The First Step in Geoscience! Let's Master the "Overview of the Earth"
Hello, everyone! Welcome to the world of "Geoscience." Our first theme is the "Overview of the Earth." Let's explore what kind of shape this planet we call home actually has and what it’s like inside!
You might be thinking, "Geoscience sounds difficult with all the memorization..." but don't worry! If we take it one step at a time and compare things to familiar objects, you'll definitely be able to understand it. Let’s start by getting a rough picture of the Earth as a whole!
1. Shape and Size of the Earth
Do you think the Earth is a perfect sphere? Actually, strictly speaking, it's a little bit distorted.
① Shape of the Earth: Geodetic Ellipsoid
Due to the centrifugal force caused by its rotation, the Earth bulges slightly at the equator, taking the shape of an "oblate spheroid." We call this the geodetic ellipsoid.
Think of it like a "tangerine that has been slightly squashed from the top and bottom, making it bulge around the middle."
【Key Points】
・Equatorial radius (approx. 6378 km) > Polar radius (approx. 6357 km)
・This slight difference (approx. 21 km) is evidence that the Earth is rotating!
② Geoid (The shape based on sea level)
When considering the "shape of the Earth," another important concept is the geoid. This refers to a hypothetical surface formed by extending the sea level under the continents, assuming the ocean is still. Think of it as a surface that ignores the Earth's uneven surface and connects points perpendicular to the direction of gravity (the plumb line).
【Trivia】 Eratosthenes' Calculation
Around 230 B.C., a man named Eratosthenes calculated the circumference of the Earth using nothing more than "sunlight entering a well" and the "length of a shadow." With the technology of that time, his margin of error was only a few percent! Isn't that amazing?
【Summary of this section】
The Earth is a geodetic ellipsoid that bulges at the equator due to the centrifugal force of its rotation!
2. Internal Structure of the Earth (Let's compare it to an egg!)
We cannot look directly inside the Earth. However, by studying how seismic waves travel, we know that the interior is "layered." The structure of the Earth is often compared to a "hard-boiled egg."
① Crust: The "shell" of the egg
The thin, outermost layer where we live.
- Continental crust: Approx. 30–50 km thick. Mainly composed of granite (lighter rock).
- Oceanic crust: Approx. 5–10 km thick. Mainly composed of basalt (slightly heavier rock).
② Mantle: The "egg white"
The layer extending from below the crust to a depth of about 2900 km. It accounts for about 80% of the Earth's volume!
It is mainly made of a rock called peridotite. While it is solid, it flows (convects) very slowly over long periods of time.
③ Core: The "egg yolk"
The center of the Earth, mainly made of iron and nickel.
- Outer core: Liquid. The movement of liquid iron generates the Earth's magnetic field.
- Inner core: Solid. Because of the extremely high pressure, it remains solid even though it is very hot.
【Common Mistake】
People often think "the mantle is a molten liquid," but it is actually "solid rock!" Just be careful to remember that it only moves very, very slowly. Only the outer core is liquid.
【Summary of this section】
From the outside in: Crust → Mantle → Core (Outer core & Inner core). Only the outer core is liquid!
3. Structure of the Earth's Atmosphere
The Earth's surface is surrounded by a layer of air (the atmosphere). The atmosphere is divided into four layers based on temperature changes. Let's look at them starting from the one closest to the ground.
① Troposphere
The layer from the surface up to about 11 km.
- Characteristics: Temperature decreases as you go higher (approx. \( 0.6^\circ C \) per \( 100m \)).
- Phenomena: Because it contains water vapor, weather phenomena like cloud formation and rain occur here.
② Stratosphere
The layer from about 11 km to 50 km high.
- Characteristics: Temperature increases as you go higher.
- Reason: The ozone layer absorbs ultraviolet rays from the sun and releases heat.
③ Mesosphere
The layer from about 50 km to 80 km high. Here, the temperature decreases again as you go higher. It contains the coldest place in the atmosphere (approx. \( -90^\circ C \)).
④ Thermosphere
The layer above 80 km. Because it receives strong radiation directly from the sun, the temperature becomes very high. This is also the layer where the aurora occurs.
【Memorization Tip】
Remember the temperature changes in a zigzag (Left, Right, Left, Right)!
1. Troposphere (Decreases ← Left)
2. Stratosphere (Increases → Right)
3. Mesosphere (Decreases ← Left)
4. Thermosphere (Increases → Right)
【Summary of this section】
Weather occurs in the troposphere. The ozone layer is in the stratosphere. Temperature changes are a zigzag!
4. The Earth's Surface (Land and Oceans)
Finally, let's confirm the layout of the Earth's surface.
● Ratio of Land to Sea
The Earth's surface area is composed of land and sea in a ratio of approximately 3 : 7. The ocean covers the vast majority of the planet.
● Elevation Distribution (Hypsographic Curve)
If you graph the elevations and depths of the Earth, two large peaks appear:
- Around 1 km above sea level (the average elevation of continents)
- Around 4–5 km below sea level (the average depth of the deep sea floor)
【Closing】
It might be hard to memorize the terms at first, but it gets fun once you visualize the "Earth as a living, hard-boiled egg!" This "overview" will be the foundation for the topics of earthquakes and volcanoes that we will study next. First, make sure you have the names of the internal structures and the four layers of the atmosphere down pat!
Great work today! Let's do our best together next time, too.