Chapter 13 - Volcanoes

1. Chapter 13 - Volcanoes Section 1: Volcanoes and Plate Tectonics

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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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/oceanexplorer.noaa.gov/explorations/04fire/background/volcanism/pillow_lava_220.jpg&imgrefurl=http://oceanservice.noaa.gov/education/yos/multimedia/oceanexplorer.noaa.gov/oceanexplorer.noaa.gov/explorations/06fire/background/volcanism/volcanism.html&usg=__hXQKTNkmPcCI1nilHA5oK0I0iPs=&h=165&w=220&sz=8&hl=en&start=5&tbnid=LW1Sb-8d9jRnKM:&tbnh=80&tbnw=107&prev=/images%3Fq%3Dpillow%2Blava%26gbv%3D2%26hl%3Den

10. 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.

11. 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