The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Sunday, October 5, 2014

October 5. Karoo volcanics

When we talked about the Karoo Supergroup of rocks in South Africa on September 28, I indicated that parts of it extended up into the Jurassic. The Jurassic part of the Karoo rocks includes the Drakensberg Volcanics, lava flows from about 180 million years ago that originally may have covered as much as 300,000 square miles. The flows extended at least into South America and Antarctica and beyond, all of which were still attached to South Africa at this time. 

Map of Karro-Ferrar and related volcanics from: The links between large igneous provinces, and continental break-up: evidence reviewed from Antarctica, by Bryan C Storey, Alan P M Vaughan, and Teal R Riley
The origin of the Drakensberg Volcanics is similar to the lavas and sills in the Newark Group and the Palisades, which we talked about yesterday. Eastern Gondwana, Antarctica, India, and Australia, was beginning to rift away from the western part of Gondwana, Africa and South America combined. The initial pulse of this rifting included cracks in the earth that resulted in magma reaching the surface.

The Drakensberg Escarpment is a huge, long mountain front, the erosional edge of the South African plateau. The resistant cap of the escarpment contains various rocks along its 1,000-kilometer length, including hard sandstones and other rocks of the Karoo Supergroup, including the Drakensberg Volcanics in Lesotho and eastern Cape Province. 

In places the Drakensburg lavas are more than 1,600 meters thick – just about a mile of uninterrupted lava. The outpourings appear to have lasted for a couple million years. Based on what we’ve said about massive volcanic eruptions, you should be wondering, “where’s the associated extinction?” Well, there is a likely contender for that extinction. It’s called the Toarcian event, for the Toarcian age of the early Jurassic when it occurred. It wasn’t really huge, although it was global in extent. It seems to have mostly affected marine life, especially ammonites.

The volcanics extend to both the Atlantic and Indian Ocean sides of South Africa, suggesting that the initial rifting that would ultimately form both the Indian Ocean and the South Atlantic was beginning. But this wasn’t a smooth, continuous event, and the “action” on the South Atlantic side especially wouldn’t really take off for a long time yet. It’s suspected that a mantle plume, an upwelling of heat from deep in the earth, may have contributed to the initiation of these rifts that were about to help dismember Pangaea, but as you may recall from previous episodes, the ultimate causes of continental break-up are still debated.

* * *

M. King Hubbert was born October 5, 1903, in San Saba, Texas. He lad a long geological career in academia and with Shell Oil Company. He’s probably most remembered for creating the Hubbert Peak Theory, which says that for any geographic area, from a single field to the whole planet, the oil production rate tends to follow a bell-shaped curve. This is one of the fundamental concepts in the idea of Peak Oil, that we are near or at the peak of that bell-shaped curve, which would mean that half the oil that can be produced has been produced, and half remains to be produced. Hubbert predicted in 1956 that the peak of oil production in the United States would occur about 1970. He was correct, much to almost everyone’s surprise. The theory cannot accommodate things like major changes in technology, both in exploration, such as horizontal drilling, and in things like refining processes.

—Richard I. Gibson

Map of Karro-Ferrar and related volcanics from: The links between large igneous provinces, and continental break-up: evidence reviewed from Antarctica, by Bryan C Storey, Alan P M Vaughan, and Teal R Riley

Karoo-Ferrar Igneous Province

No comments:

Post a Comment