Earth Science

Earthquakes

Merrill Chapter 15 Notes

15-1 Volcanoes and Plate Tectonics

 

A volcano is a mountain that forms when layers of lava and volcanic ash erupt and build up.

 

Causes of Volcanic activity:

  1. Magma is less dense than the rock around it, so it is very slowly forced upward toward the Earth’s surface.
  2. Eventually it reaches the Earth’s surface and flow’s out through an opening called a vent.
  3. As lava flows out it cools and hardens, forming layers of igneous rock around the vent. The opening at the top of a volcano’s vent is the crater.

 

Occurrence:

Volcanoes occur at divergent boundaries, at convergent boundaries, and at locations not at plate boundaries called hot spots.

  1. Divergent boundaries: Where plates separate they form long, deep cracks called rifts. Magma flows from rifts and is instantly cooled by the seawater. As more lava flows it builds up from the seafloor. In some cases the eruptions build volcanoes that emerge from the sea- example Iceland.
  2. Convergent boundaries: Magma that is created in the subduction zone is force upward to the surface, forming the volcanoes of the Cascades. The area around the Pacific Plate where earthquakes and volcanoes are common is called the Pacific Ring of Fire.
  3. Hot Spots: Areas in the mantle where magma is hotter then surrounding areas. Melt rock above it and erupts onto the crust. Example are the Hawaiian Islands.

 

 

15-2 Geothermal Energy and Volcanoes

The geothermal energy that is stored in magma could be used to generate electricity. The heat could be used to boil water to produce steam that would turn a turbine in a power plant.

 

15-3 Eruptions and Forms of Volcanoes

Types of Eruptions:

2 important factors that determine whether an eruption will be explosive or quiet.

  1. The amount of water vapor and gases that are trapped in the magma. (Think pop can that is shaken before open and one that is not)
  2. Whether the magma is basaltic or granitic. Basaltic is very fluid and typically produces quiet lava flows. Granitic, with its high silica content, is thick and does not flow well it traps gasses more easily that leads to an explosive eruption.
  3. The more water trapped in the magma/lava the more explosive it it.

 

Three Forms of Volcanoes:

  1. Shield: produced by quiet eruptions of basaltic lava that spread out in flat layers. It has gently sloping sides.
  2. Cinder Cone: formed by explosive eruptions that produces particles of lava that cool quickly (tephra). These build up around the vent to form steep-sided volcanic cones.
  3. Composite: is produced by a cycle of gentle eruptions and violent tephra eruptions. Located primarily near subduction zones.

 

 

15-4 Volcanic Features

 

Intrusive Features:

  1. Batholith: a large underground chamber of magma that cooled before reaching the surface of the earth.
  2. Dike: magma that is squeezed into a vertical crack that cuts across rock layers and hardens.
  3. Sill: magma that is squeezed into a horizontal crack that cuts between rock layers and hardens.
  4. Lacolith: is a dome that is produced when a sill continues to push the layers of rock above it.

 

Surface features:

  1. Volcanic Neck: the remains of a volcano that has been reduced by erosion.
  2. Caldera: a very large opening produced when the top of a volcano collapses into the empty lava chamber below.