More blue water - why is the Nil Diya Pokuna blue?

On my last visit to Sri Lanka, I was keen on exploring some lesser-known attractions and decided to visit Nil Diya Pokuna (නිල් දිය පොකුණ) located close to Ella in the Uva Province. I was impressed and fascinated by the massive underground cave complex and the blue water pond at the end of the 850m hike through the cave. This was the second time I saw clear blue water in Sri Lanka, the first being in a limestone quarry.  

The usual reason for ponded water to appear bright blue or turquoise in colour is the fine particulates that selectively scatter light through water (the same reason why the sky is blue). In the case of the limestone quarry the fine particulates are minute calcite crystals and in the case of glacial lakes they are finely ground rock particles known as glacial flour. 

Nil Diya Pokuna has a very interesting geology, with several different rock types present around and within the caves, and I wanted to understand what gives the water its blue colour. Caves of this scale are usually formed by the action of weathering and erosion of sedimentary rocks such as limestone. However, this region of Sri Lanka consists of primary of metamorphic rocks. This blog post by Dr Jayasingha describes the geological origins of the cave complex containing Nil Diya Pokuna. According to it, the caves have been formed by the initial dissolution of Marble, which leads to weakening of rock joints and bedding planes and subsequent collapses of the other rock masses creating the large underground caverns. 

Marble is formed by the metamorphosis of limestone, and its dissolution would lead to the release of calcite crystals. There are stalactites formed at several places within the cave, as seen in the photos below, that confirm the occurrence of marble or limestone dissolution. Therefore, it is reasonable to conclude that the reason for the blue coloured water in Nil Diya Pokuna is the calcite crystals that are accumulated in the water as it flows through the joints and fissures in rock containing marble or limestone before making its way into the pond. Below are some photos from my visit:

Stalactites in the cave indicating marble or limestone dissolution
 

Evidence of weathering and staining in the rock

Visible bedding planes and smooth joint surface of a possible collapse leading to cave formation

Blue water and more stalactites

High water levels were blocking off some more expansive areas of the cave

The water was a little murky due to recent rains

Backfill for underground infrastructure: Soil strength or corrosivity? Which do we choose?

This post is inspired by one of my research findings published recently. My PhD research involved the evaluation of underground corrosion from a unique viewpoint combining the two traditionally separate fields of soil mechanics and electrochemistry.

Corrosion requires both water and oxygen to occur, and in the case of underground corrosion, soil supplies these reactants for the reactions to proceed. It follows that the ability of the soil to store and supply water and oxygen will to some extent, govern the rate of corrosion of metal buried in soil. It is known that there is a critical level of moisture at which corrosion is soil is maximized. Too much water will stifle corrosion because the supply of oxygen to metal is severely limited under waterlogged conditions.

As part of my research, we investigated these mechanisms in detail. Buried infrastructure such as pipelines and storage tanks fail due to corrosion, causing large economic losses and environmental damage due to leaks and bursts, exposing contents such as oil and gas to the surrounding environments. An understanding of underground corrosion and its prevention helps increase longevity of such buried assets and is a step towards sustainable development. While corrosion and its prevention are handled by electrochemical practitioners, the installation of buried infrastructure and backfilling is under the purview of geotechnical engineers. Given that the problem of underground corrosion overlaps both these fields, its study should be an interdisciplinary effort.

Our experiments showed that the critical level of moisture for corrosion is related to a relationship known as the soil water retention curve, or the soil water characteristic curve. Combining results from several electrochemical experiments and soil tests we were able to identify the behavior of water in soil that governs corrosion. We saw that in different soil types, the continuity of air and water phases change differently with the degree of water saturation, and that the transition point at which the air phase becomes occluded coincides with the critical degree of saturation for corrosion (where corrosion is maximized) for each soil type. What was more interesting was that this critical water content for corrosion is the same as the optimum water content for soil compaction. It has been shown that the same mechanisms for air entrapment occurs at the optimum water content during soil compaction.

