6th Grade Science Chapter 9 – Flashcards
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What are earthquakes?
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the vibrations in the ground that result from movement along breaks in the Earth's lithosphere
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What is a fault?
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a break in the Earth's lithosphere where one block of rock moves toward, away from, or past another
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Why do rocks move along a fault?
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The forces that move tectonic plates also push and pull on rocks along the fault
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What has a greater chance of occuring with increasing forces applied to a fault?
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An earthquake
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Where do most earthquakes occur?
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in the oceans and along the edges of continents
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What is the relationship between earthquakes and plate boundaries?
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Earthquakes result from the buildup and release of stress along active plate boundaries
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Which plate boundaries are associated with earthquakes deep below the earth's surface?
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Along convergent plate boundaries where an oceanic plate is subducting into the mantle
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how far below the surface do deep earthquakes occur?
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more than 100 km below the Earth's surface
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What determines how much energy is released by an earthquake?
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The size of the fault, the motion of the fault, and the strength of the rocks that break
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Why are earthquakes that occur along convergent plate boundaries often disasterous?
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Because they release large amounts of energy
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Where are shallow earthquakes common?
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Where plates separate along a divergent plate boundary or along transform plate boundaries
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What kind of earthquakes occur where continental plates collide?
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earthquakes of varying depths
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What is rock deformation?
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When a rock changes shape due to forces applied along plate boundaries.
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Rock deformation can result in ______.
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ground displacement
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In what direction do the rocks need to move along the fault for an earthquake to occur?
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Any direction
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What determines the direction in which the rocks will move along a fault?
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the forces that are applied to the fault
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What are three types of faults?
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Strike-slip, Normal, and Reverse
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What is the rock movement at strike-slip faults?
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Two blocks of rock slide horizontally past each other in opposite directions
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Where do strike-slip faults occur?
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Transform plate boundaries
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What is the rock movement at normal faults?
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Two blocks of rock are pulled apart. The block of rock above the fault moves down relative to the block below the fault
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Where do normal faults occur?
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Divergent plate boundaries
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What is the rock movement at reverse faults?
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Two blocks of rock are pushed together. The block of rock above the fault moves up relative to the block below the fault.
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Where do reverse faults occur?
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Convergent plate boundaries
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What happens when rocks move along a fault?
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They release energy that travels as seismic waves
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What are seismic waves?
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Energy that travels as vibrations on and in Earth
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Where do seismic waves originate?
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Where rocks first move along the fault, at the focus
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What is the focus?
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a location inside Earth where seismic waves originate and rocks first move along a fault
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At what depth can an earthquake occur?
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Anywhere from the surface to 600 km below the surface
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What is the epicenter?
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The location on Earth directly above the earthquake's focus.
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What produces seismic waves during an earthquake?
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A rapid release of energy along a fault
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In which direction do seismic waves travel?
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In all directions through rock
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What do seismic waves transfer to the ground?
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Energy that travels as vibrations on and in Earth
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Where is the energy of seismic waves the strongest?
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At the epicenter
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What do seismic waves do as they move away from the epicenter?
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They decrease in energy and intensity
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The farther away you are from an earthquake, the ____ the earth moves.
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less
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What do scientists use to classify seismic waves?
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wave motion, wave speed, and the type of material that a wave travels through
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What are three types of seismic waves?
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Primary, secondary, and surface
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What is another name for a primary wave?
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P-wave
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What is another name for a secondary wave?
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S-wave
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What is a primary wave?
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A wave that causes the particles to move in a push-pull motion in the same direction that the wave travels.
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Put the seismic waves in order of speed.
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P-wave, S-wave, surface wave
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Which wave do you feel first following an earthquake? Second?
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P-waves then S-waves
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What are secondary waves?
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Waves that cause particles to move up and down perpedicular to the direction of the wave.
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What are surface waves?
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Waves that cause particles to move up and down in a rolling motion, similar to ocean waves
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In what materials can p-waves and s-waves travel?
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p-waves can travel in solids and liquids, but s-waves can only travel in solids
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Where do surface waves travel?
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Only on Earth's surface closest to the epicenter
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Which waves cause the most damage at the Earth's surface?
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surface waves
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Which waves can travel to the interior of the earth?
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s-waves and p-waves
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What are seismologists?
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Scientists that study earthquakes
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How have scientists determined the composition of Earth's layers?
