Tectonic Hazards: Earthquakes and Volcanoes
FoundationThe lithosphere is broken into plates that drift apart, collide or grind past one another at boundaries, building fold mountains, trenches and mid-ocean ridges; earthquakes and volcanoes therefore line up along plate boundaries, most densely on the Ring of Fire.
- The lithosphere is split into plates driven by mantle convection, moving a few centimetres a year
- Three boundary types: divergent (ridges, rift valleys), convergent (trenches, island arcs, fold mountains), transform (faults)
- Earthquakes and volcanoes line up along boundaries; the Ring of Fire holds about 80 per cent of large quakes and three quarters of active volcanoes
文體小知識：自然系統說明文——講解地球內部結構與板塊移動如何塑造地貌並引發地震和火山爆發，是「機會與風險」單元的科學基礎。
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1The Earth's interior has three layers: the crust, the mantle and the core. The outermost lithosphere — the crust plus the stiff uppermost mantle — is about 100 km thick and broken into plates of different sizes. The plates rest on softer mantle material and are driven by mantle convection, moving a few centimetres per year.
2Where two plates meet is a plate boundary. Boundaries fall into three types: divergent boundaries where plates pull apart, convergent boundaries where plates collide, and transform boundaries where plates grind past each other. The world has more than a dozen major plates, including the Pacific, Eurasian and Indo-Australian plates; activity is most intense along their boundaries.
3Different boundaries build different landforms. Along convergent boundaries, continental crust is squeezed into fold mountains such as the Himalayas; where oceanic crust dives beneath another plate, ocean trenches, island arcs and volcanoes form. Along divergent boundaries, rising magma hardens into mid-ocean ridges and rift valleys.
4Earthquakes and eruptions are not scattered randomly but form belts. The most important runs around the Pacific rim — the Ring of Fire — concentrating about 80 per cent of the world's largest earthquakes and about three quarters of its active volcanoes; another belt crosses southern Eurasia. These belts match the plate boundaries closely.
5An earthquake is the shaking released when rocks suddenly fracture and slip; a volcanic eruption throws magma, gases and fragments onto the surface. Most earthquakes and volcanoes sit on plate boundaries, but a few lie far away — Hawaii in the mid-Pacific, for example, sits over a mantle hotspot, an exception that must not be ignored.
1Summary 重點摘要
The lithosphere is broken into plates that drift apart, collide or grind past one another at boundaries, building fold mountains, trenches and mid-ocean ridges; earthquakes and volcanoes therefore line up along plate boundaries, most densely on the Ring of Fire.
2Vocabulary cards Vocabulary
The theory that the lithosphere is broken into plates which, driven by mantle convection, shape landforms and cause tectonic hazards.
Where two plates meet; crustal activity is most intense, so earthquakes, volcanoes and tectonic landforms concentrate here.
A boundary where plates move apart; rising magma creates new crust, forming mid-ocean ridges and rift valleys.
A boundary where plates collide; oceanic crust subducts, forming trenches, island arcs, fold mountains and volcanoes.
A boundary where plates slide past one another without crust being made or destroyed; friction stores energy released as earthquakes.
A long, deep zone where oceanic crust subducts beneath another plate; the deepest places on Earth.
Mountains formed when rock layers are squeezed and buckled upwards, such as the Himalayas.
A fixed, unusually hot zone in the mantle where magma keeps rising; plates moving over it form volcanic chains such as Hawaii.
The earthquake and volcanic belt around the Pacific rim, holding about 80 per cent of large earthquakes and three quarters of active volcanoes.
The point on the surface directly above the focus; shaking and damage are usually strongest nearest to it.
3Step by step Steps
- Step 1: Inside the Earth and its platesThe Earth consists of the crust, mantle and core. The lithosphere on the outside — crust plus the top of the mantle — is about 100 km thick and split into more than a dozen major plates. Beneath it, mantle material is soft and flows slowly. For example, think of breaking the skin on a bowl of custard: the pieces drift on the flowing layer below. Plates move on mantle convection at a few centimetres a year — thousands of kilometres over tens of millions of years.
- Step 2: The three types of boundaryBoundaries fall into three types by direction of movement. At divergent boundaries plates pull apart and magma rises to make new crust; at convergent boundaries plates collide, with denser oceanic crust subducting or continental crust squeezing together; at transform boundaries plates grind past each other. For example, the San Andreas Fault in California is a famous transform boundary: the Pacific and North American plates slip past one another and stored friction energy is released as earthquakes.
- Step 3: Landforms at convergent boundariesLandforms here depend on the crust involved. Where oceanic crust dives beneath a continent, a deep trench forms with island arcs and volcanoes rising outside — the Mariana Trench in the western Pacific is over 10 km deep. Where two continents collide, rock layers buckle into fold mountains. For example, the Indo-Australian Plate crashed into the Eurasian Plate, squeezing ancient seabed sediments into the Himalayas, which still rise a few millimetres a year.
