The Cretaceous Period, the third and last period of the Mesozoic Era, gets its name from the chalk of Cretaceous age in parts of Europe. Chalk is a soft, granular variety of limestone, calcium carbonate, that usually represents an accumulation of the microscopic shells of coccolithophores, which are single-celled phytoplankton – floating algae. The Latin word for chalk is creta, which gives its name to this geologic period. The geologic symbol used on maps is K, from the German word for chalk, Kreide.
Belgian geologist Jean d'Omalius d'Halloy proposed the name in 1822, basing it on rocks from the Paris Basin.
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|Fire and tsunami resulting from|
the 1755 Lisbon earthquake. Public Domain.
The earthquake was centered at least 150 miles southwest of Lisbon, in the Atlantic Ocean, along the transform fault that marks the boundary between the European Plate and the African Place. That boundary is along the Azores-Gibraltar fault zone, a poorly understood and vaguely defined break in the crust. It’s poorly defined because Africa and Europe are pretty close to being locked together – but definitely not quite. They are still sliding obliquely alongside each other, with Europe moving to the east relative to Africa. Another way of thinking about the boundary is that the Atlantic Ocean, expanding from the mid-Atlantic Ridge, is not opening in a completely uniform way, so that the northern part, including Europe, is moving away from the ridge at a slightly faster rate than the southern part that includes Africa. The differential between them is enough to make significant earthquakes along the boundary.
—Richard I. Gibson
Image: Fire and tsunami resulting from the 1755 Lisbon earthquake. Public Domain.