Volcanoes: Fire From Within the Earth

In 79 CE, the Roman city of Pompeii was buried under several metres of volcanic ash in less than 24 hours. The eruption of Mount Vesuvius was so sudden that people were preserved in the exact positions they were standing in. Today Vesuvius still looms over the city of Naples, Italy — and nearly three million people live within 30 km of its summit. Why would anyone choose to live next to an active volcano? Keep that question in your mind as you work through this lesson.

**Inside the Earth** Deep below Earth's surface, temperatures are so high that rock melts into a thick, slow-moving liquid called magma. Magma collects in an underground pool called a magma chamber. Because magma is less dense than the solid rock around it, it is slowly pushed upward — a bit like an air bubble rising through water. **From Magma to Lava** When magma forces its way through cracks and weaknesses in Earth's crust, it reaches the surface. At the moment it breaks out, we call it lava. Lava pours from an opening called a vent. Over many eruptions, layers of lava and ash build up around the vent to form the cone shape we recognise as a volcano. The bowl-shaped dip at the very top is the crater. **Plate Tectonics and Volcanoes** Most volcanoes are not random. Earth's outer shell is broken into giant slabs called tectonic plates, which move very slowly — a few centimetres per year. Volcanoes form mainly in two situations: 1. **Convergent boundaries** — where one plate dives beneath another (subduction). The descending plate melts, producing magma that rises to form explosive volcanoes. The Pacific Ring of Fire is a belt of such volcanoes encircling the Pacific Ocean, including those in Japan, Indonesia, and the west coast of the Americas. 2. **Divergent boundaries** — where plates pull apart and magma wells up to fill the gap, forming gentler shield volcanoes. Iceland sits on the Mid-Atlantic Ridge and is built almost entirely from such eruptions. 3. **Hot spots** — columns of unusually hot mantle rock that burn through the plate above, regardless of boundaries. Hawaii's volcanoes formed this way. **Types of Volcano** - **Shield volcanoes** have gently sloping sides built from runny, low-silica lava that flows far before cooling. They erupt frequently but less explosively. Example: Mauna Loa, Hawaii. - **Stratovolcanoes (composite volcanoes)** have steep, layered sides built from alternating ash and viscous lava. They erupt less often but far more violently. Example: Mount Fuji, Japan. **Primary and Secondary Hazards** A volcanic eruption produces primary hazards — those caused directly by the eruption — including lava flows, pyroclastic flows (super-fast avalanches of hot gas and rock), and ash fall. Secondary hazards happen as a result: lahars (volcanic mudflows when ash mixes with water), tsunamis triggered by flank collapse, and disruption to air travel from ash clouds. **Why Do People Still Live There?** Volcanic soils are exceptionally fertile because lava and ash break down into minerals that plants need. Millions of farmers in Indonesia, Italy, and elsewhere depend on this rich soil for their crops. Geothermal energy — heat drawn from volcanic activity — provides Iceland with most of its electricity and heating. Tourism also brings income to communities near famous volcanoes.