This finding is important because, it is usual practice among geotechnical engineers to compact soil to its optimum water content to maximize its strength. In the installation of buried infrastructure and the subsequent compaction of backfill, if this usual practice is followed, we will be inadvertently creating the most conducive conditions for corrosion of the buried metallic asset. So, the question is whether we increase our soil strength, of the rate of deterioration of the buried metal. One possible solution is to compact soil in the drier side of the optimum water content. But there may be other factors at play that need to be considered. What is more important is to identify that this problem is interdisciplinary in nature and needs to be solved that way. Corrosion engineers need to be aware of this in planning prevention techniques such as cathodic protection, and geotechnical engineers need to know the effect of soil compaction on the corrosion of buried metallic infrastructure.

Like many of our modern problems, underground corrosion needs to be viewed and solved by taking a multidisciplinary approach. Confining our engineering efforts to the traditionally isolated fields is likely to worsen the problem rather than solving it, and our knowledge and forces need to combine to achieve sustainable progress.

The point of maximum corrosion was identified as the degree of saturation at the inflection point of the water retention curve also coincident with the degree of saturation at the optimum moisture  content (OMC) in the compaction curve

Geochemical Environment

The Geochemical Environment is defined by the surrounding conditions of pressure, temperature, and abundance of chemical components. The geochemical environment influences the ore body formation and dispersion. Therefore a clear understanding of the geochemical environment is essential in mineral exploration.

The geochemical environment can be classified into two types based on the conditions of temperature, pressure and chemistry. They are,
1. Endogenic Environment.
2. Exogenic Environment.

The Endogenic Environment is the deep-seated environment in which forces driven by earth's internal heat predominate. Since rock formation often takes place in this environment, it is also called the "Primary Environment". Metamorphic conditions, magmatic, igneous conditions, activities in the vicinity of plate boundaries and sometimes deep seated sedimentary conditions are geological activities that are prominent in the endogenic environment. The endogenic environment can be characterized by,
- high temperature
- high pressure
- lack of free oxygen
- lack of free water and CO2
- restricted movement of fluids

The Exogenic Environment is the surficial environment in which forces are mainly driven by solar energy. It is also termed the "Secondary Environment". Geological activities present in the exogenic environment are, weathering, erosion, transportation and sedimentation. The exogenic environment can be characterized by,
- low temperature
- low pressure
- abundant free oxygen
- abundant water and CO2
- free movement of solutions

These two environments are connected to each other and material gets transported from one to another creating a closed system. Driven by the natural forces described above this dynamic system can be simplified and depicted in the diagram below. A good understanding of this cycle can help decision making in the mineral exploration process.

the geochemical cycle(Image from "Geochemistry in Mineral Exploration" by Rose et al)

Basics of Photogeology

The interpretation of Aerial Photographs for geological purposes is termed Photogeology. This a form of Remote Sensing. The primary objective of Photogeology is, to identify geological structures and rock types of an area and to prepare a geological map of the area. Photogeology is usually employed before field geological work in order to get a general idea of the geological features of the area. The observations and inferences obtained from Photogeology are then confirmed by using field data.

The analysis of aerial photographs in photogeology is done using the same elements of interpretation. "Tone" is used in the identification of rock type. Generally lighter tones indicate rock types like Quartzite. Tone is also used in the identification of the density of vegetation which in turn provides clues about the underlying rock. Usually dense vegetation can be seen in area where the rock Khondalite is present and in areas where Quartzite is abundant, very little or no vegetation is present.

