Brief Review from yesterday• We claimed that the Earth has 3 main layers Crust Mantle Core

• 2 of these layers (Mantle & Core) are broken into subgroups due to differences in composition and physical properties

Mantle consists of (order from least to greatest depth) lithosphere, asthenosphere, and mesosphere

The core consists of an “outer core” and an “inner core.” Though both are part of what we call the core due to their composition, they have very distinct physical properties

• Because we just barely “scratched the surface” yesterday, let’s break these layers down a bit and get a “deeper” understanding

2 puns in one sentence without intention….not bad….not good…..but not bad ;)

The Crust

• Though we generally refer to the crust as any continental plates, geologists separate the crust into continental and oceanic crust for 3 reasons:

Difference in density and therefore composition Difference in actual depth Difference in formation and therefore age

Characteristics Continental Oceanic

Density Less dense More dense

Depth 20-70 km About 7 km

Age Around 4 billion yrs Around 200 m.y.

The Crust

The oceanic crust at the island of Hawaii is about 5 km thick Thickness of the continental crust under eastern California ranges from 25 km

under the Great Valley to 60 km under the Sierra Nevada where Mount Whitney is

The Crust

• So why is oceanic crust much thinner AND younger than continental crust?

• Unlike continental crust, oceanic crust is actively being created

• At mid-ocean ridges, magma erupts on ocean floor and horizontally pushes previously created ocean crust in a conveyor-belt fashion

• When oceanic crust encounters continental crust, it becomes “subducted” because of its greater density

• Oceanic crust then returns back to mantle where it is re-melted into magma

The Mantle

• Constitutes about 84% of Earth’s volumeLithosphere The uppermost mantle plus

overlying crust RigidAsthenosphere Resembles a much more “plastic-

like” substance due to the T & P conditions that allow rock to become bendable w/o breaking

Area of low density Important to tectonic plate

movementsMesosphere Lower part of mantle Though the rock is hot, the

pressure makes the rock solid

The Core

• Huge ball of liquid and solid iron (mostly)Outer Core Radius of 3400 km Liquid due to low enough pressure (which is still huge

amounts!) and extreme temperature Contains Iron and Nickel Creates Earth’s magnetic field which protects life from

solar radiationInner Core Radius about 1200 km Solid ball of iron-nickel alloy Temperatures comparable to the sun! (over 10,000

degrees Fahrenheit) SLOWLY growing in size due to the cooling of the outer

core where the 2 cores meet

Sounds good, but how do we know it’s true?

• What about the “hollow Earth theory?” • http://www.youtube.com/watch?v=ZlkjtiebZw

k• Look for the evidence!!

Why waves???

One of the properties about any type of wave is that it will change its speed & direction as it moves from one substance to another

If we have a good understanding of how certain waves move through certain substances, we can use this information to infer what kind of substance a wave moves through without having even seen the actual substance

Represent different layers in the Earth

Notice how waves are “bent” each time it encounters a new layer Time it takes waves to reach microphone also allows

us to determine depth

Oil companies use this concept Help determine where the oil is Help determine depth Provides information as to what material they will need to drill through

So lets get back to the inside of the Earth

Here was the hypothesis: IF the crust and the mantle were composed of the same material, then the wave

traveling the shortest distance would be the first to register at the seismograph

But that’s not what actually happened…So what did happen? The seismic waves that traveled through the mantle, which was a greater distance,

actually arrived at the seismograph FIRST!

• Andrija Mohorovicic, a Croatian seismologist discovered this in 1909 Moho realized that the waves must have been traveling through a different substance,

specifically a substance with greater density, in order to arrive earlier Labeled this boundary between the crust and mantle as Mohorovicic Discontinuity or

simply “Moho”

What about the rest of the Earth?Only high energy seismic waves from nuclear explosions or earthquakes can travel all the way through the Earth and back to the surface• If the interior of the Earth was made of the same substance and density, we would

expect to see the seismic waves travel in a path like the diagram below

However…..Instead, what we see is something like this… What this means is that something caused these waves to bend and travel at

different velocities The increase in density increases the speed at which the waves travel

Therefore, the Earth is not made of the same material and has different density throughout!

In addition….We noticed something else very interesting…. Some of the waves, called S waves, never reached the opposite side of the Earth Scientists knew from previous experiments that S waves CANNOT travel through

liquid, only through solid

What this meant was that something inside the Earth stopped these S waves from traveling through Therefore, there must be a

liquid layer inside the Earth (outer core)

Size of the shadow zone helped us understand the size of the layer that was liquid

What about the inner core?We cannot rely on S waves to reveal the inner core because, as we saw, they stopped at the liquid outer core Because of this, we have to rely on another type of seismic wave called a P wave P waves can travel through both liquid and solid

The waves bend AGAIN after passing through outer core! This has to mean that

there must be another layer within the outer core

To summarize….

Mostly made of basalt

Mostly made of granite

What I expect you to know about Earth’s interior

• Names of the different layers• General depths (which is bigger and smaller)• General physical properties of different layers

(pressure and state of matter)• Explain how we know of the existence of

different layers Use evidence to support such claims

So why do scientists laugh at such a theory as the hollow Earth?

• Use of seismic waves Seismic waves Vibrations that carry energy from the source

of the shaking outward in all directions Concept is similar to the “pebble in pond”

experience we’ve all had To no surprise, an earthquake is a much

more complicated process and therefore has much more varied seismic waves

So what waves are we talking about then? P waves (primary) S waves (secondary)

P and S waves

P & S wavesWhen an earthquake occurs, the seismic waves (P & S) spread out in all directions through Earth’s interior

Scientists analyze this seismic data by specifically looking for a few things The velocity the waves travel at throughout the

interior of the Earth Whether or not they actually travel through

them at all

What are some characteristics of P and S waves while traveling through interior?• Depend on the material properties such as composition, temp., pressure, and density• Travel more quickly through denser materials • Hot areas slow down seismic waves• Seismic waves move more slowly through a liquid than a solid• Molten areas slow down P waves and stop S waves (cannot travel through liquid)

It’s all about the MohoMohorovicic Discontinuity (Moho) Boundary between the crust and the

mantle Discovered by Andrija Mohorovicic, a

Croatian seismologist, in 1909 The word “discontinuity” in geology is

used for a surface at which seismic waves change velocity

Moho saw this increase in velocity and inferred that the acceleration must be caused by a higher density material (a different layer)

Has anyone ever seen the Moho?• No one has ever been deep enough into the Earth and no wells have ever been

drilled deep enough• Drilling wells to that depth (avg. of 32 km) are insanely expensive and very difficult• Deepest well ever drilled about 12 km