At 11:30 in the morning on 20 February 1835, a huge earthquake, later estimated at about magnitude 8.5, struck near the Chilean cities of Concepción and Talcahuano. A tsunami followed and destroyed Talcahuano. HMS Beagle was working along the coast at the time. Charles Darwin was lying down in a wood near Valdivia when the ground shook. He and the ship's captain, Robert FitzRoy, then studied the damage and the changes to the land, and their records became key early evidence that earthquakes can lift the Earth's crust.
Darwin was on shore at Valdivia, resting in a wood, when the shock came. In his Beagle diary he called it a day "memorable in the annals of Valdivia." The worst damage was further north. When the Beagle reached Concepción and Talcahuano, the travellers found the cathedral in ruins. The tsunami had wrecked the port. The quake's worst lesson was about the ground itself: Darwin wrote that the earth, "the very emblem of solidity," had moved beneath him like a thin crust floating on liquid.
FitzRoy took careful measurements at Isla Santa María, in the Gulf of Arauco. They showed the island had risen by roughly two to three metres. Darwin described a rocky flat off the island that had been "at one blow upheaved above high-water mark". It was left covered with gaping, rotting mussel shells, still attached to the rock where they had lived. To Darwin this was proof that coastlines and mountains could be built by sudden uplift, one earthquake at a time.
Nearly the same stretch of fault broke again 175 years later, in the magnitude 8.8 Maule earthquake of 2010. That quake lifted Santa María by about two metres. Using FitzRoy's charts, geologists were able to reconstruct a complete seismic cycle for the first time.
Key people: Charles Darwin, Robert FitzRoy