Volcanoes Chapter 13

Volcanoes & Plate Tectonics Volcanic eruptions can be caused by the movement of tectonic plates Plates are moved by the internal heat of the Earth A volcanic eruption can be more powerful than an atomic bomb

Formation of Magma Magma – liquid rock that forms under Earth’s surface Comes from melted crust and mantle

Volcanism Magma naturally rises to the surface because it is less dense The amount of magma increases because it melts rock along the way

Volcanism Volcanism – any activity that includes the movement of magma onto Earth’s surface Lava – magma that has reached the surface Volcano – the vent in Earth’s surface where magma reaches the surface

Major Volcanic Zones Like earthquakes, volcanoes tend to occur near convergent or divergent plate boundaries

Subduction Zones Where one plate goes beneath another Oceanic crust + oceanic crust = island arc chain Oceanic crust + continental crust = mountain range

Mid-Ocean Ridges The largest amount of magma comes to the surface at mid-ocean ridges Forms under water volcanoes Water quickly cools the lava creating pillow lava

Hot Spots Volcanically active areas in the interior of a tectonic plate The plate moves over the hot spot, causing a volcano to form That volcano eventually moves away and a new one forms

Intrusive Activity Magma that cools and solidifies under the ground (intrusion) Forms plutons Dikes (thin areas) Batholiths (large areas >100 km2) Devil’s Tower Wyoming

Exit Ticket 1. What is the difference between lava and magma? 2. Which of the following is the most accurate definition of volcano: A. Any activity that includes the movement of magma toward or onto Earth’s surface. B. Magma that flows onto Earth’s surface; the rock that forms when lava cools and solidifies C. A mountain of rock that spews hot lava onto Earth’s surface D. A vent or fissure in Earth’s surface through which magma and gases are expelled.

(Use I think… because… form) ! Do Now Create a hypothesis that answers the question: Which liquid will be able to trap the most bubbles when I blow into the liquid through a straw: water, cooking oil or honey? (Use I think… because… form) !

Viscosity A fluids resistance to flow. Magma can have different viscosities, which affect the force with which a particular volcano will erupt.

Volcanic Eruptions 2 types of magma Mafic – magma or rock rich in magnesium and iron; dark colored Felsic - magma or rock rich in silicates; light colored

Types of Eruptions Quiet Eruptions Usually oceanic volcanoes (ex. Hawaii) Lava has a low viscosity (runny) Gases easily escape from the lava Formed from mafic magma

Types of Eruptions Lava Flows Crust may form on top of the lava If the lava keeps flowing, it forms wrinkles called pahoehoe Aa lava – surface breaks into jagged chunks Blocky lava – lava is more viscous (thicker)

Types of Eruptions Explosive Eruptions Usually continental volcanoes (ex. Mt. St. Helens) Lava has a high viscosity (thick & sticky) formed from felsic magma Gases trapped inside lava Pyroclastic material – fragments of rock that form during a volcanic eruption Thrown into the air during an explosive eruption

Pyroclastic Material Volcanic ash Lapilli Volcanic bombs <2mm in diameter Lapilli <64mm in diameter “little stones” Volcanic bombs Form when red hot lava is thrown and cools in the air Volcanic blocks Largest material Pieces of rock blasted from the vent

Quick Check Using the term viscosity, explain why a milkshake is harder to drink with a straw then a can of pop.

Calderas Can form when the magma chamber empties and the cone collapses Lakes can form in the caldera basin Crater Lake (Mount Mazama) Oregon

Predicting Volcanic Eruptions Increase in strength and frequency of earthquakes may be a sign of an eruption Before an eruption, the surface of a volcano may bulge outward from the magma Scientists compare a volcano’s previous behavior to current measurements

Types of Volcanoes The shape of a volcano is related to the composition of its matter. http://www.youtube.com/watch?feature=player_embedded&v=uZp1dNybgfc

Types of Volcanoes Shield volcano Broad base; gently sloping sides Covers wide area Quiet eruption Layers of mafic lava build up cone Ex: Hawaiian Islands

Types of Volcanoes Cinder cones Very steep slopes Rarely less than a few hundred meters high Explosive eruptions Cone made of pyroclastic material Ex: Parícutin, Mexico

Types of Volcanoes Composite volcano Cone made of alternating layers of… lava flows (quiet eruptions) pyroclastic material (explosive eruptions) Form large volcanic mountains Ex: Mt. Hood, Mt. St. Helens, Mt. Rainier, Mt. Fuji

Exit Ticket The depression that results when a volcanic cone collapses over an emptying magma chamber is a: A. Crater B. Caldera C. Vent D. Fissure Compare the three main types of volcanic cones: Shield, Composite, and Cinder-Cone.