- Step 4: Landforms at divergent boundariesAt divergent boundaries plates are pulled apart; magma wells up along the crack and hardens into new oceanic crust, forming mid-ocean ridges that run tens of thousands of kilometres across the seafloor. If rifting happens inside a continent, the crust drops into a rift valley with steep walls. For example, the Mid-Atlantic Ridge is where the American plates separate from the Eurasian and African plates; Iceland sits right on the ridge, where cracks, hot springs and volcanoes can be seen side by side.
- Step 5: How an earthquake happensPlate movement stresses rocks at boundaries until they deform; when rocks fracture and slip, energy is released as seismic waves in all directions, shaking the ground. The source underground is the focus, and the point on the surface above it is the epicentre. Strength is described on two scales: magnitude measures the energy released, usually on the Richter scale, while intensity measures the degree of damage at a place. For example, the 2008 Wenchuan earthquake in Sichuan reached magnitude 8; intensity and damage were highest near the epicentre.
- Step 6: Eruptions and the distribution patternSubducting plates carry water-bearing minerals deep into the mantle; heating releases fluids that lower the melting point of the surrounding rock, producing viscous magma. Magma rises and accumulates until pressure drives an eruption. Volcanoes, like earthquakes, form belts: the Ring of Fire holds about 80 per cent of large earthquakes and three quarters of active volcanoes. For example, Japan, the Philippines, Indonesia and western South America all lie on this belt — the great 1991 eruption of Pinatubo in the Philippines occurred above a subduction zone.
- Step 7: The exception away from boundaries — hotspotsNot every volcano sits on a plate boundary. Fixed plumes of unusually hot rock in the mantle, called hotspots, keep feeding magma that burns through the plate; as the plate drifts over the hotspot, a chain of volcanic islands is left behind. For example, Hawaii lies in the middle of the Pacific Plate, thousands of kilometres from the nearest boundary; the south-eastern island still has active volcanoes, and islands grow older to the north-west — recording the plate's direction of motion. Citing hotspots explains the exceptions on distribution maps.
4Diagrams Diagrams
5Worked example Example
Map reading and data comparison: the map shows the world's plate tectonics with boundaries marked by heavy lines. (a) State the relationship between global earthquake belts and plate boundaries. (b) Both the 2008 Wenchuan earthquake in Sichuan (magnitude 8) and the 2011 '3.11' earthquake in Japan (magnitude 9) were huge tectonic events. Using plate tectonics, compare their causes.
- Read the question: part (a) asks for a distribution pattern — use words like 'closely match'; part (b) asks for a comparison of causes — locate each quake first, then analyse
- For (a): the global belts — the Ring of Fire and the Trans-Eurasian belt — almost coincide with plate boundaries, because boundaries are where rocks are stressed and fractured most often
- For (b), locate them: Wenchuan lay on the Trans-Eurasian belt, on an inland fault squeezed by the collision of the Indo-Australian and Eurasian plates; 3.11 lay on the Ring of Fire, where the Pacific Plate subducts beneath the North American plate
- Compare causes: both were sudden releases of energy as rocks slipped; but Wenchuan was a continental-collision quake, while 3.11 was a giant subduction-zone rupture with a far larger fault area — hence magnitude 9 and a tsunami
- Conclude: distribution is decided by plate boundaries; the difference in causes comes from boundary type and crustal nature — both layers, 'pattern plus cause', are needed
- Answer: For part (a), the world's earthquake belts — the Ring of Fire and the Trans-Eurasian belt — closely follow plate boundaries: plates moving apart, colliding or grinding past each other stress the rocks most along boundaries, so fractures and earthquakes line up in belts. For part (b), both quakes were sudden energy releases when locked rocks slipped. Wenchuan, however, lay on the Trans-Eurasian belt: the Indo-Australian Plate's collision with Eurasia squeezed an inland continental fault; it reached magnitude 8 and mainly brought shaking and landslides. 3.11 lay on the Ring of Fire: the Pacific Plate subducted beneath the North American plate and ruptured hugely; it reached magnitude 9, and the large seabed displacement triggered a tsunami. Distribution therefore follows plate boundaries, while the differences in cause follow boundary type and crustal nature.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the three types of plate boundaries?
Q2Which major landforms form at convergent boundaries?
Q3Why are earthquakes and volcanoes common around the Pacific?
Q4Name a volcanic region away from plate boundaries and explain it.
Q5What is the difference between magnitude and intensity?
8Recap Recap
- The lithosphere is split into plates driven by mantle convection, moving a few centimetres a year
- Three boundary types: divergent (ridges, rift valleys), convergent (trenches, island arcs, fold mountains), transform (faults)
- Earthquakes and volcanoes line up along boundaries; the Ring of Fire holds about 80 per cent of large quakes and three quarters of active volcanoes
- Magnitude (Richter) measures energy; intensity (Modified Mercalli) measures damage at a place
- Hotspots explain volcanoes far from boundaries, such as Hawaii