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 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