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By studying the speed of s-waves and p-waves in the earth. The density of a material determines how fast the wave moves.
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How did scientists determine that the Earth's outer core is liquid?
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By observing that the s-waves do not travel through it.
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What is the composition of the inner and outer cores and how did seismologists figure it out?
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Mostly iron and nickel; by studying the speed of p-waves
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What in the mantle have seismologists used seismic waves to model?
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Convection currents
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What properties of a material change the speed of a seismic wave travelling through it?
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temperature, pressure, chemistry of the rocks
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Where in the mantle are seismic waves slower and faster?
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Slower in the areas beneath mid-ocean ridges and faster in cool areas near subduction zones
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What is a seismometer?
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an instrument that measures and records ground motion and determines the distance that a seismic wave travels
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What is a seismogram?
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A graphical illustration of seismic waves
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What method is used to locate an earthquake's epicenter?
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triangulation
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How does triangulation work?
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The speed and travel times of seismic waves is used to determine the distance to the epicenter from at least 3 seismometers.
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What is the first step in triangulation?
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Determine the difference in arrival time between the p-wave and s-waves from a seismogram
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What is the lag time?
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the number of seconds between the arrival of the first p-wave and first s-wave
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What is the second step in triangulation?
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Find the distance to the epicenter by converting the lag time to a distance using a graph
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What is the third step of triangulation?
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Draw a circle on a map with a radius=the distance from the second step and a center=the location of the seismometer.
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What is the last step of triangulation?
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Do the first three steps two more times from other seismometers. The intersection of the circles is the epicenter.
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What are the three scales scientists use to describe earthquakes?
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Richter scale, moment magnitude scale, modified Mercalli scale
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What does the Richter scale measure?
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the amount of ground motion at a given distance from an earthquake
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What is the minimum and maximum of the Richter scale?
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Minimum is zero and there is no maximum
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What is the largest earthquake ever measured?
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9.5 on the Richter scale in Chile in 1960
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1 unit on the Richter scale equals how much more ground motion?
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10 times
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What does the moment magnitude scale measure?
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the total amount of energy released by the earthquake
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1 unit on the moment magnitude scale equals how much more energy released?
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31.5 times more. The scale is exponential.
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What does the modified Mercalli scale measure?
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the earthquake intensity based on the descriptions of it's effects on people and structures
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What does a I mean for the Modified Mercalli scale and what does a XII mean?
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I means shaking is not noticeable and XII means everything is destroyed.
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How will an earthquake's intensity differ in a region covered by loose sediment versus on solid bedrock.
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the intensity of the earthquake will be higher on loose sediment
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Where is the earthquake risk highest in the US?
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On the transform plate boundary in California and on the convergent plate boundaries in Oregon, Washington, and Alaska
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Do all the earthquakes in the US occur on plate boundaries?
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no
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How many earthquakes with a magnitude greater than 7.0 occur worldwide every year?
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10
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Why do seismologists study the probability that an earthquake will occur in a given area?
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So that people can design buildings to be more resistant to earthquakes
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What factors determine the risk of an earthquake?
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Probability of an earthquake occuring, past earthquakes, geology around a fault, population density, building design
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What is a volcano?
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A vent in the Earth's crust through which melted or molten rock flows
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What is magma?
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Molten rock below the Earth's surface
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How do volcanoes shape the Earth's surface?
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By forming large mountains, creating new crust, and creating destruction
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What is lava?
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Molten rock that erupts onto Earth's surface
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How do volcanoes form at convergent plate boundaries?
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Thermal energy below the surface and fluids from the subducting plate melt the mantle which rises in cracks in the crust
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How do volcanoes form at divergent plate boundaries?
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As the plates separate, magma rises through the opening in the crust that forms between them
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Where does more than 60% of all volcanic activity occur?
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along mid-ocean ridges
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What are hot spots?
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Volcanoes that are not associated with plate boundaries
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Give some examples of hot spot volcanoes
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Hawaii, the Galapagos Islands, and Yellowstone National Park
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What is a plume?
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A rising current of hot mantle
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What forms hot spots?
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a rising convection current from deep within the Earth's mantle
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What happens when a tectonic plate moves over a hot spot?
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A chain of volcanoes will form, with the oldest volcano furthest from the hot spot. Old ones are dormant.
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Where are most volcanoes?
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close to plate boundaries
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What is the Ring of Fire?
