## The internal of the earth

The inside of the earth is hardly accessible for direct observations.

While the radius of the earth is 6.3 Mm the deepest shaft they have ever has been dugged

is less than five kilometers deep. Incidentally: 1 Mm = 1 mega meter = 1,000,000 m = 1000 km.

Moreover, the number of deep shafts is to a handful.

The deepest troughs stabbing about eleven miles below the ocean surface.

Slowly but surely information about the interior of the earth is becoming known by physical means.

1. Density

The circumference of the nearly spherical earth is known. So the radius and the volume of the solid

sphere as well.

The mass of the earth, we can not determine by a (spring0 balance but with the aid of the movement

of the moon. (For the uninitiated people it will look strange.)

We learned that mass equals density times volume orm = ρ • V.

In this case we speak of the average density of the earth,

because we can not assume homogeneity.

2. To calculate a bit

The circumference of the earth is by outdated definition 40,000 km exactly.

The mass of the earth is six kwadriljoen kg.

In this notation, the value is easy to remember.

More accurately that mass is 5.976 × 10^{24}kg.

We calculate from these two data the average density of the Earth being 5520 ton/m^{3}is.

We use

- a ton is 1000 kg.

- circumference = 2πrandV_{sphere}= (4/3)πr^{3}andm = ρ×V. In itρthe mean density of the earth.

The density of ocean water is approximately 1.03 tons/m^{3}. It varies with temperature,

the depth and salinity (concentration of salt).

The density of the different lithologies varies from 2 to 5 ton/m^{3}. To come to an average of 5,520 ton/m^{3}

the density of deeper parts are about 8 ton/m^{3}.

So from the surface to the center of the earth the density increases from 1 (water) to 8 ton/m^{3}.

3. Magnetism

The earth behaves like a giant bar shaped magnet. This is evident from the behavior of a compass needle.

The earth is then either a giant electromagnet or a permanent magnet, made from a magnetizable material.

The first choice is not very obvious. So we think about magnetized iron, nickel, or other natural and

magnetic material. There are only a few other occurring magnetizable substances in nature!

The magnetic north and south poles move slowly and erratically over the earth's surface.

This makes it probable that the magnetic material is liquid and swirls.

4. To combine

The densities of pure nickel and iron are respectively 8.90 and 7.87 ton/m^{3}.

Which come close to the calculated value of the density of the interior of the earth.

Therefore, it is plausible that the interior of the earth has been made of nickel and/or iron.

Due to the high temperature the metal has been molten.

5. Waves

A wave moves through a certain type of substance.

If there is a boundary layer with a different substance which is reached by the wave non-orthogonally

then the wave splits into two parts:

a portion of the wave is reflected back from the boundary layer and

the other part passes through the boundary layer where wave refraction occurs.

The wavelength and the wave speed are changed, the frequency is maintained.

6. Earthquakes

With earthquake waves the Earth can be 'X-rayed' and so we can learn more about the earth's interior.

7. Sources of earthquakes

The location of the source of an earthquake hypocenter is almost always close to the surface,

right under the epicenter of the earth's surface.

Huge amounts of kinetic energy of the continental plates are gradually converted into spring energy

as a result of protrusions hooking to each other. The kinetic energy is quite large despite the speed

of centimeters per year but thanks to the incredible mass of the continental plates.

During unhooking the slowly accumulated spring energy suddenly transforms into kinetic energy:

an earthquake source.

The shearing can be both horizontally and vertically. In the latter case, one nature plaice presses

the other one up or down. Most earthquakes are caused by the above-described process.

Also collapsing caves, volcanic eruptions, artificial underground explosions and crashing meteors

can cause earthquakes.

8. Kinds of earthquake waves

Two types of earthquake waves come from the hypocenter and go through the interior of the Earth.

The one kind is a transversal wave that only can go through solid state.

The second type is a longitudinal wave or a pressure wave that also can travel through fluids.

The one wave velocity is roughly twice as large as the other speed.

The third kind is a wave, which moves over the surface of the earth.

The stone topsoil then moves about like a wave. That wave has a different speed again.The seismographs record the progress of an earthquake. The event is recorded on paper or as data inVictims

Many survivors of earthquakes have severe trauma to a natural disaster.

It's good to just thinking about it.

a computer. Later in peace precise measurements and data can be evaluated as exactly as possoble.

A sudden, brief vibration of the earthquake source always gives at least three records at a seismometer:

each of the three aarbevingsgolven has its own speed. They are therefore in their own time to.

Why there are more than three vibrations are observed is explained in the following model.

9. A simple model

We assume the earth is a homogeneous stone spherical shell. Moreover, we assume the speed of each of

the waves is independent of the direction of the wave through the rock. The latter is not obvious!

(Think about anisotropy.)

In the remaining hollow, spherical space, the core, is a homogeneous liquid.

See the schematic drawing of this model of the earth.

Bron = source

1 is the direct wave from the hypocenter to a seismometer.

2 and 3 are waves each reflecting once.

4 undergoes two refractions during the travel.

5 shows four refractions and a reflection.

Seismometers

One picture represents a seismometer for the vertical vibration records.

The other picture registers horizontal vibrations.

The waves 1, 2 and 3: each arrives twice, because both occur transversal and longitudinal.

The waves 4 and 5 are only longitudinally.

In the seismometer we can record as a result of a tremor of the hypocenter least eight times a

vibration. There are also the surface waves.

10. The reality

In reality the happening is much more complicated: in the solid part of the earth, many boundary

surface from one type of rock to another. Also these boundery faces are not all spherical.

The hypocenter does not give off a clearly defined "signal". A series of shocks is quite normal.

The analysis of the received vibrations at the seismometer, which partially overlap each other

is not a very easy job.

11. Global cooperation

By comparing the observations of all cooperating seismological stations we gradually know

how the earth has been built up in its layers more and more accurately.

Actually the earth is scanned by earthquake waves.

Summery

The reflections on the density, magnetic and seismic properties give a fairly detailed picture of

the interior of the earth.

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