Inside the Earth

Actual Condition Inside the Earth:

Now, what are conditions like inside the Earth? How can we describe the structure inside the

Earth? I’d like to start by giving a quiz. Which of the following answers do you think best describes conditions inside the Earth?

1: All areas inside the Earth are just homogeneous.

2: The inside of the Earth has a concentric circular structure like an onion. In other words, a layered structure extends itself from the inside towards the outside with layers composed of different substances.

3:The inside of the Earth has a structure made of different radial blocks.

The correct answer is 2.

The Earth has a structure like an onion. From the perspective of different substances, the entire Earth can be broadly classified into three layers. The first layer is right under our feet. Known as the crust,

it is formed of rocks. Underneath the crust is the mantle, which is the second layer. The mantle is also composed of rocks and composes 85% of the Earth’s total volume. Under the mantle is the third layer, the core. The core is a mass of metal which composes the very center of the Earth. So, to review, the Earth is composed of a metallic core at the center. This is surrounded by the mantle, which is made of rocks.

The outside layer is the crust, which is very thin and is made of rocks that differ from the rocks of the mantle. When divided very roughly, the Earth consists of these three layers. The next question to consider is whether the inside of the Earth is composed of solid substances or thick molten substances. I’m sure that all of you have seen photographs of magma erupting from a volcano. This magma rises up from deep beneath the volcano. This indicates that, at the very least, some portion of the Earth is molten. The real question is whether the Earth’s inside is almost all molten, or if molten substances constitute only a small portion of our planet. Also, assuming that only a portion of the Earth is molten, then what is the approximate percentage of that portion?

The answer is that only 15% of the Earth’s volume is molten. The remaining 85% is solid. To give a slightly more detailed explanation, the crust under our feet is, generally speaking, completely solid. The underlying mantle is also solid. Magma is not located evenly throughout the entire interior of our planet instead, only a small amount of magma exists directly under active volcanoes. The entirety of the crust and mantle is basically solid.

On the other hand, the metal lump which is located at the Earth’s center and is known as the core is mostly molten. Although a solid area known as the inner core exists at the very center, this inner core composes only 4% of the core’s total volume. This means that the core is almost completely molten.

An overall rough image of the Earth can be stated as follows: the Earth consists of a central molten core surrounded by solid layers known as the mantle and crust.

Data on the three layers of the earth:

Data on the three layers of the earth is summarized in the table below. Try using the data to imagine more concretely the form of each layer.

Methods for Examining Inside Earth:

1. Dig it up?

The deepest hole ever dug by humans is the 12.262 km deep hole on Russia’s Kola Peninsula. They began digging in 1970 as part of an earth science survey, and continued until 1992. They originally planned to dig to 15km, but the temperature in the earth reached 180°C at 12km, and it was predicted that the drill would stop operating, so further excavation was stopped. (180°C at 12km is actually relatively mild for a temperature increase in the earth interior) The boundary between granite and basalt was surpassed, but they were unable to cross the earth crust/mantle boundary.

2. Take a look!

Is it therefore impossible to obtain peridotite?

There are actually places on earth where rocks from deep within the earth have been driven up to the surface by past crustal movements. These are called ophiolite. By observing these places we can learn about rocks from 20 or 30 km deep.

3. Pick it up!

So how can we learn about even deeper places?

The answer is to find xenolith. Xenolith is rock from deep within the earth that gets carried along by rising magma. In many cases it has been altered by heat, but not always. Xenolith made presumptions about the chemical composition of the upper mantle possible.

A seismic wave observation:

A method for investigating the entire earth is to use seismic waves.

Earthquakes have a very scary image, but at the same time they are an excellent tool for investigating the inside of the earth.

So why can we understand the earth interior with seismic waves?

Because waves have the following properties.

(1) The arrival velocity and location of a wave changes depending on the state of the materials it passes through.

(2) If there is a liquid layer in the earth interior, a shear wave cannot pass through it.

(3) Reflections and refractions occur depending on the boundaries and density changes of layers.

Now let’s take a look at a shorter example of waves being used to “see what cannot be seen.”

1. Fish finder and sonar of submarines:

The deep sea is a pitch black world. You cannot make out obstacles and schools of fish lurking in the distance. By emitting sounds from a ship, and observing the repercussions, it can be known where obstacles or schools of fish are. (If there is an obstacle, the sound bounces back. The closer it is, the sooner the sound bounces back!) In this way the invisible deep sea can be made visible.

2. Inside the stomach of a pregnant woman:

Waves are also useful when looking inside a womb of a pregnant woman. By emitting ultrasound waves from a machine, and observing the repercussions, the form of the baby becomes visible.

3. Seismograph:

We have thus learned how even in our everyday lives, waves are useful for “seeing” places which cannot be seen. A device for emitting waves and a device for observing waves are necessary to make things visible with waves.

It is not possible to artificially create waves the size of the earth. Therefore earthquakes are used as the wave source. The observation device is a globe-spanning seismic wave observation network. This observation network not only protects people from earthquakes, but also teaches people the figure of the earth.