【Basic Earth Science】Chapter 1: Earth's Activity 〜What’s happening beneath our feet?〜

Hello everyone! Let’s learn about "Earth’s activity" together.
You might feel like "Earth science has so much to memorize," but don't worry. If you imagine the Earth as a living "creature," everything becomes much easier to understand. Let's uncover the dynamic mechanisms behind how the land we live on was formed and why earthquakes and volcanoes occur!

It might feel tricky at first, but once you grasp the core points, this can become one of your strongest subjects, helping you score well on the Common Test. Let’s get started!

1. Internal Structure of the Earth: The Earth is like a "Hard-Boiled Egg"!?

What is the inside of the Earth like? Actually, it is divided into several layers based on depth. To understand this, it is best to imagine a "hard-boiled egg."

  • Crust: Corresponds to the "shell." It is the thin outer layer of rock.
  • Mantle: Corresponds to the "egg white." This layer, made of rock, accounts for about 80% of Earth’s volume.
  • Core: Corresponds to the "yolk." Located at the center, it is composed mainly of iron and nickel. The outer part, the "outer core," is liquid, while the inner part, the "inner core," is solid.

Key Point: Two Types of Crust

There are two types of crust: the continental crust, where we live, and the oceanic crust, which lies at the bottom of the ocean. This is a common exam topic!

① Continental Crust: Thick (about 30–50 km) but low density (lighter). It is mainly composed of granitic rocks.
② Oceanic Crust: Thin (about 5–10 km) but high density (heavier). It is mainly composed of basaltic rocks.

💡 Fun Fact: Remember the rule, "the heavier one sinks." Since the oceanic crust is heavier, it subducts beneath the continental crust when they collide.

【Summary of this section】
・The Earth consists of the crust, mantle, outer core, and inner core, in order from outside to inside.
・Continental crust is "thick and light"; oceanic crust is "thin and heavy."

2. Plate Tectonics: The Moving Earth

The Earth’s surface is covered by over a dozen massive rock slabs called plates. The theory that these plates float on the mantle and move slowly (a few centimeters per year, about the speed at which fingernails grow) is called plate tectonics.

Plate Boundaries: 3 Patterns

When plates move, intense activity occurs at their boundaries. Let's learn these three patterns!

① Divergent Boundary (Mid-ocean ridge):
A place where plates are pulling apart. The mantle rises to create new crust. Iceland is a rare place where this can be seen on land.

② Convergent Boundary (Trench):
A place where plates collide. The area where the heavier oceanic plate subducts is called a trench, and areas where continents collide and buckle upward are called fold mountains (like the Himalayas).

③ Transform Boundary:
A place where plates slide horizontally past each other. The San Andreas Fault in the U.S. is a famous example.

⚠️ Common Mistake: Don't mix up "ridges" and "trenches"!
・Ridge (Mid-ocean ridge): The "starting" point that forms a mountain-like shape.
・Trench: The "ending" (subduction) point that forms a deep, trench-like shape.

【Summary of this section】
・The Earth's surface is made of plates.
・There are three types of boundaries: "divergent (ridge)," "convergent (trench)," and "transform."

3. How Earthquakes Work and How Waves Travel

Earthquakes occur when the movement of plates applies stress to underground rock, causing the rock to eventually snap (creating a fault).

P-waves and S-waves: Which arrives first?

When an earthquake occurs, two types of waves are transmitted:
・P-wave (Primary wave): The first wave. It is fast and has small vibrations (vertical motion).
・S-wave (Secondary wave): The second wave. It is slower but has larger vibrations (horizontal motion).

★ Memory Tip:
Think of P for "P-fast!" and S for "Slow and Super (strong vibration)!"

Preliminary Tremor Duration (PS Time)

The time between the arrival of the P-wave and the S-wave is called the preliminary tremor duration. The further you are from the epicenter, the longer this time becomes.
It can be expressed with the following formula:
\( d = k \times t \)
(\(d\): Epicentral distance, \(k\): Omori constant, \(t\): Preliminary tremor duration)

Difference between "Seismic Intensity" and "Magnitude"

Be careful, this is easy to confuse!
・Seismic Intensity (Shindo): The "strength of shaking" at a specific location. It is expressed on a scale of 0–7 (5 and 6 are divided into 'lower' and 'upper').
・Magnitude (M): The "amount of energy (scale)" of the earthquake itself.

Analogy: It’s easy to understand if you compare it to a light bulb.
The "brightness of the bulb itself (wattage)" is the magnitude.
The "brightness at a location away from the bulb" is the seismic intensity.
Even a high-magnitude earthquake will have a low seismic intensity if you are very far away.

【Summary of this section】
・P-waves are fast (preliminary tremor); S-waves are slow (main motion).
・The distance to the epicenter can be calculated from the difference in arrival times of P and S waves.
・Magnitude represents the scale of the earthquake, while intensity represents the strength of shaking.

4. Volcanoes and Magma: The Secret of the Goo

Rocks melted deep underground are called magma. The shape of a volcano and the style of its eruption depend on the properties of the magma.

Magma Properties and Volcanic Shapes

The viscosity (thickness) of magma changes depending on the amount of silica (\(SiO_2\)) it contains.

① Basaltic Magma (Low viscosity):
Runny. Gases escape easily, so eruptions are gentle. This forms shield volcanoes (like Mauna Loa) that spread out horizontally.
Color: Dark/Blackish

② Andesitic to Rhyolitic Magma (High viscosity):
Sticky/thick. Gases get trapped, causing explosive eruptions. This forms raised lava domes (like Showa-shinzan) or beautifully conical stratovolcanoes (like Mt. Fuji).
Color: Light/Whitish

💡 Fun Fact: Think of it this way: the sticker the magma, the harder it is for gas to escape, making it prone to "BOOM!" explosions. It's like the tension of opening a bottle of soda that’s been shaken.

【Summary of this section】
・Whitish magma = high viscosity = explosive eruption = dome-shaped.
・Blackish magma = low viscosity = gentle eruption = flat-shaped.

Closing

Great work! You have mastered the basics of "Earth’s Activity."
The trick to studying basic Earth science is to "try drawing the diagrams and pictures yourself." Once you can quickly sketch the plate diagrams or the different volcano shapes, the knowledge will stick firmly.
It might feel difficult at first, but try to enjoy your review by thinking of it as a grand, epic story happening right beneath your feet. I’m cheering for you!