By using a stereoscope to view the three dimensional topography of the area structural features such as ridges, valleys and slopes can be identified. Due to differences in hardness and the extent of weathering, certain rock types show characteristic structural features. For example, Marble and Hornblendebiotite Gniess are found at the bottom of valleys because they are highly prone to weathering. Rocks that are much harder and are resistant to weathering like Quartzite, Granite and Granitic Gneiss usually form ridges.

geological features in an aerial photograph

When viewing outcrops from above, for example in a ridge like structure, by looking at the texture on either side of the ridge, the direction of dip and strike can be inferred. Since the side where multiple layers of rock are exposed undergoes differential weathering that side become ragged and rough. This side is termed the escarpment face. The side on which a single layer of rock is exposed undergoes uniform weathering and therefore is relatively smooth. This side is the Dip slope. Therefore, generally the side of a ridge that has a relatively smooth texture indicates the direction of dip.

cross section of a ridge

The drainage pattern and shapes of marshes lakes etc also provide clues about the geological structure of the area. A drainage pattern like shown in the first diagram indicates a homogeneous and relatively flat rock while a pattern like in the second diagram indicates a highly jointed rock. In addition to this, the displacement of any feature along a line indicates a rock fault.

Once all possible features are identified form the aerial photograph, a geological map of the area is plotted. This map is referred and then the data is confirmed by using field data.

 

Stepping into Field Geology

While Geology is a field of study that takes place both outdoors in the field and indoors in laboratories, Field Geology is a primary constituent of Geology. Most of the activities that are involved in Geology like sample collecting, mapping and recording takes place in the field. When moving into field geology for the first time, several important points need to be considered.

The first thing that needs to be considered is, Safety in the field. Mostly this means to be aware and protect one's self against threats from nature and wild animals. For this purpose a thorough knowledge about the surroundings, proper equipment and first aid facilities are essential. In the case of snake bites it is useful to know the closest hospital or medical center equipped with anti-venom serums. In addition to the above, proper usage of tools like the hammer and chisel will reduce the likelihood of an accident.

Once the safety is taken care of, the right tools and equipment related to field work must be made available. The common tools used in field geology are, Geological Compass(Brunton pocket transit or Silva compass), Hand lens(10X), Hammer and chisel, waterproof field book, mineral testing kit and sample bags. In addition to these basic tools, additional tools such as camera, GPS, binoculars  gold pans, metal detectors and Geiger  counters may be used for specific requirements.

The Geological Compass is arguably the most useful tool at the disposal at the Geologist or Engineer. In addition to the obvious purpose of locating the direction of north and calculating the bearing, a geological compass has the following functions.
- Finding the dip and Strike of geological features such  as foliation, joints bedding planes etc.
- Measuring slope angles
- approximating heights of objects
- a level
When using a compass, it is essential to keep away from vehicles, power lines and any other magnetized objects. This means that the Hammer and chisel also needs to be kept aside when using the compass. With the emergence of smartphones with magnetic sensors and accelorometers, applications that perform similar to a geological compass have been developed. One such application that can be used for traversing, calculating Dip and Strike along with a host of other useful functions is, Rocklogger which is available for the Android mobile platform.

Another important aspect of Field Geology is Sample Collection. When collecting rock samples, an approximate sample size of 3"x3"x3" or 4"x4"x4" is preferred. Weathered rock samples are generally avoided unless it is a specific requirement. Once collected, sharp edges should be trimmed off before bagging the sample to avoid tearing the bag. Cloth bags made out of a cotton fabric are usually used for this purpose. Samples should also be numbered systematically. While there is no hard and fast rule for this, a meaningful numbering system incorporating information such as collection year, area, and collectors initials is used to avoid confusion.

Field photography is also a requirement in certain cases. When taking a picture always a scale should be used. For small objects, a pen or another small object with a known size must be in the picture. In the case of photographing macro structures, wetting the surface of the rock helps to bring out the fine details on the surface. Care should be taken to avoid shadows to fall on a part of the rock when  photographing because it could lead to misinterpretation.

Brunton Compass - image from wikipedia

A Diamond is Forever

Gemstones have been objects of desire since ancient times and are commonly seen as symbols of wealth and prosperity. The field of Gemmology aims to identify, classify and add value to gem material. Of all the gemstones identified, the diamond holds a special place due to various reasons. While already being the most sought after gem stone, efforts companies like De Beers have increased the value of diamonds even more. An example is, the De Beers advertising slogan "A Diamond is Forever". This slogan increased diamond sales to such an extent that a diamond was a part of almost every engagement ring. It was coined by Frances Gerety and is regarded as the best advertising slogan of the 20th century.

