Friday, 26 June 2015

Day 10 & 11 - Taburiente Caldera

After our trek on day 9, yesterday was a well deserved day off, involving a little wander around the island's capital of Santa Cruz a seafood lunch with a view and a bit of beach lounging on black sand.
Well you can't work and be serious all of the time!

 Lunch with this view!

Beach of black sand. The black sand is a combination of eroded lavas that surround the area and cover the island and volcanic ash. It sparkles in the sun like millions of tiny diamonds. 

Today we are off to sample Dos Aguas spring in the centre of Taburiente caldera. The caldera forms part of the north of the island, it is approximately 10km in diameter and the walls tower up to 2000m above the caldera floor. The highest point of the caldera wall is the Roque de Los Muchachos at 2426m asl. During the 15th century Spanish conquest of the Canary Islands, the caldera was the site of the last stand of the indigenous people of the islands, the Guanches. The Spaniards found the caldera impregnable and only defeated the Guanches people by coaxing their leader out of the caldera with the promise of talks. 

Although this is not an area where hazard monitoring is completed in depth, the spring is sampled twice a year, just as a precaution. Should any samples reveal any anomalous data, then further sampling of soil gases, efflux and temperatures would be collected.

Though called a caldera, this is not a caldera that has formed by the collapse of a volcanic summit due to pressure changes caused by the rapid emptying of a magma chamber. Instead this caldera formed due to deep erosion of the original summit volcanic crater.

Two images from separate points inside the caldera. 

The walk to the sample site is about 4km each way, most of which can be done along the currently dried up river bed of the Barranco de las Angustias. This is a treat because the deep incision caused by years of extreme erosion has exposed the upper part of the Seamount Series. Thus allowing for the observations of some fabulous spilite pillow lavas.

Spilite rimmed pillow lavas (Paulo's foot is at the left of the image (scale). 

Spilite is a term used to describe a basalt that has been hydrothermally altered by means of interaction with seawater. Spilite rims are often formed on pillow lavas as the glass rind undergoes extensive alteration to chlorite. Such interactions between hydrothermal fluids of seawater origin with basalt often results in greenschist facies mineral assemblages of albite, chlorite, sphene, actinolite and epidote. 

Also seen forming the walls of the caldera are endless and sometimes complex networks of sills, dykes and sheeted dykes (though not seafloor spreading) with compositions varying from peridotite, pyroxenite, anorthosite, gabbro and syeno-gabbro.

 Sills and dykes providing evidence of at least four different intrusive events

'Sheeted dykes' (Paulo for scale), they are sheeted in the sense that a dyke has been split by a second dyke, then a third and so on. This is evident by the presence of chilled margins along one side of the dyke only. 

Samara did the sampling at the spring while I observed. This was for two main reasons; I have never sampled from a water source of any kind before and this was the only site this would happen at, so it had to be right. 

On locating the spring and starting to record data, the first things to be recorded were the ambient and spring temperatures. Multiple samples were collected from the spring to analyse the fluids for values of dissolved gases such as Helium ratios, Hydrogen and Carbon Dioxide, Calcium Carbonate, cations of calcium, magnesium and potassium and anions, as well as completing in situ testing for pH and conductivity. Three methods will be used for analysis of the fluids; micro-chromatography, quadrupole mass spectroscopy and isotope ratio mass spectroscopy. 

After finishing our sampling we walked further in to the caldera to find the Cascada des Colores - The Waterfall of Colours.

Cascada des Colores

You definitely need to know where to go as it is tucked away up stream, that involves a little wading and rock hopping to get to it. Just a trickle now, you wouldn't want to be in here if it rained!
It's a very pretty waterfall, the colours of the rocks are due to oxidation of iron at different concentrations and over time. Oddly the water itself runs clear from the falls but turns the reddy-brown of iron within a few meters of the falls. It is also only water that follows this specific route from the base of the falls to where the stream disappears underground 2km to the southwest, that turns this colour. The water above the falls and the water from other falls in the caldera also run clear.

Orange stained water due to iron concentrations only occurs along one water route within the caldera

Every day is an adventure!

Buenas Noches de La Palma!

No comments:

Post a Comment