Friday, February 3, 2012

Supercontinent Columbia

The farther we go back into earth’s history the fuzzier it gets. Erosion and geomorphic processes erase things. Deposits burry things. Plate tectonics rips chunks of the earth surface apart, moves continents around, slams continents into one another mangling their original appearance, and restuffs ocean floor back into the mantle for recycling. Continental plate movement and cycling of the earth has caused the continents to grow in size as lighter elements have accumulated in the earth’s outer rocks through arc volcanics and scraping of lighter ocean floor sediments onto the margins of the continents. A modern example of that growth can be seen in Washington State with the Cascade volcanoes and ocean floor assemblages such as the young former ocean floor rocks in the Olympics and Willipa Hills of western Washington thrust up onto the leading western edge of North America and thus becoming the new western margin of the North American Continent.

The chunks of continents that drift around get hard to track the further back in time one tries to go. Deep time continental reassembly piecing together the ripped apart ancient continents is a specialty that calls on truly global thinking. Stuff like: part of the east coast of Australia used to be attached to what is now roughly the border of Washington and Idaho and a bit further south was east Antarctica. Or an alternative view that India and northern China were once our neighbors. Hard detective work. Lots of arm waving and really really knowing lots of rock formations from all over the planet and a good grasp of spherical geometry. And this isn’t some old science. Think about it – plate tectonics as a fundamental geologic principle is less than 50 years old. Add to that extracting from rocks and measuring very precisely miniscule amounts of trace elements for age dating and chemical comparison across the globe takes a combination of hard work and very good equipment.

The folks that try to decipher ancient plate tectonics have done an admirable job rolling the clock back by reclosing the Atlantic Ocean 180 million years ago and reversing other plate motions. However, because essentially all ocean floor older than 200 million years has been recycled, going back deeper in time requires different tools than simply running the ocean spreading ridges backwards. Complicated stuff to try to piece together ancient continents. Hurts one’s head reading papers like Counterclockwise exhumation of a hot orogen: The Paleoproterozoic ultrahigh-temperature granulites in the North China Craton (Santosh and others, 2009 in Lithos). Yikes! But fun in a geo global sort of way. Turns out those ultrahigh temperature granulites in North China happen to be similar in age and chemical signature to similar rocks in India and eastern Washington.

Rogers and Santhos (2002) proposed that most of the existing continents came together approximately 1.8 billion years ago. They proposed the name Columbia for this supercontinent as evidence supporting the existence of this supercontinent was significantly derived from rocks in eastern Washington along the east side of the Columbia Basin. We have a supercontinent named after rocks in Washington State! Captain Gray finds the mouth of a big river on a fog free day in the late 1700s and names the river after his boat, Columbia, and now a supercontinent is named Columbia. And this isn’t just any supercontinent; it is the oldest supercontinent that can be reasonably surmized.

Columbia consisted of the ancient continent of Laurentia (the core what later became North America) with West Antarctica, Asia, India and Australia attached to what is now the west side of Laurentia and Europe, Siberia, Greenland and Baltica attached to what is now the north side of Laurentia. The attaching together of continents is a collision process. A modern example is India colliding with Asia creating the Himalaya Mountain belt. The joining of ancient Laurentia with ancient continents on its west side is recorded in highly deformed rocks in Idaho and Montana that were cooked and pressurized 1.6 million years ago.

Columbia existed as a supercontinent for perhaps 200 million years. That is a bit longer than how long the Atlantic Ocean has existed. During this time the supercontinent grew in size as ocean sediments and arc volcanics added to the size of the continent along the edges of the continent.

Columbia - rotated 180 degrees - what is now the west coast was the east coast
Idaho and Australia were neighbors
(Rogers and Santhos, 2003)

1 comment:

Dominion said...

John Rogers is great! I highly recommend Continents and Supercontinents by Rogers and Santosh, I have learned so much from that book.