The value of a gemstone usually depends on its beauty, rarity and hardness. Although these are very vague terms and beauty is not quantifiable, the factors that contribute to beauty are colour and clarity. In gemstones like aquamarine, value generally increases with colour intensity. In blue saphire, the value is highest at a particular colour, termed the "optimum colour". Clear gemstones without any intrusions or impurities are usually cut so that they are faceted. translucent, opaque or sometimes clear stones with intrusions are cut with a convex top and are called "cabochons". Even opaque stones can be valued highly due to properties such as Chatoyancy (cat's eye) eg- Chrysoberyl. and Asterism eg. Star saphire. Among so many varieties of stones the clear, faceted diamond is the most sought after.

The other factor that contributes to the value of a gemstone is its rarity. while gemstones are naturally rare, the demand for it creates an increase in value. However in some cases the rarity can be increased by the producers or traders of gemstones by stockpiling and controlling the release of gemstones to the market, like what De Beers did when they held the monopoly in the diamond business at a certain period of time. These practices however, rarely take place today. Hardness is the other key property of a gem quality material. A high hardness value means high durability and therefore it stands the test of time. Whether it's the case of beauty, rarity or hardness, the diamond clearly stands on top.

image from : wikipedia

A Classification of Metamorphic Rocks

Metamorphic rocks can be classified according to the location of occurrence, chemical composition, texture and structure, and mineralogical composition.
The classification with regard to location was done in the past but is not used today. The chemical classification is primarily used for carbonate metamorphic rocks because they are easy to dissolve. This classification is not common in Silica based rocks.
Silicate rocks are classified with regard to the texture and structure of the rocks as follows
1.  Foliated Rocks
According to the degree and size of foliation, it is further subdivided into the following categories
-Slaty foliation
-Phyllitic Foliation
-Shistose foliation
-Gneissic Foliation
2.  Non Foliated Rocks
non foliated rocks are further subdivided into granular rocks where individual grains can be identified, and massive rocks in which no such features can be found.
After visual observation of the rocks and the determination of the relative abundances of minerals, the rocks are given names that indicate the principal minerals and the structural and textural features. For example, "Garnet, Sillimanite, Graphite Schist" is a rock that contains amounts of the minerals mentioned in that order and has a Shistose Foliation.

A Classification of Igneous Rocks

Igneous rocks can be classified according to several criteria such as method of formation and occurrence, mineralogical content, and textual features.

Igneous rocks can be classified into four types according to its method of solidification and mode of occurrence.
1.  Pyroclastic Rocks
these rocks re formed by the material ejected to the atmosphere by a volcanic eruption. These rocks mainly consist of fine ash particles but can have larger particles as well (known as lapilli, blocks and bombs). These are somewhat similar to Sedimentary rocks.
2.  Volcanic Rocks
These rocks are formed when lava solidifies at the surface of the earth and the rapid cooling gives rise to fine grained rocks or rocks with a glassy texture such as Obsidian.
3.  Hypabyssal
These rocks parts of igneous bodies that are close to the earth surface and usually contain a fine grained texture.
4.  Plutonic Rocks
These are the rocks that solidify beneath the surface, and the slow cooling gives rise to coarse grains.

According to the mineralogical content rocks can be classified as follows,
1.  Ultra Basic / Ultra mafic
Has less than 45% of SiO2, and dark in colour. Mafic minerals(Olivine. Pyroxene) are dominant. eg. Peridotite, Komatite
2.  Basic / Mafic
45-50% SiO2 and lighter in colour. eg Gabbro, Basalt
3.  Intermediate
Feldspar is dominant. eg. Diorite, Andesite
4.  Acidic / Felsic
Quartz is dominant eg. Granite Ryolite

