Tuesday, September 27, 2016

Another outcrop, another story

Northern New Mexico offers to see a beautiful story of volcanic eruptions associated with Rio Grande rifting that was active. The river of Rio Grande carved canyon in Taos plateau exposing extensive flows of 5 million year old basalts and minor andesites and dacites. Here's one particular outcrop that provides a snapshot of geological history of the region:

The story goes like this (from bottom up):

Paleosol (old soil) marks the period of surface erosion. Underlying rock is exposed to the atmosphere and is being broken down to produce soil. The paleosol contains remnants of root channels and other traces of  life on land. Based on the ages of underlying rocks the soil was developing about 5 million years ago.

Baked and oxidized paleosol was developed because hot lava was flowing on top of moist soil. At high temperature, available iron and water and oxygen was reacting resulting in oxidation of that iron.

Crumbly breccia, aa-lava newly erupted lava was flowing on top of the soil, cooling down quickly which resulted in partly solidified rock flowing in highly viscous lava. Solidification of the material produced crumbly, chunky aggregate commonly called 'a'a-lava. The rock was erupted about 4.8 million years ago.

Massive dacite is produced by massive outflow of dacitic lava that was hot enough to flow and cool down continuously.

Sheared dacite reflects interesting property of the silisic lava. Because of high SiO2 content dacite polymerizes more so than a mafic lava, resulting in its comparatively high viscosity. So when it cools down it viscosity is so high, that lava can't flow anymore, it starts to shear. 

Geologist has something to learn about Taos plateau volcanic field. He was erupted about 25 years ago.

Thursday, September 1, 2016

Laxford bridge: famous outcrop in Scottish Highlands

This outcrop is exposed along the roadcut of the A838 road in NW Scotland. It can be found at coordinates 58°23'24.43"N   5° 1'29.66"W. Google map streets view. Also called as multi-coloured rock stop, the outcrop became famous because it shows conspicuous cross-cutting relationship between the oldest rocks in Europe. The Precambrian rocks exposed here tells us a story: from oldest to youngest units can be distinguished by cross-cutting boundaries between them. The oldest rock in Scotland is Lewisian gneiss (grey). The original rock formed about 3 billion years ago and was metamorphosed multiple times (so now it's called gneiss). Then it was cut by mafic intrusions (black), or dykes (British spelling preserved), at about 2.5 billion years ago. Then all of these units were cut by granitic intrusions (pink), much later at about 1.8 billion years ago. After than, the whole assembly of rocks experienced metamorphism that shaped it's final look (sheared and stretched). Remember, the age of the Earth is 4.54 billion years, these rocks are really old!
Here's the outcrop with boundaries shown schematically.

Saturday, August 20, 2016

Where basalt meets seawater

Imagine a mid-ocean ridge. Between two diverging plates, a volume of molten basalt is constantly being oozed out of the mantle. The molten basalt itself is really hot, over 1000°C. In contact with cool seawater it quenches instantaneously into volcanic glass. However, in areas (open cracks, for example) where permeable freshly erupted basalt is available, seawater reacts with it at 200-500°C. After the reaction basalts acquire new look, or we say, it becomes altered, - it has new minerals in it. From igneous and water-free rock, basalt converts to water-bearing hydrothermal mineral assemblages. These areas of hydrothermal activity are partly responsible for modern seawater chemical (and isotopic) composition and its acidity. Reacting basalt supplies Mg, Si, Ca and other elements into the seawater. In turn, the reaction with basalt sucks out other elements out of seawater, such as Na and S. I will show that at some point, reacted seawater can become pretty acidic with pH of 3.8. These underwater hot springs frequently form black smokers where unusual forms of life can exist because of the supply of heat. 
Here, I wanted to show the result of a modeled experiment. I virtually took basalt of common composition (MORB) and incrementally added it to fixed amount of seawater. Starting with very small amounts increasing step by step, I could model different situations where various amount of rock is reacting with seawater. In my imagination it simulates variable amount of space where seawater could touch basalt and react with it. From very wide open cracks to porous basalt. In terms of water-to-rock ratio, the numbers will go from very high (open crack - water dominated system) to a very low (rock-dominated situation, water only in pores). The purpose of my modeling was to predict what minerals form during the reaction between basalt and seawater. Knowing thermodynamic conditions of mineral formation, the program could tell me what minerals will be found stable. 
The graph shows concentrations of products of reaction between seawater and basalt at 300°C. The right side of the graph can be interpreted as pure seawater (very little rock added), thus the water-to-rock ratio is really high. Without any involvement of basalt, seawater is oversaturated with anhydrite and brucite. Depending on the proportion of rock in the mixture, different minerals will precipitate. Moving to the left along the horizontal axis, more rock is reacting with seawater. At very high water-rock ratios, basalt turns into a mixture mostly consisting of anhydrite, serpentine, hematite, talc and chlorite. At low water-rock ratios (basalt-dominated system), the products of reacting are albite, amphibole (mostly tremolite), zeolites, pyrite, quartz and epidote. These minerals compose a large portion of seafloor, mostly it's deeper layers because at some point they underwent high temperature hydrothermal alteration at mid-ocean ridges.

In turn, the left over seawater has modified concentrations of ion dissolved in it.