Friday, July 4, 2014

Geology of Rio's Domes

Watching coverage of the World Cup, one is struck by the spectacular natural setting of Rio de Janeiro, Brazil, hemmed in and partly bisected by massive domes of rock.  The most famous of these, Sugarloaf Mountain, rises at the end of a peninsula while Corcovado, the highest summit (2300 feet) creates a magnificent backdrop for the city.

All of these rounded summits are composed of Precambrian gneiss, which formed about 570 million years ago (MYA), 150 million years before life first left the sea.  Since that time, tectonic forces have lifted this ancient bedrock and both faulting and erosion have produced the scenic landscape that we observe today.  Much of that sculpting occurred in the Permian Period (some 275 MYA), when South America collided with Africa (as Pangea formed), and during the Cretaceous Period (about 100 MYA), as the South Atlantic opened and the two Continents rifted apart.

We humans, having walked the Earth for less than 130,000 years ago, finally colonized this spectacular coastline about 18,000 years ago.  Now, through the marvels of modern technology, we view its natural beauty from homes and pubs across the globe, watching as the lofty domes of ancient rock reflect the setting sun.