【Grade 7 Science】 The Earth in Motion! Summary Notes on "Earthquakes and Volcanoes"
Hello! In this chapter, we will learn about "earthquakes" and "volcanoes," which occur right here on the ground we live on. It might feel a little scary to think about the ground shaking or mountains erupting, but these are actually signs that the Earth is alive and active.
If you're thinking, "The math looks hard..." or "I'll never remember all the names of these rocks!", don't worry. I'll break it down into simple points with easy-to-understand examples!
1. How Earthquakes Travel and the Nature of Shaking
When an earthquake happens, you first feel small, rattling vibrations, followed by larger, swaying tremors. There is a reason for this sequence.
(1) The Two Types of Seismic Waves
When an earthquake occurs, two types of waves start at the same time: "P-waves" and "S-waves."
・P-wave (Primary wave): A faster wave. It arrives first, causing the small initial vibrations (preliminary tremor).
・S-wave (Secondary wave): A slower wave. It arrives later, causing the larger shaking (main tremor).
【Pro-tip for remembering!】
Remember: P-waves arrive "P-romptly" (fast), while S-waves arrive later with "S-evere" shaking!
(2) Preliminary Tremor Duration (P-S Time)
The time interval between the arrival of the P-wave and the arrival of the S-wave is called the preliminary tremor duration.
A key characteristic is that this duration gets longer the farther you are from the earthquake's focus (source).
Example: Just like when you see lightning and have to wait a while to hear the thunder; the farther away the storm, the longer the gap between the flash and the sound.
【Key Point!】
For earthquake math problems, we use the speed formula: \( \text{Speed} = \frac{\text{Distance}}{\text{Time}} \).
For example, to find the speed of a P-wave:
\( v = \frac{d}{t} \) (where \( v \) = speed, \( d \) = distance from the source, and \( t \) = time until arrival).
Just keep an eye on your units (like km/s)!
(3) Seismic Intensity vs. Magnitude
This is a spot where many students lose points on tests, so pay attention!
・Seismic Intensity (Shindo): The "strength of the shaking" at a specific location. It is measured on a 10-level scale (0–7, with 5 and 6 split into "upper" and "lower" levels).
・Magnitude (M): The "scale or energy" of the earthquake itself.
【Think of it this way!】
Imagine a lightbulb.
The brightness of the bulb itself (the wattage) is the Magnitude. The amount of light you feel at a specific distance from the bulb is the Seismic Intensity. Even if the bulb is huge (high M), it will look dimmer (lower intensity) the farther away you go, right?
【Common Mistake】
Don't mix them up! Remember: Seismic intensity changes depending on where you are, but an earthquake only has one Magnitude value.
◎ Summary of this section
・There are fast P-waves (preliminary tremors) and slow S-waves (main tremors).
・The farther from the source, the longer the preliminary tremor duration.
・Seismic intensity measures the shaking (10 levels), while magnitude measures the energy!
2. Volcanoes and the Nature of Magma
A mountain erupts because magma from underground is forced to the surface. The shape of the volcano and the way it erupts are determined by the "viscosity" (stickiness) of the magma.
(1) Magma Viscosity and Volcano Shape
Whether the magma is runny or thick determines the shape of the volcano.
1. Low viscosity (runny): Dark color. Gentle eruptions, creating a wide, shield-like shape (e.g., Mauna Loa).
2. Medium viscosity: Intermediate properties. A beautiful conical shape (e.g., Mt. Fuji, Sakurajima).
3. High viscosity (thick): Light color. Explosive eruptions, creating a domed shape (e.g., Showa-shinzan, Mt. Unzen-Fugen).
【Did you know?】
When the magma is very thick, it clogs the vent and pressure builds up, leading to explosive eruptions. It's like baking mochi; when the air bubbles get trapped and expand, it eventually pops!
(2) Volcanic Products
Everything ejected during an eruption is called volcanic products. This includes volcanic ash, volcanic bombs, pumice, and gases like volcanic gas (mostly water vapor).
◎ Summary of this section
・Light-colored volcanoes have thick, sticky magma and erupt explosively!
・Dark-colored volcanoes have runny magma and erupt gently!
3. Igneous Rocks and Minerals (What rocks are made of)
Rocks formed when magma cools and hardens are called igneous rocks. They are divided into two main groups.
(1) The Two Groups of Igneous Rocks
The names depend on how quickly they cooled.
・Volcanic rocks: Cooled quickly near the surface. They have a porphyritic texture, which is a mix of small groundmass and larger crystals called "phenocrysts."
・Plutonic rocks: Cooled slowly deep underground. They have an equigranular texture, where all the mineral grains have grown to be large.
【The ultimate mnemonic!】
It's hard to remember the names of these rocks, but this rhyme makes it easy:
"Shinkansen wa, kariage"
(Plutonic) Shin(Sei) - ka(Granite), ri(Diorite), a(Gabbro)
(Volcanic) Kan(Zan) - ryu(Rhyolite), a(Andesite), ge(Basalt)
*Remember: "Shin" comes first (Plutonic), followed by "Kan" (Volcanic).
(2) Light-colored Minerals and Dark-colored Minerals
The grains that make up the rocks (minerals) also have categories.
・Light-colored minerals: White or transparent. Examples: Quartz, Feldspar.
・Dark-colored minerals: Black or green. Examples: Biotite, Hornblende, Pyroxene, Olivine.
【Key Point!】
Light-colored rocks contain many light-colored minerals, and dark-colored rocks contain many dark-colored minerals.
◎ Summary of this section
・Fast cooling = Volcanic rock (porphyritic), Slow cooling = Plutonic rock (equigranular).
・Use "Shinkansen wa, kariage" to master the rock names!
It might feel like there's a lot to memorize at first, but try to visualize things—like the differences in seismic waves or the colors of volcanoes—and it starts to become much more interesting! Keep reading through these notes and try to connect them to things you see in the real world. You've got this!