Earthquakes & Volcanoes
Geology - the study of solid Earth, the rocks of which it is composed, and the processes by which they change geo = Earth; ology = study of
Earth s Layers
Plate Tectonics - the theory that Earth's crust is divided into several plates that glide over the mantle
the movement of the tectonic plates is driven by convection currents in the mantle *
Stress - a force that acts on rock to change its shape or volume
Tension - stress that stretches rock so that it becomes thinner in the middle occurs where two plates are moving apart
Compression - stress that squeezes rock until it folds or breaks occurs when one plate is pushing against another
Shearing - stress that pushes masses of rock in opposite directions, in a sideways movement occurs as one mass of rock slides past another
Fault - a break in the earth s crust where masses of rock slide past each other *
Normal Fault - a type of fault where the hanging wall slides downward caused by tension
Reverse Fault - a type of fault where the hanging wall slides upward caused by compression
Strike-Slip Fault - a type of fault in which rocks on either side move past each other with little up or down motion; ex. San Andreas fault caused by shearing
Fault-Block Mountains are created when two normal faults, parallel to each other, cut through a block of rock causing the middle block to rise and the surrounding rock to fall; ex. our own Wasatch Mountains
Plateau - is formed when a large, flat block of rock is uplifted plateaus are wider than they are tall
Folding - compression shortens and thickens part of the Earth s crust folding produced some of the world s largest mountain ranges; ex. the Himalayas and the Alps
Earthquake - the shaking and trembling that results from the movement of rock beneath Earth s surface
plate movement produces stress in the Earth s crust stress increases until the rock breaks releasing all of the stored energy and forming an earthquake
Focus - the area beneath Earth s surface where rock that is under stress breaks, triggering an earthquake
Epicenter - the point on the Earth s surface directly above the focus
In an earthquake, energy is sent outwards from the focus in all directions in the form of seismic waves 3 types of seismic waves
Primary Waves (P-waves) first waves to reach a seismograph station compress and expand the ground like an accordion can travel through solids and liquids
Secondary Waves (S-waves) arrive at a seismograph station after P waves vibrate from side to side as well as up and down can travel through solids, but not liquids *
Surface Waves formed when P-waves and S-waves reach the surface slowest type of waves can cause severe damage on the surface make the ground move with a wavelike motion or from side to side
Scientists measure earthquakes using instruments called seismographs Seismographs record the strength and the time of arrival of the different waves
Information from at least 3 seismograph stations is combined to triangulate the exact epicenter of the earthquake
Scientists rate the magnitude (strength) of an earthquake by using one of three scales
Mercalli Scale - rates earthquakes based upon the level of damage at a given place the same earthquake can have different Mercalli ratings
Richter Scale - rates an earthquake s magnitude based upon the size of its seismic waves (as measured by a seismograph) works well for small, nearby earthquakes but not as well for large or distant quakes
Moment Magnitude Scale - rates earthquakes in terms of the total amount of energy released use data from seismographs to infer how much the fault moved and the strength of the rock that broke most used today
A one point increase in magnitude (5 to 6) would be 32 times stronger A two point increase would be over 1,000 times stronger Magnitude Earthquake Effects Estimated Number Each Year 2.5 or less 2.5 to 5.4 5.5 to 6.0 6.1 to 6.9 7.0 to 7.9 8.0 or greater Usually not felt, but can be recorded by seismograph. Often felt, but only causes minor damage. Slight damage to buildings and other structures. May cause a lot of damage in very populated areas. Major earthquake. Serious damage. Great earthquake. Can totally destroy communities near the epicenter. 900,000 30,000 500 100 20 One every 5 to 10 years
3 recorded earthquakes with magnitudes over 9 Chile (1960) 9.5
3 recorded earthquakes with magnitudes over 9 Chile (1960) 9.5 Alaska (1964) 9.2
3 recorded earthquakes with magnitudes over 9 Chile (1960) 9.5 Alaska (1964) 9.2 Indian Ocean (2004) 9.0
Earthquakes cause damage through shaking, liquefaction, aftershocks, and tsunamis
Shaking can cause landslides, avalanches, and damage to structures loose soil shakes more than solid rock
Liquefaction - violent shaking suddenly turns soft soil into liquid mud in areas that have lots of moisture in the soil buildings sink and pull apart
Aftershocks - earthquakes which occur after a larger earthquake in the same area weakened buildings fall can occur hours, days, or months later
Tsunami (tidal wave) - large wave produced by an earthquake on the ocean floor spreads out from an earthquake in all directions wave height is low in the open ocean, but increases as the wave approaches the shore
2004 Indian Ocean earthquake and tsunami killed over 283,000 people
Volcano - a weak spot in the crust where magma comes to the surface
Magma - a molten mixture of rock-forming substances, gases, and water from the mantle
when magma reaches the surface, it is called lava
Many volcanoes form along the boundaries of tectonic plates The Ring of Fire is a major belt of volcanoes that surrounds the Pacific Ocean
Divergent plate boundaries - two plates move away from each other Most often occur along mid-ocean ridges (sea-floor spreading)
Convergent plate boundaries - one tectonic plate collides with another plate Subduction occurs at convergent boundaries (one plate goes under the other)
oceanic plate under oceanic plate - creates an island arc (string of islands) Japan, New Zealand, Indonesia, Caribbean Islands oceanic plate under continental plate Andes Mountains, Pacific Northwest
Hot spots - areas where magma from deep within the mantle melts through the crust above it Hawaiian Islands, Yellowstone National Park
Types of Volcanic Eruptions Quiet eruptions - magma has a low viscosity and flows easily (Hawaiian Islands)
Explosive eruptions - magma has a high viscosity and does not flow easily; the magma can form a plug causing pressure to build in the volcano until it finally erupts (Mt. Saint Helens)
Pyroclastic flow - the expulsion of ash, rocks, and hot gases during an explosive eruption
Volcanoes can be active (erupting or showing signs that it may erupt in the near future),
dormant (not actively erupting, but will eventually awaken and become active),
or extinct (dead, unlikely to erupt again)