Moving plates

Continents on the move

In 1912 the German scientist Alfred Wegener pointed out that the continents fit together like jigsaw pieces, especially South America and Africa. He collected more clues: matching rock layers and mountain belts on opposite sides of oceans, the same fossils (such as Glossopteris and the small freshwater reptile Mesosaurus) on continents now far apart, and scratches left by ancient glaciers in places that are now hot, such as central Australia and India.

Wegener proposed that all the continents were once joined in one supercontinent, which he called Pangaea, and that they had slowly drifted apart. Its southern half, called Gondwana, included Australia, Antarctica, South America, Africa, India and New Zealand. Most geologists rejected his idea of continental drift because he couldn't explain what force could move whole continents. He died in 1930 on an expedition in Greenland, decades before he was proved right.

The breakthrough came in the 1950s and 1960s, when ships mapped the ocean floor. They found a chain of underwater mountains, the mid-ocean ridges, running around the world like the seam on a tennis ball. Rock near the ridges is young and rock far from them is old, and the magnetism of the rock forms matching stripes on either side. The sea floor was spreading. Wegener's continents were riding on moving plates, and the theory of plate tectonics was born.

Why plates move

The Earth's lithosphere is broken into about 15 large plates and many smaller ones. Australia sits on the Australian plate (sometimes called the Indo-Australian plate). Plates are moved by the Earth's internal heat:

• Heat from the core and from radioactive decay makes the mantle churn in slow convection currents: hot rock rises, spreads and cools, then sinks again. The plates are dragged along on top.
• At ocean trenches, old, cold, dense ocean plate sinks into the mantle and pulls the rest of the plate behind it, like a tablecloth sliding off a table. This "slab pull" is thought to be the biggest force.
• At mid-ocean ridges, the raised, hot rock helps push the plates apart.

What happens at plate boundaries

Most earthquakes, volcanoes and young mountain ranges are found along the edges of plates, where plates meet. There are three kinds of boundary:

• Divergent boundaries: plates move apart. Magma rises into the gap and cools to make new crust. Under the sea this builds mid-ocean ridges; on land it opens rift valleys, such as Iceland's Þingvellir and the East African Rift, where Africa is slowly splitting in two.
• Convergent boundaries: plates move towards each other. If an ocean plate meets a continent, the denser ocean plate sinks under it (subduction), making a deep trench, strong earthquakes and a chain of volcanoes, as in the Andes, Japan and Indonesia. If two continents collide, neither sinks far, so the crust crumples into huge fold mountains: the Himalayas formed when India crashed into Asia.
• Transform boundaries: plates slide past each other. No crust is made or destroyed, but the plates lock together, build up stress and then suddenly slip, causing earthquakes. New Zealand's Alpine Fault and California's San Andreas Fault are examples.

Plates and Australia

Australia is in the middle of its plate, a long way from the nearest boundary, so it has no active plate-boundary volcanoes and fewer big earthquakes than New Zealand or Indonesia. But it isn't quiet:

• Australia broke away from Antarctica, finally separating near Tasmania about 35 million years ago, and has been moving north ever since. It is heading towards Asia and will collide with it in tens of millions of years.
• The plate is being squeezed as it pushes north, so stress builds up in the crust and is released in earthquakes. The 1989 Newcastle earthquake (magnitude 5.6) killed 13 people, and a magnitude 5.9 earthquake near Woods Point in Victoria in 2021 was felt in Melbourne and Sydney.
• Eastern Australia has a line of old volcanoes that get younger towards the south, including Wollumbin (Mount Warning) about 23 million years old, the Warrumbungles about 17 million years and Mount Canobolas near Orange about 13 million years. As the plate moved north over a hot spot in the mantle, a new volcano formed and the old ones were carried away, like a sheet of paper being pulled over a candle.