【Science】Grade 7: The Making and Changing of the Earth
Hello everyone! Let's explore "The Making and Changing of the Earth" together.
The ground beneath our feet might seem still, but it has actually been undergoing dynamic changes over a long period of time. Volcanoes, earthquakes, and strata (rock layers)—once you understand how these are formed, the everyday scenery will look completely different!
It might feel difficult at first, but we will go through it one step at a time using familiar examples. Don't worry, you'll definitely get it if we work on it together!
1. Volcanic Activity and Igneous Rocks
Magma and Volcanic Shapes
Deep underground, melted rock is called magma. When it reaches the surface, it is called lava.
The shape of a volcano is determined by the "viscosity" (stickiness) of the magma!
・Low viscosity (runny)
Shape: Flat (e.g., Mauna Loa)
Eruption: Gentle
Color: Dark/Blackish
(Imagine: Pancake batter spreading out thin!)
・High viscosity (thick/gooey)
Shape: Dome-shaped mound (e.g., Showa-shinzan)
Eruption: Explosive
Color: Light/Whitish
(Imagine: A mound of stiff modeling clay!)
・Intermediate
Shape: Beautiful cone shape (e.g., Mt. Fuji). This is called a stratovolcano.
Types of Igneous Rocks
Rocks formed when magma cools and hardens are called igneous rocks. They are divided into two groups based on where they cool.
1. Volcanic rocks: Formed when magma cools quickly near the surface.
Characteristics: They have a "porphyritic texture," where small crystals (phenocrysts) are scattered throughout a fine-grained base (groundmass).
2. Plutonic rocks: Formed when magma cools slowly deep underground.
Characteristics: They have an "equigranular texture," where large, tightly packed crystals form.
(Tip: Taking time to cool down allows the crystals to grow large! It's like the difference between making ice cubes in a freezer versus taking your time to grow a large block of ice.)
【A Handy Trick for Memorizing!】
There is a famous Japanese mnemonic for remembering these igneous rocks:
"Shinkansen wa Kariage" (The bullet train gets a buzzcut!)
(Plutonic rocks - Shinkan-se...): Shi (Granite), n, ka (Diorite), n, se (Gabbro)
(Volcanic rocks - ...n wa Kariage): Ha (Rhyolite), ri (Andesite), age (Basalt)
*This varies slightly depending on the textbook, but learning it with this rhythm makes it much easier!
【Section Summary】
Remember that the volcano's shape depends on the magma's viscosity, and the rock's texture (appearance) depends on how fast it cools!
2. How Earthquakes Travel and the Earth's Interior
Seismic Waves and Propagation
When an earthquake occurs, two types of waves travel out.
・P-waves (Primary waves): Fast waves. They cause a small, rattling vibration (preliminary tremor).
・S-waves (Secondary waves): Slower waves. They cause a large, swaying shaking (main shock).
(Analogy: It’s like a fast runner, P, and a slow runner, S, starting at the same time. The farther they go, the larger the gap becomes between their arrival times!)
The time between the arrival of the P-wave and the S-wave is called the preliminary tremor duration (P-S time). The farther you are from the hypocenter, the longer this time becomes.
Formula: \( Distance to hypocenter = Velocity \times Time \)
Seismic Intensity vs. Magnitude (Commonly Confused!)
・Seismic Intensity (Shindo): The "strength of shaking" at a specific location. It is measured on a 10-level scale from 0 to 7 (with levels 5 and 6 split into "lower" and "upper").
・Magnitude (M): The "total energy" of the earthquake itself.
(Analogy: Think of the brightness (wattage) of a lightbulb as the Magnitude. The perceived brightness at different distances from the bulb is the Seismic Intensity!)
【Trivia】
Seismic intensity varies depending on the observation point, but there is only one Magnitude for a single earthquake!
【Section Summary】
Make sure to clearly distinguish between P-waves (fast, small) vs. S-waves (slow, large) and Seismic Intensity vs. Magnitude!
3. Strata and Past Environments
How Strata Form
Strata form through a cycle of weathering (breaking down), erosion (carving away), transportation (moving), and deposition (settling).
When particles settle on the ocean floor, larger particles (gravel) sink closer to the shore, while smaller particles (mud) settle further out (deeper water).
・Conglomerate: Particles over 2mm
・Sandstone: Particles between 0.06mm and 2mm
・Mudstone: Particles under 0.06mm
What Fossils Tell Us
Fossils found within rock layers are time capsules that tell us about the past!
1. Facies fossils: Fossils that tell us about the environment of the time.
(e.g., Coral = warm/shallow sea, Clams = shallow sea, Beech = cool climate)
(Mnemonic: "Fac-ies" means "face" or aspect of the environment!)
2. Index fossils: Fossils that tell us about the geological age (time period).
(e.g., Paleozoic = Trilobite/Fusulina, Mesozoic = Ammonite/Dinosaur, Cenozoic = Naumann Elephant/Vicarya)
(Remember these to know "what era" it is!)
Earth Changes Found in Strata
・Folding: Layers that have been bent due to pressure.
・Faults: Layers that have been shifted due to displacement.
・Tuff: Rock formed from accumulated volcanic ash. If this is found in a layer, it means there was an eruption nearby (it acts as a key bed).
【Common Mistake】
Limestone and Chert look similar, but Limestone releases gas (carbon dioxide) when you apply hydrochloric acid. Chert does not. This is a classic test question!
【Section Summary】
Generally, the lower the stratum, the older it is (unless the layers are overturned). Creating a chart to memorize the names of fossils and their corresponding eras is the best shortcut!
Great work! Learning about the Earth involves quite a bit of memorization, but it becomes much more interesting when you connect it to the rocks around you and the news about earthquakes and volcanoes. Start by learning the mnemonic tricks, and keep at it—I’m rooting for you!