volcanic eruption before it occurs › Due to increased pressure, increased temps, or fracturing of surrounding rock › Also increased strength and frequency of quakes � Patterns › › in Activity Sides of volcano may bulge as magma moves Instruments measure tilt of ground on slopes Knowledge of previous eruptions Daily measurements vs. past measurements