1. Volcanoes
Earth Science Chapter 10

2. 10.1 Volcanoes and Plate Tectonics
Magma
Molten rock beneath the earth’s surface
Contains minerals, dissolved gases, and water
Forms in the crust and upper mantle (lithosphere)
Formation depends on heat, pressure, and water content

3. Heat
Caused by friction between plates, the mantle, the core, and radioactive decay
Pressure
As rock rises through the mantle it melts due to decreased pressure
Water content
As water content increases, the melting temperature decreases which means that the rock will melt at lower temperatures.

4. Volcanoes and Plate Boundaries
Most volcanoes form at either convergent or divergent plate boundaries
Divergent Boundaries
Plates pull apart (seafloor spreading)
Magma from the mantle rises to fill the gap
Rift volcanoes (fissure)
Convergent Boundaries
Oceanic crust is subducted into the mantle
Melting occurs due to increased temperature and water content
Magma moves up through cracks forming volcanoes.

6. Oceanic – Oceanic Convergent Boundaries
The more dense oceanic crust is subducted beneath the less dense oceanic crust
Volcanoes on the ocean floor
Volcanic island arcs
Oceanic – Continental Convergent Boundaries
Oceanic crust is subducted under continental crust because it is more dense
Some of the oceanic crust is melted and rises to the surface
Continental volcanic arcs form near the coasts of continents

8. The Pacific Ring of Fire is a long belt of volcanoes that circle the Pacific Ocean
The ring is composed entirely of convergent boundaries.

10. Interplate Volcanism
Volcanic activity that occurs within a plate
Occurs where a mantle plume rises to the surface
Plume: Hot mantle material
Plumes that rise to the surface form hot spots
Hot spot: areas of volcanic activity within plates
The plate moves over the plume. The plume doesn’t move.
Examples include Hawaii and Yellowstone Nat’l Park

14. 10.2 Nature of Volcanic Eruptions
Factors that affect eruptions
Explosive eruptions
High viscosity
Less water
High silica content
Granitic or andesitic lava
More trapped gases such as water vapor and CO2
Occurs on continents

15. Quiet eruptions
Low viscosity
More water
Less silica, more magnesium
Basaltic lava
Less trapped gases that are released easily
Occur at oceanic volcanoes

16. Volcanic Material
Basaltic flows from oceanic volcanoes are very fluid because of low silica, high water content
Pahoehoe
fast moving
smooth wrinkled appearance Aa
slow moving
rough, jagged, and sharp
Granitic flows from continental volcanoes are very slow because of high silica content

18. The majority of dissolved gases in lava:
Water vapor
CO2
Sulfur dioxide
Nitrogen
Pyroclastic material is the solid particles produced in a volcanic eruption (smallest to largest)
Dust
Ash
Lapilli
Bombs
Blocks

21. Parts of a Volcano Crater: depression at the top
Vent: opening to the surface
Caldera: large depression that occurs when a magma chamber collapses
Chamber: location where magma is stored
The volcano can be composed of lava flows, pyroclastic material, or both

23. Types of Volcanoes
Shield
Form from basaltic lava flows
Form on oceanic crust
Very broad with gradual sides
Hawaii
Cinder cones
Form from pyroclastic material
Steep sides
Small
Form from a single violent eruption
Form on continental crust

24. Composite cones (stratovolcanoes)
Most dangerous
Large
Contain lava and pyroclastic material
Form from explosive eruptions
Most are found around the Pacific Ring of Fire
Cascades of the US

28. Predicting an Eruption
The most important warning signal when predicting a volcanic eruption is small earthquakes.
The number of earthquakes often increases just before an eruption.
Another signal of volcanic activity is a slight bulging of the surface above the volcano.

29. 10.3 Intrusive Igneous Activity
Plutons
structures that form from the cooling and hardening of magma below the earth’s surface
Form intrusive igneous rock
Form in continental crust
Types of Plutons
Sills: lie parallel to layers of rock
Dikes: cut across layers of rock
Lacoliths: small dome shaped pluton
Batholiths: large dome shaped pluton