Plate Tectonics and Earthquakes
FoundationThe Earth's crust is made of tectonic plates with three boundary types: divergent (pulling apart), convergent (crushing together) and transform (sliding sideways). Earthquakes cluster at plate boundaries — friction builds up energy, and a sudden release is an earthquake.
- Three boundaries: divergent, convergent, transform
- Quake = sudden stress release at boundaries
- Can't predict → mitigate with design, warnings, drills
§Text Passage
1Earth's outer shell is not one unbroken piece of rock but a dozen or so giant 'plates' fitted together like a jigsaw floating on the mantle. Each plate creeps along at only a few centimetres a year — about the speed at which fingernails grow — yet driven by convection in the mantle, that slow motion accumulated over millions of years is enough to raise towering mountain ranges or tear open an entire ocean.
2Plate boundaries come in three types. At divergent boundaries, plates pull apart and molten material wells up through the cracks to cool into new crust — the Mid-Atlantic Ridge is the classic example. At convergent boundaries, plates push together and the crumpling crust rises into high mountains — the Himalayas are the product of India colliding with Eurasia. At transform boundaries, plates grind sideways past each other along faults, which is why California's San Andreas Fault is so quake-prone.
3Earthquakes are the result of this tug-of-war. When neighbouring plates lock together, stress keeps building in the rock for decades or even centuries; once the rock can bear no more it fractures and slips, releasing the stored energy in seconds as seismic waves that shake the ground violently, bringing building collapse, landslides and fires. The 2008 Wenchuan earthquake in Sichuan left more than 80,000 people dead or missing, and quakes under the sea can disturb the water and unleash tsunamis — the 2004 Indian Ocean tsunami claimed more than 200,000 lives.
4Since earthquakes still cannot be predicted accurately, mitigation rests on preparation: building to strict seismic codes, running early-warning systems and holding regular drills. Hong Kong sits inside the Eurasian Plate, some 600 km from the nearest active boundary, so the risk of a strong quake is low — yet the fact that global earthquake belts overlap plate boundaries exactly reminds us that plate movement is the fundamental force shaping the Earth's surface.
1Summary 重點摘要
The Earth's crust is made of tectonic plates with three boundary types: divergent (pulling apart), convergent (crushing together) and transform (sliding sideways). Earthquakes cluster at plate boundaries — friction builds up energy, and a sudden release is an earthquake.
2Vocabulary cards Vocabulary
A huge slab of the Earth's crust that moves slowly.
The point on the surface directly above the focus — usually the strongest shaking.
The underground point where the earthquake starts.
Measures earthquake energy — each step up is about ×32 in energy.
The zone where two plates meet — the crust is most active there.
A boundary where plates pull apart, e.g. the Mid-Atlantic Ridge.
A boundary where plates crush together and raise mountains, e.g. the Himalayas.
A boundary where plates slide sideways past each other, e.g. the San Andreas Fault.
Energy waves released by an earthquake, spreading out from the focus.
Giant waves set off by undersea earthquakes, devastating coasts.
3Step by step Steps
- Three boundary typesPlate boundaries come in three types. At divergent boundaries, plates pull apart and molten material wells up through the cracks to cool into new crust — the Mid-Atlantic Ridge is the classic example. At convergent boundaries, plates push together, crumpling and uplifting the crust into high mountain ranges — the Himalayas are the product of India colliding with Eurasia. At transform boundaries, plates grind sideways past each other along faults, which is why California's San Andreas Fault is so quake-prone.
- Energy builds and releasesPlates creep along at only a few centimetres a year — about the speed at which fingernails grow — but when neighbouring plates lock together, stress keeps building in the rock for decades or even centuries. When the rock can bear no more, it fractures and slips, and all the stored energy is released in seconds as seismic waves that shake the ground violently. This also explains why earthquakes cluster along plate boundaries rather than in plate interiors.
- HazardsEarthquake hazards include building collapse, landslides and fires, and casualties can be devastating — the 2008 Wenchuan earthquake in Sichuan left more than 80,000 people dead or missing. In general, the closer to the epicentre, the stronger the shaking. Quakes under the sea violently disturb the water and can unleash tsunamis that crash into coasts; the 2004 Indian Ocean tsunami killed more than 200,000 people, one of the deadliest disasters of modern times.
- MitigationEarthquakes still cannot be predicted accurately, so mitigation is all about preparation: build to strict seismic codes, as Japan does; operate early-warning systems that give seconds to tens of seconds of notice before strong shaking arrives; and run regular drills so people know exactly what to do. Hong Kong sits far from active plate boundaries and faces relatively low quake risk — but never trust 'earthquake predictions' that have no scientific basis.
- Where earthquakes strikeGlobal earthquakes cluster along two major belts. The Circum-Pacific Belt, known as the 'Ring of Fire', carries Japan, Taiwan, Indonesia and Chile and hosts about 80% of the world's strongest quakes. The Eurasian–Himalayan Belt runs from the Mediterranean all the way to the Himalayas. These belts overlap plate boundaries exactly — the strongest evidence of plate activity, and the reason countries sitting on them suffer quakes so often.
- Hong Kong and earthquakesHong Kong sits inside the Eurasian Plate, roughly 600 km from the nearest active boundary, so the risk of a strong quake is low. Still, the Hong Kong Observatory records a few slight tremors every year, mostly originating from Taiwan, Guangdong and the South China Sea; the 1918 quake near Nan'ao shook the city. When shaking starts: Drop, Cover and Hold On — shelter beside a sturdy table and protect your head and neck, and never rush outside blindly while the ground moves.
4Diagrams Diagrams
5Worked example Example
Why does Japan have so many earthquakes?
- Japan sits at the junction of four plates (Pacific, Philippine Sea, Eurasian, North American)
- Multiple collision/subduction zones concentrate stress
- Hence extreme earthquake and tsunami risk
- Response: world-leading building codes and warning systems
- Answer: Japan sits at the junction of the Pacific, Philippine Sea, Eurasian and North American plates; multiple collision and subduction zones concentrate stress, so energy is released frequently and quake and tsunami risks are extreme. Quakes cannot be predicted, so Japan mitigates with world-leading quake-resistant building codes, early-warning systems and nationwide drills, keeping the damage to a minimum — an example for other countries.
6Common mistakes Mistakes
7Quick check Quick check
Q1Name the three plate boundary types with one example each.
Q2What's the difference between the epicentre and the focus?
Q3Why do earthquakes cluster at plate boundaries?
Q4Name the two major global earthquake belts.
Q5Why is Hong Kong's earthquake risk relatively low?
8Recap Recap
- Three boundaries: divergent, convergent, transform
- Quake = sudden stress release at boundaries
- Can't predict → mitigate with design, warnings, drills
- Focus underground; epicentre directly above it
- Each Richter step ≈ 32× the energy