Earth's Interior Midterm 2

Close to the source (less than 200km) the ray

on top would arrive first at 6km/sec

If the rock is homogenous, wave fonts are ______________ in cross sectio

circles

The angle which a refracted ray bends at a boundary depends on the contrast in wave velocity between the two materials in the contact at the boundary and the angle at which the ray hits the surface...

- A ray that goes from a fast moving material to a SLOWER moving material, they ray will bend DOWN...think of running and a big gust of wind comes up and SLOWS YOU DOWN its harder for you to get through and you slow down -A ray enters a faster medium and the ray will bend toward the boundary. Think of the opposite. You are running against a huge headwind and then it suddenly stoops...your stride will take off and you will be running faster and will sprint off

The Core-Mantle Boundary P-Wave Shadow Zone

-Gutenburg discovered that Pwaves from a given earthquake did not arrive at seismometers within a band between 103degrees and 143 degrees. This is called the P-wave shadow zone. IF the density of the Earth gradually increased with depth all the way to the center, the shadow zone would NOT exist, because rays passing into the interior would refract evenly so rays would reach every point on the Earths Surface

The existence of a shadow zone tells us what?

-Means that deep in the Earth a major interface exists where seismic rays abruptly REFRACT DOWN. This implies that something! That the velocity of the seismic waves suddenly DECREASES in that area. -tells us the core must be seismically slower than the mantle!

Because of seismic-volocity discontinuities, seismologists subdivide the the mantle into the

-Upper mantle (above 660km) -Lower mantle (below 660km)

The nature of the core: S-wave shadow zone

-discovered that S waves do not arrive at stations from 103 degrees to 180 fro the epicenter This is called the S WAVE shadow zone. -this means that s waves cannot pass through the core AT ALL! -s waves are shear waves and can only travel through solids THUS the fact that s waves do no pass though the core , means that the core, at least in part consists of liquid.

Recap of Seismic Waves....

-they travel through Earth (body waves) -P waves (primary/compressional) wave vibration <-> -Swaves (secondary/shear SS) Viibration direction Up and down amplitude

There are 3 layers of the earth what are they?

1. Crust-low density 2.7-3.3 g/cm3 2.mantle-intermediate density 3.3-5.7 g/cm3 3. core-high density 9.9-13.0 g/cm3

The ability of a seismic wave to travel through a material AS WELL AS the velocity at which it travels depends on what characteristics of the material?

1. Denisty-its mass /unit volume 2. rigidity-how stiff or resistant to bending a material is 3. compressibility-how easily a materials volume changes in response to squashing

Study of seismic waves has revealed what?

1. that seismic waves travel at different velocities in different rocks....makes sense 2.Waves can accelerate or slow down if they pass from one rock type to another 3.Seismic waves travel more slowly in a liquid than in a solid of the same composition. thus waves travel more slowly in magma than in solid rock and more slowly in molten iron alloy than in solid iron alloy 4. Both S and P waves can travel through a solid, BUT ONLY A P WAVE CAN TRAVEL THROUGH A LIQUID (PEE IS A LIQUID...I KNOW THATS GROSS BUT NOW YOU WONT' FORGET)

The dimensions of the shadow zone allied seismologists to calculate that this interface lies at a depth of

2900 km and interpreted it to be the Core-mantle boundary.

Core is made of an iron alloy

85% iron 5% nickel 10% lighter element probably oxygen, silicon and/or sulphur

Radioactive Decay

Also known as nuclear decay ore radioactivity. process by which the nuclear of an unstable atom loses energy by emitting radiation, including alpha particles, beta particles gamma rays and conversion electrons

Farther from the source in which the ray had to travel greater than 200 km, the ray that went through the faster mantle would get there first

Average speed was 8km/sec Crust is slower than mantle which is faster

Reflection and refraction describe the effects of a seismic wave encountering a

BOUNDARY BETWEEN DIFFERENT MATERIAL (SNELLS LAW)

Why can a p wave is the only wave to go through a liquid?

Because one part of a liquid can easily move sideways relative to another part, so the energy of a shear motion cannot be transmitted from one part of a liquid to another.

Alpha Decay

One type of radioactive decay in which the nuclear emits and alpha particle and thereby transforms (or decays) into an atom with a mass number decreased by 4 and atomic number decreased by 2

The crust mantle boundary is called what?

The MOHO- was discovered based on different average speeds of first arrivals at different seismic stations

The nature of the core

The S Wave shadow zone reveals the PHYSICAL STATE OF THE CORE -The P Wave REFLECTIONS reveals the presence of a Boundary WITHIN the core (inner and outer)

Transition zone

The lower portion of the upper mantle (between 410 and 660km) in which several small seismic discontinuities have been recognized is called this

Why is the outer core molten and the inner core is solid?

The melting temp is less than the Earth's temp in the outer core, so the outer core is molten. Liquid Iron Alloy (solid iron alloy in the inner core?

electron capture

The nucleus may capture and orbiting electron causing a proton to convert into a neutron is this process

Refracted Ray

The ray that bends at the boundary (put goes through the boundary) This is called refraction. bending=refraction

Reflected Ray

The ray that bounces off the boundary and heads back into the air This is called reflection. bouncing =reflection

Geophysics

The study of seismic waves, magnetism, gravity and other physical characteristics of the Earth

Refraction and reflection both take place where

at the interface between two materials, if the wave travels at different velocities in the two materials

Structure of the Mantle: The velocity of p waves in the mantle changes because of the physical properties of the mantle

changes with depth Between 100-200 km deep is known the LOW VELOCITY ZONE P waves slow down (hence why it bends back a bit)..why ? because peridotite, the rock comprising the mantle has undergone partial melting, so the mantle in the LVZ contains about 2 perctn melt This melt forms films on the surface solid grains. Because seismic waves travel more slowly through liquids than solids, the seismic waves slow down. This LVZ is only found under oceanic lithosphere and they have NOT found one beneath continents. meaning the base of the lithosphere is not delineated by a change in seismic velocity beneath continents.

seismic wave is the energy released by an

earthquake or explosion

The velocity of seismic waves ________________ with depth in the mantle, so rays ___________________ and fronts are __________________

increases curve oblong.

seismic-velocity discontinuities

occur at depths where pressure causes atoms in minerals to rearrange and pack together more tightly. -results in a mineral with more compact structure(process called phase change) -where such phase change takes place the overall characteristics of mantle rock change and seismic waves travel at different velocities. -straight line on the graph

wave front

the boundary between the rock through which a wave has passed and the rock through which is has not yet passed is called this

seismic ray

the changing position of an imaginary point on a wayve front, as the front moves through a rock is called this. To represent, we draw line perpendicular to the wave front -each point on a curving wave front follows a slightly different ray.

The angle at which a reflected ray bounces of a boundary IS ALWASY

the same angle at WHICH the incoming or INCIDENT RAY strikes the surface

The study of seismic waves can give us the clues to tell us what about these layers

the specific depths. By measuring how fast seismic waves travel through the Earth, and how the waves bend or reflect as they travel, it is possible to define the thicknesses of the main layers AND EVEN to recognize sublayers.

travel time

the time it takes for a wave to travel from the earthquake focus to a seismometer along a given ray is called this

beta negative (sure if the same as beta decay...but this is what he had written down) under types of radioactivity

type of radioactive decay in which a beta ray and a receptive neutrino are emitted from an atomic nucleus.