Chapter 1 · From the centre of the Earth to the crater, 6,371 km
Volcanoes
Dig to the centre
Imagine you start digging at your feet and never stop. The Earth has a radius of about 6,371 kilometres. If you could travel down at the speed of a car on a motorway, 100 km/h, the trip would take about 64 hours. First you cross the crust, only 5 to 70 km thick. Then comes the mantle, nearly 2,900 km of hot, slowly flowing rock. Then the liquid outer core and a solid inner core. Scientists estimate the centre at about 5,200 degrees Celsius, with a range of roughly 5,000 to 6,000, about as hot as the surface of the Sun. In the picture the layers glow brighter the deeper you go.
Magma looks for a way out
Magma is melted rock at about 700 to 1,300 degrees. It is lighter than the solid rock around it, so it collects in a chamber under the volcano and pushes upward through cracks. It carries dissolved gas, mostly water vapour, carbon dioxide and sulphur dioxide. As magma rises, the pressure drops and the gas turns into bubbles, just as a shaken bottle of fizzy water foams when you open it. When magma reaches the surface it is called lava. In the picture you can follow a column of glowing rock climbing through the cone, and the crater begins to smoke.
Runny or sticky
Not every eruption looks the same, and the cause is mostly the magma itself. Basalt magma has about 50 percent silica and flows like thick syrup, so its gas escapes quietly and the lava runs far. This is an effusive eruption. Magma with 65 to 70 percent silica is sticky like cold honey. It traps gas until the pressure bursts it, which gives an explosive eruption with ash, bombs and ash clouds. In the sandbox raise the silica and the gas content and watch the style change from a calm river of lava to a blast. Real volcanoes also depend on the shape of the conduit, so this is a simplified model.
Mauna Loa and Fuji
Mauna Loa in Hawaii is a shield volcano: wide, with gentle slopes of only a few degrees, built by thousands of runny lava flows. It rises 4,169 metres above the sea and about 9 to 10 kilometres from the sea floor, and it is the largest active volcano on Earth by volume. It last erupted in late 2022. Mount Fuji in Japan, 3,776 metres, is a stratovolcano: a steep cone of alternating layers of lava and ash, with slopes of up to about 35 degrees. Its last eruption was in 1707. Turn the two models in the picture and compare their slopes.
The loudest bang on record
On 27 August 1883 the island volcano Krakatoa in Indonesia exploded. The blast was heard about 4,800 kilometres away, on the island of Rodrigues in the Indian Ocean, and the pressure wave went round the globe several times. Ash rose tens of kilometres, and the tsunamis it caused killed more than 36,000 people. Compare this with eruptions of the Hawaiian kind, which are slow enough that people can watch them. Explosive eruptions are rare but hit a large area at once. The picture shows the cone collapsing into a sea-filled hollow, called a caldera, and the ring of waves spreading out.
Pompeii
In the year 79 CE Mount Vesuvius in Italy erupted and buried the Roman town of Pompeii under several metres of ash and pumice. Ash fell for hours, and then fast clouds of hot gas and ash, called pyroclastic flows, rushed down the slope at speeds that can exceed 100 km/h. The traditional date of 24 August is debated, because finds suggest that it may have happened in the autumn. Ash preserved streets, houses and even loaves of bread, which is why Pompeii is a time capsule of Roman life. Today Vesuvius is watched by instruments, and roughly 600,000 people live in its danger zone.
Islands and rich soil
Volcanoes do not only destroy. Islands such as Iceland and the Hawaiian chain were built by them. The Hawaiian islands form a line because the Pacific plate drifts over a fixed hotspot of hot mantle rock. Kauai, the oldest big island, is about 5 million years old and lies roughly 500 km from the youngest, the Big Island. Divide 500 km by 5 million years and you get about 10 cm a year, which is close to the Pacific plate's speed of 7 to 10 cm a year (all of these are estimates). Ash and lava also break down into soil rich in minerals, which is why people farm on volcano slopes. Always follow local officials near active volcanoes, and leave if they order an evacuation.
In numbers
- ≈ 5,200 °C
- estimated temperature at the centre of the Earth
- 4,800 km
- how far away Krakatoa's bang was heard, 1883
- ≈ 50 % / 70 %
- silica: runny basalt versus sticky magma