Earthquakes and volcanoes
AdvancedPlates store stress; the sudden break is an earthquake — focus underground, epicentre on the surface, magnitude on the Richter scale. Most quakes and volcanoes cluster on the Ring of Fire.
- Focus underground; epicentre on the surface
- Magnitude = energy; intensity = damage
- Safety: Drop, Cover, Hold On
§Text Passage
1Why do some places shake and erupt? This lesson links earthquakes and volcanoes to plate boundaries: an earthquake happens when rock underground suddenly breaks and releases energy that spreads out as seismic waves; a volcanic eruption occurs when molten rock finds a weak line and pushes through to the surface.
2Key ideas include focus, epicentre, magnitude and intensity: the focus is where the rock breaks, and the ground directly above it is the epicentre. Magnitude measures how much energy is released, while intensity describes how strongly a particular place shakes. The thickness of magma also decides whether a volcano erupts calmly or explodes violently.
3The examples are sobering: the 2011 Tohoku earthquake in Japan reached magnitude 9 and triggered a tsunami; the Pacific Ring of Fire concentrates most of the world's active volcanoes. Hong Kong sits away from the plate edges — the most notable earthquake nearby was in the South China Sea in 1918 — so tremors are rarely felt here.
1Summary 重點摘要
Plates store stress; the sudden break is an earthquake — focus underground, epicentre on the surface, magnitude on the Richter scale. Most quakes and volcanoes cluster on the Ring of Fire.
2Vocabulary cards Vocabulary
The underground point where the break starts and energy is released.
The point on the surface directly above the focus, usually worst hit.
The size of energy released, often on the Richter scale.
The zone with most of the world's quakes and volcanoes — the Ring of Fire.
Molten rock underground; once it erupts it is called lava.
A mountain formed when magma pushes through weak lines in the crust, erupting lava and ash.
Giant waves raised by undersea earthquakes that can flood coastal areas.
How strongly a place shakes and suffers; it varies from place to place in the same quake.
3Step by step Steps
- Where quakes come fromPlate movement keeps building up stress in the rock layers; when the stress exceeds what the rock can bear, the rock suddenly fractures and releases the stored energy — that is an earthquake. The energy spreads in every direction from the underground focus as seismic waves, and the point on the surface directly above the focus is the epicentre. Hong Kong lies on no plate boundary and its crust is comparatively stable, so earthquakes that can be felt are rare.
- Two ways to measureEarthquakes are measured with two different rulers. Magnitude (the Richter scale) measures the energy released: each step of 1 means about 32 times more energy, and one earthquake has only one magnitude. Intensity measures the shaking and damage felt at a place, and for the same earthquake it differs from place to place, weakening with distance from the epicentre. In answers, keep clear what each ruler measures, and you avoid the most common mistake.
- Distribution and self-protectionMost of the world's earthquakes and active volcanoes cluster on the circum-Pacific belt, nicknamed the Ring of Fire; Japan, Taiwan, the Philippines and the west coasts of the Americas all sit upon it. In a quake, remember the rule 'Drop, Cover, Hold On': drop down at once, take shelter under a sturdy table, shield your head and neck with your arms, and stay away from windows and objects that can fall. Practise the drills seriously, for they save lives when it matters.
- Examples to rememberTwo examples are worth memorising. In 2011 a magnitude 9 earthquake struck off north-east Japan and set off a huge tsunami, causing massive loss of life and a nuclear accident. Hong Kong, far from the plate edges, saw its most notable earthquake in the South China Sea in 1918, so tremors are rarely felt there. The contrast between the two places shows exactly how distance from a plate boundary decides the level of risk.
- Volcanoes have two tempersThe viscosity of magma decides how a volcano erupts. Runny magma flows freely and lets its gas escape, so eruptions are calm and lava pours out gently — the volcanoes of Hawaii, where visitors can watch in safety nearby. Sticky magma traps its gas until the pressure bursts out in a violent explosion of ash and volcanic bombs, like Mount St. Helens in the United States in 1980. Different eruption styles give volcanoes utterly different shapes and levels of danger.
- Undersea quakes and tsunamisWhen the focus lies beneath the sea, the sudden fracture heaves the whole mass of water upward and a tsunami is born. Far offshore the waves stand no more than tens of centimetres high and ships may not notice, but as the tsunami nears the coast and the water shallows, the waves stack up to a dozen metres or more, smashing houses and facilities ashore. The 2004 South Asian tsunami and the 2011 Japan tsunami both took a heavy toll of life. If the sea rushes out abnormally or a warning is issued, run for high ground at once and never stay to watch.
4Diagrams Diagrams
5Worked example Example
Example: why does Japan have so many earthquakes and volcanoes?
- Japan sits at the boundary of the Pacific and Eurasian plates
- The oceanic plate subducts and stress keeps building
- Sudden breaks = earthquakes; rising magma = volcanoes
- Inference: living on a boundary means high risk, which is why Japan drills for disasters
- Answer: Japan sits right on the convergent boundary of the Pacific and Eurasian plates and on the Ring of Fire: the subducting oceanic plate keeps piling up stress — sudden breaks trigger earthquakes, and magma rising through cracks builds volcanoes. Living on a boundary means high risk, which is why Japan drills so hard and builds to shake-proof standards; most nations on the Ring run disaster-prevention systems.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the difference between focus and epicentre?
Q2What is the world's main earthquake–volcano belt called?
Q3Roughly how much more energy does each Richter step release?
Q4Why do Hawaii's volcanoes erupt calmly while Mount St. Helens in the USA exploded violently?
Q5How does a tsunami form, and what should you do when a tsunami warning is issued?
8Recap Recap
- Focus underground; epicentre on the surface
- Magnitude = energy; intensity = damage
- Safety: Drop, Cover, Hold On
- Most active volcanoes and strong quakes cluster on the Ring of Fire
- Hong Kong lies far from plate boundaries, so quake risk is low