Mount Halla and Mount Paektu are volcanoes, built long ago when magma from deep underground rose to the surface and piled up.
Magma, rock melted deep underground, is pushed upward, and when the pressure becomes too great it bursts out of a crater; magma that flows out onto the surface is called lava.
An earthquake is the shaking of the ground when the force built up by huge plates covering Earth's surface pushing against each other is released all at once.
In the first picture, change how strongly the volcano erupts; in the second, increase the force pushing the plates and see when an earthquake happens. You will also learn how to stay safe in an earthquake.
Why do volcanoes erupt?
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A volcano cut in half: magma rises from the chamber up the vent and erupts from the crater
🌋 Why volcanoes erupt
①Deep underground it is so hot that rock melts into magma
②Magma is lighter than the rock around it, so it rises and collects in a magma chamber
③When the gas and pressure in the magma grow too great, it bursts up the vent and out of the crater
④A volcano gives out not only lava but also volcanic ash and gases
Rocks made by volcanoes
Volcanic and plutonic rock
Volcanic rock and plutonic rock
Feature
Volcanic rock
Plutonic rock
Where it forms
On or near the surface (cools fast)
Deep underground (cools slowly)
Grain size
Small, hard to see with the naked eye
Large, visible with the naked eye
Examples
Basalt, rhyolite
Granite, gabbro
Look
Often full of holes left by gas
Sparkling grains you can see
💡 Cooling fast or cooling slowly?
①Magma that cools fast on the surface has no time to grow grains, so they are small (volcanic rock)
②Magma that cools slowly deep underground grows large grains (plutonic rock)
③Basalt, which Jeju's stone grandfather statues are carved from, is a volcanic rock full of holes
④Granite, often used for stone pagodas and floors, is a plutonic rock with large grains
Why do earthquakes happen?
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Two plates push against each other until the built-up force is released all at once in an earthquake
🌍 Why earthquakes happen
①Earth's surface is split into several huge plates that move very slowly
②Where plates meet, the force of them pushing against each other builds up little by little
③When the rock can no longer hold the force, it suddenly breaks or slips and the ground shakes
④In an earthquake, get under a table and protect your head, and once the shaking stops, go down the stairs and move to an open space
Summary
How a volcano erupts
Magma (underground) → pressure rises → erupts from the crater → lava
Rock melted deep underground comes up to the surface
How an earthquake happens
Plates move → force builds up → released at once → earthquake
Force built up by the plates pushing is suddenly released and the ground shakes
🎯 Key points
①A volcano is where magma from underground erupts from a crater
②Magma that cools fast has small grains, and magma that cools slowly has large grains
③An earthquake is the ground shaking when force built up by the plates pushing is released at once
④In an earthquake, protect your head under a table and use the stairs to leave once the shaking stops
⑤Volcanoes and earthquakes are both caused by energy inside Earth
Examples and Unit Test
Example 1
When a volcano erupts, what is the magma called that comes out of the crater and flows over the ground?
1
Hot magma rises from deep underground and the pressure builds up.
2
Magma that cannot hold the pressure bursts out of the crater, and magma flowing over the surface is called lava.
▸ Lava
It is called magma while it is underground and lava once it comes out onto the surface.
Example 2
Why do earthquakes happen? Can you explain using plates?
1
Earth's surface is split into plates, and as they push against each other, force builds up little by little.
2
When the built-up force is released all at once, the ground shakes; that is an earthquake.
▸ Because the force built up by plates pushing against each other is released all at once
This is why earthquakes are common in places on plate boundaries, such as Japan and Chile.
Unit-test style
Which statement about volcanoes and earthquakes is correct?
①Magma that erupts from a volcano is called an earthquake
②An earthquake happens when force built up by moving plates is released
③A volcano erupts because the inside of the Earth gets cold
④The more slowly a rock cools, the smaller its grains
⑤In an earthquake, you should leave quickly by lift
▸ ② An earthquake happens when force built up by moving plates is released
1
When force built up by plates pushing against each other is released all at once, the ground shakes in an earthquake.
2
Erupted magma is lava (①), a volcano erupts because of the pressure of hot magma (③), the more slowly rock cools the larger its grains (④), and in an earthquake you leave by the stairs (⑤).