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Chapter 8 NEW BOOK



o   *Earthquake-the shaking and trembling that results from the movement of rock beneath Earth’s surface.

o   *Stress-a force that acts on rock to change its shape or volume.

o   *Shearing, tension, and compression work over millions of years to change the shape and volume of rock.

o   *Shearing-the stress that pushes a mass of rock in two opposite directions.

o   *Tension-the stress force that pulls the crust, stretching rock so that it becomes thinner in the middle.

o   *Compression-the stress force that squeezes rock until it folds or breaks.

o   *Deformation-any change in the volume or shape of Earth’s crust.

o   *Fault-a break in Earth’s crust where slabs of crust slip past each other.

o   *Faults usually occur along plate boundaries, where the forces of plate motion compress, pull, or shear the curst so much that the crust breaks.

o   *Strike-slip fault-the rocks on either side of the fault slip past each other sideways with little up-or-down motion.

o   *Normal Fault-The fault is at an angle, so one block of rock lies above the fault while the other block lies below the fault.

o   Hanging wall-The half of the fault that lies above.

o   Footwall-The half of the fault that lies below.

o   Reverse Fault-Has the same structure as a normal fault, but the blocks move in opposite direction.

o   Folds-bends in rock that form when compression shortens and thickens part of Earth’s crust.

o   Plateau- a large area of flat land elevated high above sea level.


o   Focus-The point beneath Earth’s surface where rock that is under stress breaks, triggering an earthquake.

o   Epicenter-The point on the surface directly above the focus.

o   Seismic waves carry the energy of an earthquake away from the focus, through Earth’s interior, and across the surface.

o   There are three categories of seismic waves: P waves, S waves, and surfaces waves.

o   P waves-Earthquake waves that compress and expand the ground like an accordion.

o   S waves- Earthquake waves that vibrate from side to side as well as up and down.

o   Surface waves-When P waves and S waves reach the surface, some of them are transformed to surface waves. These move more slowly than P waves and S waves, but they produce the most severe ground movements.

o   Seismograph- records the ground movements caused by seismic waves as they move through the Earth.

o   Magnitude- a measurement of earthquake strength based on seismic waves and movement along faults.

o   Mercalli scale- was developed in the 20th century to rate earthquakes according to their intensity.

o   Richter scale – another type of scale to rate the intensity of earthquakes

o   The severe shaking produced by seismic waves can damage buildings and bridges, topple utility polls, and fracture gas and water mains.


o   Aftershock- An earthquake that occurs after a larger earthquake in the same area.

o   *Tsunamis- Water displaced by the quake that forms large waves.

o   To reduce earthquake damage, new buildings must be made stronger and more flexible. Older buildings must be modified to withstand stronger quakes. ‘SEISMIC SAFE’ indicates that a building can stand up to an earthquake

o   Base-isolated buildings- A building designed to reduce the amount of energy that reaches the building during an earthquake.

o   During an earthquake- The best way to protect yourself is to drop, cover, and hold.


o   To observe these changes, geologists put in place instruments that measures stress deformation in the crust

o   Geologists can determine earthquakes risk by locating where faults are active and where past earthquakes have occurred.


Volcanoes- Cone shaped mountains where magma, solids, and gas are spewed out.


Some volcanic eruptions are violent while others are quiet



Volcanoes form at weak spots in the earth’s crust where magma comes to the surface.

-         Magma: molten rocks, gases and water from the mantle

-         Lava: magma reaching the surface


Locations of volcanoes:

-         600 active volcanoes

-         Many lie beneath the sea

-         Occur in belts across continents and oceans


Pacific ocean has the “ ring of fire.”


Volcanoes occur from divergent plate boundaries and in subduction zones at convergent plate boundaries.


Divergent plate boundary volcanoes.

-         Form along the mid-ocean ridge

-         Lava pours out of cracks in the floor


Convergent plate boundary volcanoes:

-         Subduction zones cause oceanic crust to sink into the mantle. Crust melts and forms magma which rises to the surface

-         Where oceanic plates collide, the older plate goes under the newer, creating a trench. The trench melts and forms magma, eventually forming volcanoes

-         Island arcs are formed this way


Hot spots are areas were magma melts through the crust like a blowtorch. Usually lie in the middle of an oceanic and continental plate.



Volcano- A weak spot in the crust where molten material or magma comes to the surface.


Magma- A molten mixture of rock forming substances, Gases, and water from the mantle.


Lava- When magma reaches the surface.


Ring of fire- One major volcanic belt in the Pacific Ocean. Think of the Johnny Cash song to help you remember J


Island arc- The resulting volcanoes that create a string of islands.


Hot spot- An area where magma from deep within the mantle melts through the crust like a blow torch.


Magma chamber- Beneath the volcano where magma collects in a pocket.


Pipe- a long tube in the ground that connects the magma chamber to the earth’s surface.


Vent- Molten rock and gas leave the volcano through a vent.


Lava flow- The area covered by lava as it pours out of a vent.


Crater- A bowl shaped area that may form at the top of a volcano around the volcanoes central vent.


Silica- A material that is formed from the elements oxygen and silicon is one of the most abundant materials in the earth’s crust and mantle.


Pahoehoe- Fast moving hot lava.


Aa- lava that is cooler and slower moving


Pyroclastsic flow- when an explosive eruption hurls out ash, cinder, and bombs as well as gases.


Active- live volcano


Dormant- or sleeping volcano

Geothermal energy- water heated by magma can create a clean reliable energy source.


Extinct- or a volcano that is unlikely to erupt.


Hot spring- ground water heated by a nearby body of magma rises to the surface and collects in a natural pool.


Shield volcano- places on the earth’s surface; thin layers of lava pour out of a vent and harden on top of previous layers. Such lava flows gradually build a wide, gently sloping mountain.


Cinder Cone Volcano- a steep cone shaped hill or mountain. If a volcanoes lave is thick and stiff it may produce ash cinders and bombs.


Composite Volcanoes- tall cone shaped mountains In which layers of lava alternate with layers of ash.


Calderas- huge hole left by the collapse of a volcanic mountain.


Volcanic Neck- it forms when magma hardens in a volcano’s pipe.


Dike- Magma that forces itself across rock layers to harden.


Sill- when magma squeezes between layers of rock.