6th Grade Science Chapter 9

What are earthquakes?
the vibrations in the ground that result from movement along breaks in the Earth’s lithosphere

What is a fault?
a break in the Earth’s lithosphere where one block of rock moves toward, away from, or past another

Why do rocks move along a fault?
The forces that move tectonic plates also push and pull on rocks along the fault

What has a greater chance of occuring with increasing forces applied to a fault?
An earthquake

Where do most earthquakes occur?
in the oceans and along the edges of continents

What is the relationship between earthquakes and plate boundaries?
Earthquakes result from the buildup and release of stress along active plate boundaries

Which plate boundaries are associated with earthquakes deep below the earth’s surface?
Along convergent plate boundaries

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where an oceanic plate is subducting into the mantle

how far below the surface do deep earthquakes occur?
more than 100 km below the Earth’s surface

What determines how much energy is released by an earthquake?
The size of the fault, the motion of the fault, and the strength of the rocks that break

Why are earthquakes that occur along convergent plate boundaries often disasterous?
Because they release large amounts of energy

Where are shallow earthquakes common?
Where plates separate along a divergent plate boundary or along transform plate boundaries

What kind of earthquakes occur where continental plates collide?
earthquakes of varying depths

What is rock deformation?
When a rock changes shape due to forces applied along plate boundaries.

Rock deformation can result in ______.
ground displacement

In what direction do the rocks need to move along the fault for an earthquake to occur?
Any direction

What determines the direction in which the rocks will move along a fault?
the forces that are applied to the fault

What are three types of faults?
Strike-slip, Normal, and Reverse

What is the rock movement at strike-slip faults?
Two blocks of rock slide horizontally past each other in opposite directions

Where do strike-slip faults occur?
Transform plate boundaries

What is the rock movement at normal faults?
Two blocks of rock are pulled apart. The block of rock above the fault moves down relative to the block below the fault

Where do normal faults occur?
Divergent plate boundaries

What is the rock movement at reverse faults?
Two blocks of rock are pushed together. The block of rock above the fault moves up relative to the block below the fault.

Where do reverse faults occur?
Convergent plate boundaries

What happens when rocks move along a fault?
They release energy that travels as seismic waves

What are seismic waves?
Energy that travels as vibrations on and in Earth

Where do seismic waves originate?
Where rocks first move along the fault, at the focus

What is the focus?
a location inside Earth where seismic waves originate and rocks first move along a fault

At what depth can an earthquake occur?
Anywhere from the surface to 600 km below the surface

What is the epicenter?
The location on Earth directly above the earthquake’s focus.

What produces seismic waves during an earthquake?
A rapid release of energy along a fault

In which direction do seismic waves travel?
In all directions through rock

What do seismic waves transfer to the ground?
Energy that travels as vibrations on and in Earth

Where is the energy of seismic waves the strongest?
At the epicenter

What do seismic waves do as they move away from the epicenter?
They decrease in energy and intensity

The farther away you are from an earthquake, the ____ the earth moves.
less

What do scientists use to classify seismic waves?
wave motion, wave speed, and the type of material that a wave travels through

What are three types of seismic waves?
Primary, secondary, and surface

What is another name for a primary wave?
P-wave

What is another name for a secondary wave?
S-wave

What is a primary wave?
A wave that causes the particles to move in a push-pull motion in the same direction that the wave travels.

Put the seismic waves in order of speed.
P-wave, S-wave, surface wave

Which wave do you feel first following an earthquake? Second?
P-waves then S-waves

What are secondary waves?
Waves that cause particles to move up and down perpedicular to the direction of the wave.

What are surface waves?
Waves that cause particles to move up and down in a rolling motion, similar to ocean waves

In what materials can p-waves and s-waves travel?
p-waves can travel in solids and liquids, but s-waves can only travel in solids

Where do surface waves travel?
Only on Earth’s surface closest to the epicenter

Which waves cause the most damage at the Earth’s surface?
surface waves

Which waves can travel to the interior of the earth?
s-waves and p-waves

What are seismologists?
Scientists that study earthquakes

How have scientists determined the composition of Earth’s layers?
By studying the speed of s-waves and p-waves in the earth. The density of a material determines how fast the wave moves.

