Plate Margins & Why People Stay

Three Types of Plate Margin

A plate margin is where two tectonic plates meet. There are three main types, and each one produces a different combination of hazards. Understanding what is happening at the boundary is the first step to predicting what hazards a place will face.

Constructive Margins

At a constructive margin two plates pull apart. Magma rises through the gap, cools and creates new crust — which is why these are also called divergent boundaries. The Mid-Atlantic Ridge runs the length of the Atlantic Ocean, splitting it open at about 2 cm a year. Iceland sits directly on top of this ridge, which is why it has so many active volcanoes despite being in the middle of the Atlantic. Eruptions here tend to be effusive: runny basalt lava flows steadily rather than exploding. Earthquakes are usually shallow and small, because the plates are sliding apart smoothly.

Destructive Margins

At a destructive margin two plates move towards each other — these are also called convergent boundaries. When an oceanic and a continental plate collide, the denser oceanic plate is forced down into the mantle in a process called subduction. Friction and pressure build up before the plate jolts free, releasing massive earthquakes. Heat melts the descending plate, and the magma forces its way to the surface as explosive volcanic eruptions. The Andes Mountains in South America formed where the Nazca Plate is being subducted beneath the South American Plate.

When two continental plates collide, neither is dense enough to subduct. Instead the rock buckles upwards, building mountain ranges. The Himalayas formed where India is still pushing into Eurasia at about 5 cm a year — powerful earthquakes shake Nepal, but there are no volcanoes here because no plate is descending into the mantle to be melted.

Conservative Margins

At a conservative margin the plates slide past each other rather than colliding or pulling apart. No crust is created or destroyed — that is why they are 'conservative' — but the plates rarely move smoothly. Friction locks them together until enough stress builds, then the plates suddenly jolt free in a powerful earthquake. The San Andreas Fault in California is the best-known example: the Pacific Plate is sliding north past the North American Plate at about 5 cm a year. There are no volcanoes here, because no plate is being subducted, but the earthquake risk is severe.

Why Do People Live in Hazard Zones?

Around 800 million people live within range of an active volcano, and tens of millions live close to active earthquake faults. Why do they stay? The answers come in three groups: people don't choose to be there, the benefits outweigh the risks, and they trust that they can manage the danger.

• Family and culture. Many people are born in hazard zones. Their families, work, schools and places of worship are all there. Moving means starting over, and not everyone can.
• Fertile soil. Volcanic ash, once weathered, makes some of the richest farmland on Earth. Around Mount Vesuvius in Italy and the slopes of Java in Indonesia, ash-rich soil supports dense farming and high populations.
• Economic opportunities. Cities like San Francisco, Tokyo, Mexico City and Naples are major economic centres. The jobs, infrastructure and connections drew people in long before the hazard risks were fully understood.
• Tourism. Volcanoes and hot springs attract visitors. Iceland's Blue Lagoon and the geysers of Yellowstone bring billions in tourism revenue.
• Geothermal energy. Heat from beneath plate boundaries powers homes — Iceland gets nearly all its electricity from geothermal and hydroelectric sources.
• Confidence in management. In high-income countries, people trust that warning systems, building codes and emergency services will keep them safe.