EarthquakesVolcanoesEarthquakes, 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 boundariesWhen 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