The sands of time show impact of great quake of Lisbon in 1775 | Geology
Usually geologists analyze events that happened deep in the past, but a recent bit of research applied the same kind of analysis to a historic event.
At around 9:40 local time on the morning of Nov. 1, 1755, the southwestern coast of Europe was hit by a magnitude 8.5 earthquake and its subsequent tsunami. In some places, the waves were 50 feet high, ravaging coastal towns and cities of Spain, Portugal and Morocco.
Called the great quake of Lisbon, the tsunami also nearly destroyed the city of Cadiz, Spain. Estimates of the death toll range from 12,000 to 50,000, making it one of the deadliest in recorded history.
The new research analyzed a 30-foot-long core taken from a lagoon in the area of the Rock of Gibraltar, at the southern end of the Iberian Peninsula and the entrance to the Mediterranean from the Atlantic.
Scientists analyzed its sediments (for example, sand versus mud) and organic remains including plants, clams, snails, and foraminifera, commonly called forams. Forams are single-celled organisms that make wonderfully complex shells, called tests, that vary between species. Species differ in different environments, such as fresh versus saltwater. The core was divided into six layers based on those characteristics.
The bottom sediments of the core were deposited in an intertidal open bay to beach environment, as shown by the fine to medium sand, the forams, and shells of clams and snails, a few of which indicated periodic incursions of freshwater. That ended about 6,400 years ago.
Shells and forams in the next layer indicated a more-marine lagoon environment with numerous species of clams and snails that live exclusively in the ocean towards the bottom but with increasing brackish and freshwater species towards the top. That lasted until about 4,100 years ago.
Sediments and shells in the third layer indicated a very restricted lagoon that was occasionally above sea level, as shown by thin layers of peat. That episode lasted until about 2,800 years ago.
The fourth layer is the one representing the Lisbon quake of 1755. Its base is an irregular erosional surface on which was deposited numerous ripped-up pieces of the peat layers from below, an increase in the grain size of the sediments, and numerous fragmented shells of snails and clams.There are also occasional shells of species that are known only from much deeper layers, ripped up by the waves. That erosion eliminated about 2,600 years of the sedimentological record.
Above the tsunami deposit was a projectile fragment that came from a six-pound hand-grenade used in hostilities between Spain and Great Britain in 1779-1781. Above that, asphalt and other modern human-made debris was abundant.
Evidence of the 1755 quake and its resulting tsunami has been found in other areas of southwestern Spain, but very little evidence has been found of it in the Mediterranean. This was the first in-depth study to show that it affected that area, too.
The 1755 quake was not the first big one to hit that area of Europe – others occurred in 1321 and 1531 – and it won’t be the last.
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University. Contact him at gnidovec.1@osu.edu
This article originally appeared on The Columbus Dispatch: New research show impact of 1775 European earthquake, tsunami
