Illinois State Museum

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Formulating a Theory

In the early 1900s, scientists noticed that the continents seemed to fit together like the pieces of a puzzle. Continents that had the best fit, such as Africa and South America, also had similar rocks and fossils along their opposing coastlines. Then it was discovered that mountain chains occurred along the edges as if the continents had collided.

In the 1950s, deep sea explorations revealed ridges running through the oceans. Magma, oozing from the ridges, formed mountainous bands of solidified lava on either side of the ridges. The bands were increasingly older, moving away from the ridges. This evidence confirmed that the sea floor was spreading, and the continents were moving. The discovery of radioactivity generating heat in the Earth's core suggested that heat-driven currents were moving the continental plates.