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an area of earthquake and volcanic activity that surrounds the Pacific Ocean
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How many active volcanoes are there in the US and where are they mostly located?
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60, in the ring of fire
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How do scientists evaluate the possibility of eruptions of a volcano?
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Earthquake activity, changes in the shape of the volcano, gas emissions, and the past history of eruptions
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What is the USGS?
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The United State Geological Survey, which monitors the potential for future volcanic eruptions in the US
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Name three volcanoes that have erupted in the last 30 years
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Mount St. Helens, Kilauea, or Mount Pinatubo
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Name two active volcanoes
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Mount Redoubt in Alaska and Mount Rainier in Washington
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What are the three main types of volcanoes?
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Shield volcano, composite volcano, and cinder cone volcano
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What is a shield volcano?
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a large volcano with gentle slopes of basaltic lavas
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What contributes to the shape of a volcano?
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Magma composition and the eruptive style
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Where do shield volcanoes usually form?
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Divergent plate boundaries or at oceanic hot spots
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What is a composite volcano?
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A large, steep-sided volcano that result from explosive eruptions of andesitic and rhyolitic lava and ash
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Where do composite volcanoes form?
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Convergent plate boundaries
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What is a cinder cone?
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A small, steep-sided volcano made from moderately explosive eruptions of basaltic lava
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What is a caldera?
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a large volcanic depression formed when a volcano summit collapses or is blown away by an explosion
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Give an example of a caldera
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Yellowstone Caldera
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What is a supervolcano?
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Volcanoes that have very large and explosive eruptions
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What is volcanic ash?
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tiny particles of volcanic rocks and glass
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What determines the eruption style of a volcano?
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Magma chemistry, amount of dissolved gases, the silica content
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What is the main chemical compound in all magmas?
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Silica, SiO2
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What does the silica content of magma determine?
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the magma thickness and viscosity
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What is viscosity?
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a liquid's resistance to flow
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What is magma with a low silica content like?
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It has a low viscosity and flows easily
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What happens when a low silica magma erupts?
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The magma flows as fluid lava that cools and form basalt
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Where are low silica magma eruptions found?
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Along mid-ocean ridges and oceanic hot spots
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What is magma with a high silica content like?
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It has a high visocity and doesn't flow easily
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What happens when a high silica magma erupts?
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Explosive eruptions
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When does high silica content magma form?
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When magma from the mantle mixes with continental crust and rocks rich in silica melt
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Where do you find high silica content magmas?
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Subduction zone volcanoes and continental hot spots
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What rocks form when intermediate and high silica magmas erupt?
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Andesite and rhyolite
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What do all magmas contain?
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Dissolved gases such as water vapor, carbon dioxide, and sulphur dioxide
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Why do dissolved gases want to leave the magma as it reaches the surface?
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At the surface, the pressure from the weight of the rock above decreases, so the magma can hold less gases
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How do explosive eruptions result from dissolved gases?
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When gases can't get out of low visocity magmas, the magma can explode to release the gas
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What forms in volcanic rocks due to dissolved gases escaping?
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Holes
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How do volcanoes affect the Earth?
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They enrich the soil with valuable nutrients, regulate climate, and cause lots of destruction
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What are the effects of lava flows?
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They aren't deadly because they are slow moving, but they threaten surrounding communities
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What are the effects of ash fall?
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Ash can disrupt air traffic, cause serious breathing problems, and cause climate change by blocking the sun
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Why does ash disrupt air traffic?
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Planes can stall out because shards of ash can fuse to engine blades
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What are the effects of mudflows?
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they can wipe out towns and forests
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What is a volcanic mudflow?
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When thermal energy from a volcano melts snow and ice which mixes with mud and ash
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What is another name for a mudflow?
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Lahar
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Give an example of where there was a mudflow
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Mount Redoubt
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What is a pyroclastic flow?
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An avalanche of gas, ash, and rock moving at more than 100 km/hr and with 1000C temperature
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What is the effect of a pyroclastic flow?
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It will destroy anthing in its path, including people
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Give two examples of pyroclastic flow eruptions
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Mount St. Helens and Mount Mayon in the Phillippines
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What events predict the eruption of a volcano?
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Ground deforms, the shape of the volcano changes, an earthquake swarm, gas emissions increase, nearby water becomes more acidic
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How do volcanic eruptions influence the climate?
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Ash blocks the sun and sulfur dioxide gases form droplets in the upper atmosphere that also block the sun
