Chap 7  Earthquakes and Volcanoes


Chapter Objectives:  3 sections with 8 total objectives

                 Section 1:

1.   Explain how earthquakes are formed by a buildup within earth’s crust

2.   Compare and contrast primary, secondary and surface waves

3.   Recognize earthquake hazards and how to prepare for them

Section 2:

4.   Explain how volcanoes can affect people

5.   Describe how types of materials are produced by volcanoes

6.  Compare how three different volcano forms develop

Section 3:

7.   Explain how the locations of volcanoes and earthquakes epicenters are related to tectonic plate boundries.

8.  Explain how heat  within the Earth’s crust causes Earth’s plates to move.


Chap 7  Earthquakes and Volcanoes


Section 1 Earthquakes


What Causes an Earthquakes

      Elastic rebound

             Applying force to a rock causes it to strain

                   To change shape

             Apply enough force and it will break/snap – like a piece of lettuce

             A vibration occurs when it snaps

                   Called elastic rebound

             This changing shape (deforming) takes long periods of time

             The deforming builds up potential energy in the rocks

             If the rock breaks under strain the potential energy is released

                   This release causes an earthquake

      Types of Faults

             When a rock breaks either side of the rock might move due to the elastic rebound

             On the surface of a break is called a fault

             There are three types of faults – type is determined by the cause of the fault not the change of position or end result

                   Normal – caused by  rock layers  pulling apart – (as in a divergent tectonic plate) but faults are smaller and occur just within the crust

                   Reverse – caused by  rock layers  pushing together

                   Slip strike – when the rocks are moved side by side

             Faults are caused by the motion of the earth’s plates, sections of Earth’s crust and upper mantle



Making waves

                           (Drantch’s note – you should be able to do a ven diagram and t charts – comparing and contrasting or describing these waves )


                        Seismic wavesthe energy waves created by earthquakes transmitted thru materials in the earth and at the surface  

      Earthquake Focus and Epicenter

             The strain along a faiult line is released by movement – it releases potential energy – which moves out from the fault

                  This energy release is an earthquake

                  The point where the energy release (movement) first occurs is the focus

                  The epicenter is the place on earth’s surface directly above where focus is – (where the waves started)


      Seismic Waves

          Waves are grouped in two ways:

                                                   –1.  by their time of origin from the start (P & S)

                                                     2. by their travel location ( interior & surface)

                      Waves are described in two waves:       

1. by their speed

2. by their type of vibration                                                  

             Seismic waves travel out in all directions from the focus – the greatest damage is by those that travel along the crust

                  There are two types of seismic waves

                           P waves – the primary waves – travel the fastest – cause material to vibrate – rock back and forth – move in the same direction as the waves

                           S waves – the secondary waves – cause vibration in materials  at right angles from the direction the wave is moving

                  Surface waves (those moving outside on the surface) – slowest and largest of the seismic waves – cause the most destruction

                           Complex movement – meaning they have many forms and effects

                                             Some cause rock and soil to move back and force    

                                                              Some cause soil to move roll as a wave does in the ocean

                                             Some cause soil to sway – move side to side – these are the most damaging   

                                             Buildings crack and fall down

                                                      Bridges and highways collapse



Learning From Earthquakes

                        Speed that earthquake waves travel help us to find their location (or distance to the epicenter)


                   Earthquake Measurements

 A seismograph is a tool used by seismologists

                                    Seismograph  - tool used to gather and chart (by tracing) seismic waves from all over the planet

                                    Seismologist – person that studies seismic waves – earthquakes

                  Magnitudethe strength of seismic wave (based on the height ( amplitude)  of the line traced on the recording part of a seismograph)

      Epicenter Location

             When seismic waves are charted on a seismograph:

                           The further apart the different types of waves are from each other helps to calculate distance from the epicenter


How Strong are Earthquakes

                           There are two scales to measure earthquakes:

                                    Richter – by the energy strength (amplitude on a seismograph)

