Under (geo)pressure

The Trinidad saga continues today with a theme of geopressurized fluids in the south.

Our day started off visiting Vessigny Beach as a structural geology appetizer to Pitch Lake, which we visited afterwards. Vessigny Beach had a great outcrop of a small anticline that happened to contain lignite coal, from biological material. We even found a leaf and stem fossils! Some of the sediments were a strange red which has been interpreted to be baked from the coal above lighting on fire due to natural explosive combustions.

Next was the famous Pitch Lake. Pitch Lake is the world’s largest single oil sea. Oil from Cretaceous sediments buried deep below the surface have managed to breach the surface, although how this happened is a bit of a mystery. Pitch lake is positioned on a faulted anticline structure. The pitch being dug from the lake is composed of 40% oil, 40% sand and 20% water. Currently, the pitch extraction rate is greater than the production rate resulting in a roughly 10-meter reduction in depth since 1867, when extraction first began. The breached anticline structure that formed the lake was produced due to hydraulic fracturing due to hydrothermal pressure applied by the overlaid rock onto a trapped unit of gas. One theory for why the trapped pressure was released, is that the overlaid rock eroded, reducing the downward force being applied on the gas and allowing it to explode.

Interestingly, the community’s daily routine has been guided by the daily operations of the factory. In the morning a steam whistle sounds and wakes the community at 7 am, shortly after it sounds again to alert the workers that they should already be in the building. Throughout the day, like a clock tower, or a high school, it structures the community’s routines. We had the pleasure of visiting the lake during the Trinidad dry season which allowed us to walk upon the soft rock surface. During the wet season, the community utilizes the sulphur waters and clays that accumulate on the surface and in trenches as a DIY Trinidadian spa. Additionally, the entire town surrounding the pit is subject to the daily and seasonal shifting of the surface due to thermal expansion and contraction during the hotter and cooler hours of the day.

_________________

After our lengthy but incredibly interesting Pitch Lake encounter and a brief lunch, we headed to a tiny fishing village, Frank Bay, on the south shore of Trinidad to orient ourselves with outcrops on the sea edge and as an introduction to mud volcanoes. Mud volcanoes are areas where mud and water from deep cracks in the earth get pressurized and bubble up to the surface, creating small cone volcanoes entirely of mud. Off the coast of Trinidad there have been enough of these eruptions to even create entire new islands! This spot on the south shore is also theorized to be the southernmost border of plate movement between the Caribbean plate and south American plate, with about 3mm of movement a year.

__________________

The next stop was the most impressive mud volcanoes to date, with real bubbling action! The Erin Bouffe mud volcanoes lie somewhat perpendicular to an anticlinal structure and have had a large eruption in the last year. Many small craters with mud bubbling to the surface scatter a small area in the forest of the Trinidadian countryside. During our visit, we counted 17 holes which were actively releasing saline waters, mud and natural gases, but this number fluctuates throughout the year as new trapped pressure sites are able to break free.

Last but not least we saw the remains of a landslide that affected the same area the Erin Bouffe volcanoes were located in. The landslide was on August 28th, 2018. It totally washed out a road, but vegetation has grown back fairly quickly. Using Radar data taken before and after the slide, we could see that the event resulted in an 18-meter horizontal shift. This was the same date as an earthquake in Venezuela, which is probably what triggered the slide.

The day was long and hot, but we learned a lot about this interesting and varying area of the island.

Until next time,

Naomi and Keenan

This man fell in the pitch lake.