How did scientists determine that the Earth’s outer core is liquid?
By observing that the s-waves do not travel through it.

What is the composition of the inner and outer cores and how did seismologists figure it out?
Mostly iron and nickel; by studying the speed of p-waves

What in the mantle have seismologists used seismic waves to model?
Convection currents

What properties of a material change the speed of a seismic wave travelling through it?
temperature, pressure, chemistry of the rocks

Where in the mantle are seismic waves slower and faster?
Slower in the areas beneath mid-ocean ridges and faster in cool areas near subduction zones

What is a seismometer?
an instrument that measures and records ground motion and determines the distance that a seismic wave travels

What is a seismogram?
A graphical illustration of seismic waves

What method is used to locate an earthquake’s epicenter?
triangulation

How does triangulation work?
The speed and travel times of seismic waves is used to determine the distance to the epicenter from at least 3 seismometers.

What is the first step in triangulation?
Determine the difference in arrival time between the p-wave and s-waves from a seismogram

What is the lag time?
the number of seconds between the arrival of the first p-wave and first s-wave

What is the second step in triangulation?
Find the distance to the epicenter by converting the lag time to a distance using a graph

What is the third step of triangulation?
Draw a circle on a map with a radius=the distance from the second step and a center=the location of the seismometer.

What is the last step of triangulation?
Do the first three steps two more times from other seismometers. The intersection of the circles is the epicenter.

What are the three scales scientists use to describe earthquakes?
Richter scale, moment magnitude scale, modified Mercalli scale

What does the Richter scale measure?
the amount of ground motion at a given distance from an earthquake

What is the minimum and maximum of the Richter scale?
Minimum is zero and there is no maximum

What is the largest earthquake ever measured?
9.5 on the Richter scale in Chile in 1960

1 unit on the Richter scale equals how much more ground motion?
10 times

What does the moment magnitude scale measure?
the total amount of energy released by the earthquake

1 unit on the moment magnitude scale equals how much more energy released?
31.5 times more. The scale is exponential.

What does the modified Mercalli scale measure?
the earthquake intensity based on the descriptions of it’s effects on people and structures

What does a I mean for the Modified Mercalli scale and what does a XII mean?
I means shaking is not noticeable and XII means everything is destroyed.

How will an earthquake’s intensity differ in a region covered by loose sediment versus on solid bedrock.
the intensity of the earthquake will be higher on loose sediment

Where is the earthquake risk highest in the US?
On the transform plate boundary in California and on the convergent plate boundaries in Oregon, Washington, and Alaska

Do all the earthquakes in the US occur on plate boundaries?
no

How many earthquakes with a magnitude greater than 7.0 occur worldwide every year?
10

Why do seismologists study the probability that an earthquake will occur in a given area?
So that people can design buildings to be more resistant to earthquakes

What factors determine the risk of an earthquake?
Probability of an earthquake occuring, past earthquakes, geology around a fault, population density, building design

What is a volcano?
A vent in the Earth’s crust through which melted or molten rock flows

What is magma?
Molten rock below the Earth’s surface

How do volcanoes shape the Earth’s surface?
By forming large mountains, creating new crust, and creating destruction

What is lava?
Molten rock that erupts onto Earth’s surface

How do volcanoes form at convergent plate boundaries?
Thermal energy below the surface and fluids from the subducting plate melt the mantle which rises in cracks in the crust

How do volcanoes form at divergent plate boundaries?
As the plates separate, magma rises through the opening in the crust that forms between them

Where does more than 60% of all volcanic activity occur?
along mid-ocean ridges

What are hot spots?
Volcanoes that are not associated with plate boundaries

Give some examples of hot spot volcanoes
Hawaii, the Galapagos Islands, and Yellowstone National Park

What is a plume?
A rising current of hot mantle

What forms hot spots?
a rising convection current from deep within the Earth’s mantle

What happens when a tectonic plate moves over a hot spot?
A chain of volcanoes will form, with the oldest volcano furthest from the hot spot. Old ones are dormant.