                                                 Mercalli – by the damage done by the earthquake in a specific location

                      Major earthquakes can cause much loss of life and destruction

                                    Taiwan – 2400 dead, many injured and more homeless

                                                  Current day in Haiti

                      It leaves people homeless ffor many miles from the epicenter

                                    Ie Mexico city – 400 km from the epicenter

      The Richter Scale

             Describes how much energy an earthquake releases

                  The greatest amplitude measured is a 10 on the scale

                         Each number is 32 times more powerful then the number before it

                           A 7.5 is 32 times more destructive (powerful) then a 6.5

                     Earthquake Damage

                   Measured by the Mercalli scale  - I thru XII

                                    A Richter 6.7 has caused  a Mercalli IX = severe building damage & cracks in the ground              


                                    An earthquake on the ocean floor causes a huge (long length and tremendous volume of water compared to regular beach waves)

                                             In deep water they can be a small height but very fast

                                                      About three feet tall

                                                      950 km per hour

                                                        As they get near land they slow down but get very tall

                                                      30 meter in height – nearly one hundred feet tall

                                                  An early sign of a tsunami is the the shoreline moves far in from the edge of the beach (it goes away from the place where the people would be)

                                    The waves travel out from the epicenter within and along the ocean

                                             They can travel out from many hundreds of miles


Earthquake Safety

                      Earthquake dame can be decreased or prevented prevetive building and knowledge

                            (Drantch note – these preventions were not in place in Haiti and so much damage occurred)

                           Is your home seismic safe

                          Prevention means:

                                             Heavy objects on bottom shelves

                                             Secure any gas  appliances

                                                      Water heater, dish wasker, water boiler

                                             Stay away from windows

                                             Protect from fire hazards by having gas shut offs

                   Seismic-Safe Structures    

                          Have strict building codes which can make buildings (tall high rise apartments and office buildings) earthquake proof       

                                             Steel and rubber foundations to deal with the vibration

                                             Bendable pipes so they don’t snap and release gas or water

                                             Bridges and roadways have reinforcement to prevent collapse                       


Section 2 Volcanoes


How Do Volcanoes Form

                       Raising magma and hot gases rise up thru the rock layers form cone shaped mountains called volcanoes

                              Lava is magma which has flowed out of the vent (opening ) of the volcano

                        Tephra is rock and solidified magma that rains down from the sky after a volcanic eruption (explosion) –it comes out in  two forms

                                    Can be:

 dust like material called cinder (like ashes)

 large rocks called bombs or blocks

             Where Plates Collide

                   Volcanoes can form from collisions of the Earth’s crust

                           At plate subduction zones the older and denser  oceanic plate (crust with mantle)  is pushed under the less  dense newer plate

                                    The friction at this site melts the crust forming pools of magma

                                     Some Caribbean islands formed this way

                                    Montserrat island

             Eruptions on a Caribbean Island

                   Soufrierre Hills Volcano – on island of monseratte

                                    1995 but exploded was thought dormant

                                             Ash rose to 10,000 meters

                                                      Covered the capital and many other cities

                                                      Magma avalanches and hot boiling mudflows covered cities and destroyed the port

                                    1998 explosion volcanic ash reached 14,000 meters

                                             ash covered entire island

                                             heavy rain followed and caused   mudflows

                                    Pyroclastic flows – massive avalanches of hot rock flowing ontop of a cushion of intensely hot gases

                                             Can reach speeds of 200/mph            

             Volcanic Risks

                   Maps can be made to demonstrate risk of volcanic activity by location around a volcano and its island

Forms of Volcanoes

                                    Volcanoes can cause destruction but also can deposit new rock

                                             New rock deposition occurs many ways

      What Determines How a Volcano Erupts

             The magma composition (what is made of) determines the type of eruption

                           Lava with lots of silica (silca and oxygen) is thicker and resists flow

                           Lava with iron and magnesium (less silica) flows easily

                  The amount of water vapor in the lava will also influence (effect) lava eruption

                  Sticky, silica rich magma traps much water vapor

                           The trapped steam (of water vapor) tends to build up pressure (like a container of soda) and can cause a powerful eruption (explosion)

                  In the Lesser Antilles Volcano

                           Plate subduction brought water into the mantle which got trapped in magma

      Shield Volcanoes

             Basaltic lava – flows in broad, flat layers  (Mona Loa)

(high in iron and magnesium) – thinner less dense vs silica magma high in silca – thicker denser

