1) "When seismic wave hit the Moho (crust-mantle boundary), the speed of the wave increases. The increase in speed as the waves reach the mantle indicates that the mantle is denser than its crust."
__The mantle is densier than the crust, so seismic waves speed up as they reach the mantle! How come seismic waves speed up as they travel through denser layers? (Should it be harder to pass through instead, if it is densier?)

2) In my book, it also says "Seismic waves travel faster through more rigid materials"
__Is rigidity directly proportional to density?

Please help, I don't understand! :confused:

1) "When seismic wave hit the Moho (crust-mantle boundary), the speed of the wave increases. The increase in speed as the waves reach the mantle indicates that the mantle is denser than its crust."
__The mantle is densier than the crust, so seismic waves speed up as they reach the mantle! How come seismic waves speed up as they travel through denser layers? (Should it be harder to pass through instead, if it is densier?)



I believe that they speed up mainly because their going through a dense cool rock rather than a less dense hot rock. Correct me if im wrong somebody :mad:

But when seismic waves from the crust hit the mantle, the rocks there are HOTTER and DENSER, how come seismic waves speed up instead of slowing down when reaching the Moho (crust-mantle boundary)? This doesn't make sense to me...when for example, light waves, passing from a less dense material to a denser material, it is going to SLOW DOWN...why seismic waves are having the complete oppsite behaviour? (they are both waves)

Seismic waves can be thought of as sound waves. What is Sound? Sound is vibration through the air from molecule to molecule... that is why there is no sound in space... there is nothing there.

So, in a vacuum there is no sound... in the atmosphere, sound travels at a certain speed, in water, it travels faster yet, and in solids it travels incrediblly fast. In solids, think of it as hitting a tuning fork.

I guess the best way I think of it is that waves travel faster in denser material simply because the molecules are closer together... you bump one, and it immediately bumbs the next one... whereas in air, it might have to travel a distance before bumping the next molecule.

Make sense?

Later
T

Seismic waves can be thought of as sound waves. What is Sound? Sound is vibration through the air from molecule to molecule... that is why there is no sound in space... there is nothing there.

So, in a vacuum there is no sound... in the atmosphere, sound travels at a certain speed, in water, it travels faster yet, and in solids it travels incrediblly fast. In solids, think of it as hitting a tuning fork.

I guess the best way I think of it is that waves travel faster in denser material simply because the molecules are closer together... you bump one, and it immediately bumbs the next one... whereas in air, it might have to travel a distance before bumping the next molecule.

Make sense?

Later
T
Hi Tristan,

Yes, that makes more sense!

So seismic waves and sound waves both travel faster through denser materials, but why light waves (also a wave) travel slower through denser materials? For example, glass is denser than water so glass travel slower through glass...

Now there are 2 facts about seismic waves:
"Seismic waves travel faster through denser materials"
"Seismic waves travel faster through more rigid materials"
Are these 2 statements both meaning the same thing? Is density usually proportional to rigidity?

Edit: I have found a good explanation on this on a web site
"The velocity of a P wave is proportional to:


Velocity P wave ~ ((B + G)/Density), where:

B = the bulk modulus - the resistance to change in volume

G = the Shear modulus - the resistance to change in shape (rigidity)

Density = mass/volume

Velocity S wave = (G/Density)

From equations (a) and (b) we can formulate some important generalizations about these earthquake waves.

Note that Density appears in the denominator of both (1) and (2). If the numerators of these expressions are held constant, we would predict that the velocities of both waves would decrease with increasing depth of penetration since we know that pressure and density increase with increasing depth. However, observations show that both waves speed up with increasing depth. Therefore, the numerators must be increasing at a greater rate than density; rocks get stronger with increasing depth of burial. The comments above assume that both waves are traveling in the same material."

http://www.uh.edu/~jbutler/physical/chapter18notes.html

Another:
"Since the density of the Earth increases with depth you would expect the waves to slow down with increasing depth. Why, then, do both P- and S-waves speed up as they go deeper? This can only happen because the incompressibility and rigidity of the Earth increase faster with depth than density increases. [2] "
http://72.14.203.104/search?q=cache:4uZxhqRMOaAJ:ourworld.compuserve.co m/homepages/dp5/inner1.htm+moho+crust+mantle+speed+up&hl=en