Earthquakes Volcanoes Earthquakes, volcanoes and plate tectonics

 Elastic Rebound: when rocks are put stress they may break causing elastic rebound  Rocks change shape or deform slowly over a long period of time  Energy is released and vibrations move through rock which is known as earthquakes

 The surface of a break where rocks move as a result of elastic rebound is called a fault  Faults can be either normal, reverse or strike-slip depending on whether they are pulled apart, move together, or move past each other (shear forces)

 Earthquakes release waves and are transmitted through the earth  These waves are called seismic waves

 When the potential energy is released from strained rocks seismic waves are released  The point at which the the energy is released is called the focus  The point above the earthquake focus (at the surface) is called the epicenter

 These seismic waves travel from focus throughout the earth’s interior(3 waves released)  P-waves or primary waves are the fastest waves to move through the interior  They move back and forth in the same direction as the waves are moving  S-waves or secondary waves move at a slower pace and move in an up and down pattern at right angles to the direction of the wave  L-waves or surface waves arrives last and cause the most damage for buildings and structures at the surface (at surface they move side-to-side)

 Seismograph is an instrument that records seismic waves  Seismic waves are recorded by a pen on a pendulum that increases a line in a graph according to the magnitude of the earthquake

 Move all heavy objects to lower shelves  Secure your gas hot-water heater and appliances  Seismic safe structures assured by building codes helps to build structures so they can ride out vibrations of earthquakes  Steel and rubber supports help riding out the vibrations  Spiral re-enforcment rods on cement pillars help keep bridges and buildings up during earthquakes

 Small seismic activities are better than no seismic activity in an earthquake prone area  If an earthquake prone area has no earthquake activity it could lead up to one large slippage where damage at surface is great

 How do volcanoes form  Rising magma, solids, and gases spew out onto earth’s surface to form cone-shaped mountains are called volcanoes  Magma (molten rock) that reaches the surface through vents is called lava  Volcanoes have circular holes near their summits are called craters  Tephra are bits of rock or solidified lava dropped from the air (could be ash, cinders, or larger rocks called bombs or blocks)

 Volcanic islands form when oceanic crust and mantle collides  Older denser oceanic crust subducts or sinks beneath less dense mantle, where it melts forming a magma pool and rises to form volcanic islands  Pyroclastic flows are massive avalanches of hot glowing rock flowing on a cushion of intensely hot gases

 Composition of magma influences how destructive a volcano can be  The more silica in the magma, the thicker the magma and more chance that it will have a violent eruption  Iron and magnesium rich magma is more fluid and erupts quietly (low silica content)  Water vapor and gases trapped in magma by silica rich magma leads to violent eruptions

 Basaltic lava (high in iron and magnesuim and low in silica) flow in broad flat layers  Shield volcanoes have broad bases with gently sloping sides  Hawaiian islands are an example

 Lower altitude volcanic mountains that form as a result of layers volcanic ash, lava, and cinders (usually less than 300 m in height)  Moderate to violent eruptions occur  Gases are important to formation of cinder cone volcanoes

 Steep sided mountains composed of alternating layers of lava and tephra  Erupt violently releasing large quantities of ash and gas  Then lava layers flow in between the tephra layers (mountains formed because of subduction zones and magma rise to surface)  Cascade Mountains are composite volcanoes

 Very fluid magma can ooze from cracks or fissures in earth’s crust  Low viscosity of lava allows it to flow like water across the surface  Flood basalts form lava plateau like the Columbia River Basalt flows  Built up in some areas 2 miles thick extending from Canada to California to Wyoming

 Where volcanoes form: most volcanoes form along plate boundaries  Divergent plate boundaries: Where plates move apart long cracks form (rifts)  When plates move apart, stress is placed on the crust that allows cracks to form where magma rises to those weaken fractures  Fissures are formed where magma flows as lava reaches the surface (primarily at rifts)  Basalt is most common rock at rifts zones

 Dense oceanic crust dives under continental crust at convergent boundaries  When one plate dives under another plate, basalt and sediment are carried deep under earth’s surface  The material eventually melt and rises through weakness and cracks above to the surface  Pacific rim volcanoes like the Cascade Mts are examples

 There are areas on earth’s surface that is hotter than others where there is a pool of magma below  Plates moves over these hot spots and allows the magma to rise and reach the surface  Volcanoes are formed above these hot spots  Hawaiian islands form in the middle of convergent and divergent zones

 80% of earthquakes occur along the Pacific Rim of Fire  Earthquakes are a result of pressure and stress built up by moving plates at convergent and divergent boundaries