Sunday, January 10, 2016

Summary of the Geology of the Antarctic Peninsula

A prompt from Jack S. in Boston reminded me that I yet to write about the generalized geology of the Antarctic Peninsula. It is rather simple, with the Peninsula having been located on the active western margin of the Gondwana supercontinent from about 400 Ma to its breakup about 150 Ma. Thus, the Peninsula exposes rocks related to arc magmatism and volcanism.

Note: All of the diagrams shown in this post were part of my lecture given on board called, "Landscapes of Antarctica: Rocks, Ice, and Time."

Interesting rock texture from Pleneau Island, where granite magma invaded the mid-crustal level of the Antarctic Peninsula and incorporated portions of the pre-existing rocks into the melt.

This National Geographic map depicts the details of the ocean floor around Antarctica, located by tradition and perhaps convenience at the bottom of the earth (for all those north-centric persons). Antarctica is 98% ice covered but the 2% that is exposed allows for an interpretation of its generalized geologic history.

Earth's fifth largest continent is divided into East and West Antarctica, with the 2,500 mile-long Trans Antarctic Mountains separating the two. It might seem strange for a place located in the far south to have east and west components but this is based on where the Greenwich Meridian intersects the continent (from the top in this projection). We are traveling exclusively in West Antarctica and in the most northerly portion of the entire continent. The Antarctic Peninsula is sometimes referred to as the "banana belt" of Antarctica since it is that part farthest north from the South Pole.

If the ice of Antarctica were removed, its topography would look like this. Compare with the satellite image above - West Antarctica is an archipelago separated from the mainland. However, if these ice-free conditions lasted for a mega-annum or more, much of the land surface would rebound above sea level.

Antarctica is larger than most people assume, being the combined area of Mexico and the contiguous United States!

The positions of the southern continents today. And...

...their positions at about 200 million years ago during the time of the Gondwana supercontinentThe blue X in East Antarctica marks the ancient location of the modern South Pole. Subduction occurred where the word Peninsula is shown and this was the setting for when most of the rocks in the Peninsula were formed. 
A cross-section showing the tectonic setting for the Antarctic Peninsula between late Jurassic and Eocene time (about 160 to 50 Ma).

An example of intrusive granite (far left of rock outcrop) into bedded and folded rhyolite tuff (center) in the Lemaire Channel. The geology is relatively simple. Essentially, the region lay along the western coast of Gondwana where subduction occurred. It was along this western margin that an arc complex and associated volcanism where present. Most of the rocks we have been seeing are these intrusive granites and extrusive lavas. All this occurred during a time when rocks such as the Coconino Sandstone, Kaibab Limestone, the Moenkopi and Chinle formations, and the Navajo Sandstone were being laid down in the American Southwest, on the other supercontinent of the time, Laurasia.

 Satellite image of the Antarctic Peninsula showing the areas we have been exploring from bottom of image to top.

Overlay on the same image showing a back-arc basin that has split the South Shetland Islands from the mainland. Spreading here is what produces the recent volcanism at Deception Island.

Watch my next posting as we visit huge, tabular icebergs in Antarctic Sound and another Pleistocene volcano on the Peninsula!


Dr. Jack Share said...

Voila! Thank you.

Dr. Jack Share said...

So the back-arc locale of Deception Island must account for the bimodal distribution of the volcanics.