The textural classification is as below,
1.  Phaneritic
Coarse grained texture - produced by slow cooling
2.  Aphanitic
Fine grained texture- produced by rapid cooling
3.  Glassy
Non crystalline - very rapid cooling
4.  Porphyritic, Vitrophirc - Phenocrysts in fine grained and glassy matrix respectively.
5. Cavity textures - Vesicular, Amygdaloidal, miarotitic.
6.  Intergrowths - Graphic textures

Diagenesis

Diagenesis is the set of processes that convert deposited sediments into a sedimentary rock. These processes include chemical, physical and biological changes of the sediment. Diagenesis takes place at a depth of a few kilometers in the upper crust. For instance deposited mud turns to shale, sand turns to sandstone and gravel turns to Conglomerate as the result of Diagenesis.
The following distinct processes are included in Diagenesis.
1.  Compaction
The pressure due to the overburden causes the compresses the sediments forcing air and water out. This is also known as Consolidation.
2.  Re-crystallization
This refers the formation of new mineral crystals over existing grains. For example, Quartzite sandstone is formed by the development of silica on quartz grains.
3.  Cementation
This is the solidification of loose grains as a result of another mineral acting as a cement. Common cements include Iron oxide, Clay, Silica, and Calcium Carbonate.

A Classification of Sedimentary Rocks

Sedimentary rocks are the rocks formed by sediments. Sediments are  formed through the destruction and breaking down of previously formed rocks. Sediments are created by the processes of weathering and erosion, undergo transportation, accumulation and deposition and finally undergo a process known as diagenesis to produce Sedimentary Rocks. Physical factors such as Wind (aeolian), Water (alluvial), and Gravity(colluvial) aid the sedimentation process.

Sedimentary rocks are usually characterized by their structure and appearance. features such as bedding planes, ripple marks, mud cracks and fossils not only help identify sedimentary rocks but they also help to determine other properties of the rock such as age, environment of deposition, and the orientation of the original bedding.

image from -  wikipedia

The environments where sedimentation takes place, are classified as Continental environments, Shoreline environments and Marine Environments. According to the method of formation of sedimantary rocks in these environments Sedimentary rocks are classified as follows,

1.  Detrital/Clastic Sedimentary Rocks

Detrital sedimentary rocks are formed by the accumulation of solid particles and debris from pre-formed rocks (detritus). These rocks consist of fragments of rock and minerals. Depending on the Size and Shape of these fragments, several types of rock can be identified.
A cementation of large, rounded pebbles or gravel is identified as a Conglomerate. If the gravel and fragments are sharp and angular, it's called Breccia. According to the grain size of the rock and whether it contains sand sized particles or mud or silt the rocks are named as Sandstone, Siltstone or Shale. These rocks are formed mainly in Continental and Shoreline Environments.

2.  Chemical Sedimentary Rocks
Chemical Sedimentary rocks are formed by the precipitation of material from solutions. These rocks can be formed in water basins as well as in underground formations involving water such as Limestone Caves. According to the chemicals that precipitate to create these rocks are classified into groups such as, Carbonates(Limestone, calc tuffa, travertine), Silicates(Opal, Chalcedony, Agate), Ferruginous(limonite, Goethite), Allitic(laterite, bauxite), Halides(rock salt), Sulphates(Gypsum), and Phosphates(phosphorite).

3.  Biogenic Sedimentary Rocks
Biogenic sedimentary rocks are formed through the vital activity of plants and animals. The building up of animal tissue by using materials in water leads to the formation of these types of rocks. The accumulation of skeletal remains of organisms like marine vertebrae and even diatoms also end up in the formation of biogenic sedimentary rocks. Another point worth noting is that fossils of prehistoric animals can be found only in sedimentary rocks. Carbonates such as fossileferous limestone, siliceous diatomite and caustobioliths such as coal belong to this type. These rocks are primarily formed in Marine environments

4.  Sedimentary Rocks of Mixed Origin
Some sedimentary rocks exhibit properties that belong to more than one of the above types. These rocks are complex and contain detrital, as well as biogenic properties described above to a certain extent. Examples of these rocks are Marls, Calcareous sandstone and Siliceous Clay.