Volcanoes
FoundationMagma rising through weaknesses at plate boundaries erupts as volcanoes — mostly at divergent or convergent boundaries. They destroy (lava, ash, pyroclastic flows) yet also bring fertile soil, geothermal energy and tourism.
- Volcanoes at boundaries or hotspots
- Hazards: ash, pyroclastic flows, lava; Benefits: soil, geothermal, tourism
- Same geology, both costs and gifts
§Text Passage
1Most volcanoes form at plate boundaries: the magma near a boundary is less dense than the surrounding rock and climbs upward through cracks, erupting once the pressure has built high enough. Molten rock underground is called magma and renamed lava once it erupts; as lava and ash cool, they pile up layer upon layer into a cone. The other cause is a hotspot, where a plume from deep in the mantle burns straight through the middle of a plate — the Hawaiian island chain was built island by island as the Pacific Plate drifted slowly over one.
2Volcanic hazards take many forms. Lava flows advance slowly enough for people to escape, yet they burn every forest and house in their path. Ash hurled into the sky can cripple jet engines — the cloud from Iceland's 2010 eruption paralysed European aviation for days — and thick falls crush roofs. The deadliest hazard of all is the scorching, fast-moving pyroclastic flow, which engulfs everything so quickly that residents have no time to flee.
3Yet volcanoes also give. Weathered ash turns into fertile soil that draws farmers to settle on the slopes; underground heat is harnessed as geothermal energy, heating about 90% of Icelandic homes; and hot springs with dramatic scenery boost tourism. By activity, volcanoes are classed as active, dormant or extinct — and High Island at Sai Kung in Hong Kong is the remains of a supervolcano about 140 million years old, its spectacular hexagonal rock columns the evidence that ancient eruption left behind.
4Eruptions usually give warning first: minor tremors multiply, the ground swells and deforms, and gas releases such as sulphur dioxide rise. Volcanologists track these changes from monitoring stations around the clock, buying time for evacuation; before Mount Pinatubo in the Philippines erupted in 1991, the authorities used monitoring data to evacuate tens of thousands of residents in time, sharply reducing casualties. Volcanic belts often overlap earthquake zones, because both are products of plate movement.
1Summary 重點摘要
Magma rising through weaknesses at plate boundaries erupts as volcanoes — mostly at divergent or convergent boundaries. They destroy (lava, ash, pyroclastic flows) yet also bring fertile soil, geothermal energy and tourism.
2Vocabulary cards Vocabulary
Molten rock underground is magma; once erupted it's lava.
Fine rock dust that can ground flights and collapse roofs.
Using underground heat for electricity or heating.
Decomposed ash is mineral-rich and great for farming.
A mantle plume burning through the middle of a plate, e.g. Hawaii.
The underground pocket of molten rock, feeding the crater through the main vent.
The opening at the volcano's top where lava and ash erupt.
Scorching gas mixed with rock debris racing downhill — the deadliest volcanic hazard.
Tracking tremors and gas changes to warn and evacuate early.
3Step by step Steps
- FormationMost volcanoes form at plate boundaries: near the margins, magma is less dense than the surrounding rock and climbs upward through cracks, erupting once the pressure builds high enough — which is why divergent and convergent boundaries are lined with volcanoes. The other cause is a hotspot: a mantle plume burns straight through the middle of a plate, and the Hawaiian island chain was built island by island as the Pacific Plate drifted slowly over such a hotspot.
- StructureInside a volcano, a magma chamber deep underground stores molten rock and connects to the surface through the main vent, opening at the crater; eruptions throw out lava, ash and volcanic gases. Watch the names: molten rock underground is called magma, renamed lava once it erupts. Lava and ash pile up layer upon layer as they cool, gradually building the cone, whose steepness depends on how sticky the lava is.
- HazardsVolcanic hazards take many forms. Lava flows advance slowly enough that people can usually escape, but they burn everything in their path, from forests to houses. Ash thrown high into the sky can cripple jet engines — the ash cloud from Iceland's 2010 eruption paralysed European aviation for days — and thick falls crush roofs and foul water supplies. The deadliest hazard is the scorching, fast-moving pyroclastic flow, which engulfs everything in an instant and leaves residents no time to flee.
- BenefitsVolcanoes are not all harm. Weathered volcanic ash turns into fertile soil, drawing many farmers to settle and cultivate on the slopes; underground heat is harnessed as geothermal energy — about 90% of Icelandic homes are heated geothermally, and the heat also generates electricity; hot springs and dramatic volcanic scenery further boost tourism and bring local income. Volcanoes are therefore the classic case of costs and benefits coexisting — the key is managing the risks.
- Volcano typesBy activity, volcanoes fall into three classes. Active volcanoes still erupt from time to time, such as Mount Etna in Italy. Dormant volcanoes have long been quiet but have erupted in history and may do so again — Mount St Helens in the United States, dormant for over a century, erupted violently in 1980. Extinct volcanoes show no eruption for 10,000 years. High Island in Sai Kung, Hong Kong is the remains of a supervolcano about 140 million years old, and its spectacular hexagonal rock columns are the evidence that eruption left behind.
- Monitoring and evacuationEruptions usually give warning signs first: minor tremors multiply, the ground swells and deforms, and gas releases such as sulphur dioxide rise. Volcanologists set up monitoring stations to track these changes around the clock, buying time for evacuation. Before Mount Pinatubo in the Philippines erupted in 1991, the authorities used monitoring data to evacuate tens of thousands of residents in time, slashing casualties — an international model of monitoring-based disaster reduction.
4Diagrams Diagrams
5Worked example Example
Why is Iceland both volcanic and a geothermal powerhouse?
- Iceland sits on the Mid-Atlantic Ridge (divergent boundary)
- Rising magma → frequent eruptions and quakes
- But the same heat → electricity and heating
- ~90% of homes heated geothermally
- Answer: Iceland sits right on the Mid-Atlantic Ridge, a divergent boundary: magma keeps rising through cracks, bringing frequent eruptions and quakes — yet the same underground heat generates electricity and heating, warming about 90% of homes, and hot springs draw tourists. One geological setting brings both hazard and clean energy: Iceland guards against eruptions with monitoring and evacuation while turning its 'weakness' into geothermal strength.
6Common mistakes Mistakes
7Quick check Quick check
Q1Magma vs lava — what's the difference?
Q2Give two volcanic hazards and two benefits.
Q3Hawaii's volcanoes aren't at a plate boundary — what causes them?
Q4How do active, dormant and extinct volcanoes differ?
Q5Does Hong Kong have volcanic remains?
8Recap Recap
- Volcanoes at boundaries or hotspots
- Hazards: ash, pyroclastic flows, lava; Benefits: soil, geothermal, tourism
- Same geology, both costs and gifts
- Deadliest: pyroclastic flows and ash; lava is slower and avoidable
- Monitoring tremors and gases buys time to evacuate early