Shield volcanoes are basaltic lava

Gentle slopes of basaltic layers 

The largest types of volcanoes 

Form when magma is forced between layers from the depths     

Or when there are divergent plates

      Cinder Cone Volcanoes

                                    Rising magma gathers gases on its way to the surface

                                    Moderate to violent explosions of:

                                             Volcanic ash, cinders & lava blown high in the air (hot lava quickly cools in mid air)

                                                 This tephra forms small cone

                                             300 meters high and usally form groups with with other larger volcanoes

                                             tends to be short lasting because of the sudden gas release

                                    Sunset Crater

      Composite Volcanoes

             Steep sided with alternating layers of tephra and and lava

 They occur where one plate sinks below another

                  Noising eruptions lead to tephra deposits

                       Quiet eruptions lead to the gentle lava flows

                           Examples are:

 Mt. St. Helens

Soufriere   Hills

Mt. Rainer                                


      Fissure Eruptions

             Magma that is highly fluid, low viscosity (it flows easily- more like liquid ice cream then ketchup) can ooze from cracks and fissures

                  This magma is associated with fissure eruptions

                  A flood basalt is a low flat ooze that has hardened with millions of years of erosion

It is called a lava plateau

The Columbia River Plateau

Keep this simple – don’t over analyse or over study the below material- use flash cards (know the differences and similarities without a lot of memorizing)

                  Just have some favorite facts in case you want to write a story

Choose a composite volcano, a cinder volcano, and a shield volcano

– be able to describe its:

                  Eruptive force, silica content, gas content, eruption products

Good examples are: Krakatoa, Paricutin, Kilauea                


Section 3 Earthquakes, Volcanoes, and Plate Tectonics


Earth’s Moving Plates

      Earthquakes are the vibrations within the tectonic plates as they move apart, together or slide past each other

         These conditions can also cause volcanoes   

Where Volcanoes Form

      Most volcanoes form at plate boundaries

         Plate movement can help explain the formation of volcanoes

             Divergent Plate Boundries

                   As tectonic plates move apart (divergent plates) cracks or rifts form between them

                           The rifts contain fractures in the crust which serve as an opening thru which magma can rise

                           Rift zones are the most common place where volcanoes arise

                                    Fissure eruptions occur at rift zones

                                    They forms lava which is basaltic – the most common rock in Earth’s crust


             Convergent Plate Boundries

                   Also common place is along convergent plate boundries

                           Dense oceanic plates sink and melt below the less dense other plate

                           Mantle heat helps to melt the baslt and sediment of the oceanic plate

                                    The melted basalt (magma) is then is forced upward as a volcano

                           Where the Pacific plate collides with other plates surrounding it is  called THE RING OF FIRE

                                    A belt of volcanos surrounding the Pacific Ocean

             Hot Spots

                   Hot spot is where large bodies of magma are forced upward thru the crust.

                           The Hawaiian Islands are forming as the pacific plate passes over this hot spot

                                    The hot spot stays in one place – the plate slides over it

Moving Plates Cause Earthquakes

                       The breaking and elastic rebound as plates move together causes earthquakes

                       Three causes of earthquakes are:

                           Convergent boundaries – plates moving together

                           Divergent boundaries – plates moving apart

                           Transform boundaries - plates grinding past each other side by side                            

                           Earthquake Locations

                          Earthquakes happen most often in a common set of belts

                                    80% happen in the same belt where volcanoes form – the Pacific ring of fire

                                    There is a definite relationship between earthquake epicenters and tectonic plate boundaries                           

                   Earth’s Plates and Interior

                          Seismic waves provide much information for researchers about Earth’s interior and tectonic plates

                                    The layers of the earth have been mapped depending on the speed at which seismic waves pass thru the layers

                                    The asthenosphere was noted because seismic waves slow when the reach the base of the lithosphere

                                             The asthensophere is a warmer, softer, molten layer that the colder brittle rocky plates slide on.

                   What is Driving Earth’s Plates

                          The mantle is heated by the core

                                    As it becomes hot and less dense it rises and forms convection cells which move the plates

                                    Also as the plates they may also pass over hot spots which cause volcanic activity

         Yellowstone National Park – old faithful is over a hot spot in the North American Plate