Where are most volcanoes?
close to plate boundaries

What is the Ring of Fire?
an area of earthquake and volcanic activity that surrounds the Pacific Ocean

How many active volcanoes are there in the US and where are they mostly located?
60, in the ring of fire

How do scientists evaluate the possibility of eruptions of a volcano?
Earthquake activity, changes in the shape of the volcano, gas emissions, and the past history of eruptions

What is the USGS?
The United State Geological Survey, which monitors the potential for future volcanic eruptions in the US

Name three volcanoes that have erupted in the last 30 years
Mount St. Helens, Kilauea, or Mount Pinatubo

Name two active volcanoes
Mount Redoubt in Alaska and Mount Rainier in Washington

What are the three main types of volcanoes?
Shield volcano, composite volcano, and cinder cone volcano

What is a shield volcano?
a large volcano with gentle slopes of basaltic lavas

What contributes to the shape of a volcano?
Magma composition and the eruptive style

Where do shield volcanoes usually form?
Divergent plate boundaries or at oceanic hot spots

What is a composite volcano?
A large, steep-sided volcano that result from explosive eruptions of andesitic and rhyolitic lava and ash

Where do composite volcanoes form?
Convergent plate boundaries

What is a cinder cone?
A small, steep-sided volcano made from moderately explosive eruptions of basaltic lava

What is a caldera?
a large volcanic depression formed when a volcano summit collapses or is blown away by an explosion

Give an example of a caldera
Yellowstone Caldera

What is a supervolcano?
Volcanoes that have very large and explosive eruptions

What is volcanic ash?
tiny particles of volcanic rocks and glass

What determines the eruption style of a volcano?
Magma chemistry, amount of dissolved gases, the silica content

What is the main chemical compound in all magmas?
Silica, SiO2

What does the silica content of magma determine?
the magma thickness and viscosity

What is viscosity?
a liquid’s resistance to flow

What is magma with a low silica content like?
It has a low viscosity and flows easily

What happens when a low silica magma erupts?
The magma flows as fluid lava that cools and form basalt

Where are low silica magma eruptions found?
Along mid-ocean ridges and oceanic hot spots

What is magma with a high silica content like?
It has a high visocity and doesn’t flow easily

What happens when a high silica magma erupts?
Explosive eruptions

When does high silica content magma form?
When magma from the mantle mixes with continental crust and rocks rich in silica melt

Where do you find high silica content magmas?
Subduction zone volcanoes and continental hot spots

What rocks form when intermediate and high silica magmas erupt?
Andesite and rhyolite

What do all magmas contain?
Dissolved gases such as water vapor, carbon dioxide, and sulphur dioxide

Why do dissolved gases want to leave the magma as it reaches the surface?
At the surface, the pressure from the weight of the rock above decreases, so the magma can hold less gases

How do explosive eruptions result from dissolved gases?
When gases can’t get out of low visocity magmas, the magma can explode to release the gas

What forms in volcanic rocks due to dissolved gases escaping?
Holes

How do volcanoes affect the Earth?
They enrich the soil with valuable nutrients, regulate climate, and cause lots of destruction

What are the effects of lava flows?
They aren’t deadly because they are slow moving, but they threaten surrounding communities

What are the effects of ash fall?
Ash can disrupt air traffic, cause serious breathing problems, and cause climate change by blocking the sun

Why does ash disrupt air traffic?
Planes can stall out because shards of ash can fuse to engine blades

What are the effects of mudflows?
they can wipe out towns and forests

What is a volcanic mudflow?
When thermal energy from a volcano melts snow and ice which mixes with mud and ash

What is another name for a mudflow?
Lahar

Give an example of where there was a mudflow
Mount Redoubt

What is a pyroclastic flow?
An avalanche of gas, ash, and rock moving at more than 100 km/hr and with 1000C temperature

What is the effect of a pyroclastic flow?
It will destroy anthing in its path, including people

Give two examples of pyroclastic flow eruptions
Mount St. Helens and Mount Mayon in the Phillippines

What events predict the eruption of a volcano?
Ground deforms, the shape of the volcano changes, an earthquake swarm, gas emissions increase, nearby water becomes more acidic

How do volcanic eruptions influence the climate?
Ash blocks the sun and sulfur dioxide gases form droplets in the upper atmosphere that also block the sun

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