Dome gneiss hill near Ranchi city of India.

It is formed mainly by weathering and erosion.

by

Dr. Nitish Priyadarshi

Geologist.

Above pictures are of Dome gneiss hill. This typical dome-gneiss (locally known as dongris) 26 km from Ranchi on Ranchi-Bundu highway (85⁰ 27^” longitude and 23⁰ 14^” latitude) has slopes on all sides (the average inclination being about 40 degree to 50 degree). There is little soil but oozing moisture from rock foliae supports tuft grass except the entirely bare rocky surfaces.

A dome is a curved formation or structure. It is shaped like half of a sphere. Imagine cutting an orange in half, and placing it cut-side-down on a table. This is the shape of a dome, although most domes in nature are not perfectly rounded.

Some natural domes develop when magma from deep within the Earth pushes up surface rock layers. This type of geologic dome can form as magma intrudes between two layers of sedimentary rock. The magma creates a dome or triangle shape as it pushes the other layers apart. The hardened magma that forms this type of dome is called laccolith.

Another kind of dome is shaped primarily by weathering and erosion, which cause curved sheets of rock to separate from a large rock mass.

The dome shaped hill near Ranchi is formed mainly by weathering and erosion.

The Chotanagpur plateau region is made up mainly of Precambrian rocks but has witnessed uplifts synchronously with the Himalayan uplift in the Cenozoic. Chotanagpur plateau of Jharkhand is quoted as a typical example of domal plateau because it is studded with numerous batholithic domes which were intruded in the Dharwarian sedimentaries during Archaean period. Prolonged denudation has removed the superincumbent Dharwarian covers and these batholithic domes have been exposed on the surface.

The European geographers who attended the ‘symposium on erosion surface’ in 1968 at Ranchi felt that Chotanagpur was a fossil or dead erosion surface. There were pediments in the region during the past more arid climate. Now the climate had changed to more humid one. This has caused so much production of weathered waste in conjunction with hot climate, that the sediments were more than what could be cleared by the transporting process (mainly running water). Thus the earlier pediments and bedrock were concealed under the debris and sediment.

Fuller’s earth mining in Pakur district in Jharkhand State of India.

Bentonite may have played a major role in Fuller’s earth formation.

By

Dr. Nitish Priyadarshi

 Fig. Mining of Fuller's earth.

 Fig. Fuller's earth.

 Fig. Mining area.

Fig. Bentonite.

Abstract:

Pakur  is located in the north east corner of Jharkhand State of India. Major part of the district is characterized by undulating topography covered by basaltic flows of Rajmahal Trap. Fuller’s earth mining is limited up to a shallow depth only and it is done manually. Most of the Fuller’s earth is send to local market of Pakur and neighboring state of West Bengal. Local villagers have the ownership of the mining and the profit is distributed among them. Fuller’s earth is mainly of sedimentary origin. It occurs interstratified with geologically late sands and clays. It is thought that most fuller’s earth originated from volcanic ash. Montmorillonite and attapulgite are the principal constituents, and montmorillonite is characteristic of bentonite. There is further suggestion that it was derived from bentonite by natural leaching in surface water, assisted perhaps by plant acids and bacteria.

Discussion:
Pakur  is located in the north east corner of Jharkhand State: at 23°40' to 25°18' latitude and 86°25' to 87°57' E. longitude. Pakur is bordered by Sahibganj district in North, Dumka district in South, Godda district in West and Murshidabad and Birbhum districts (of the West Bengal) in the East. Pakur has an area of about 696 square kilometres and a population of 899,200 (2011 Census). Pakur, famous for stone and Beedi (Biri) making industry, is one of the important revenue-earning districts of newly created Jharkhand State. Its black stone chips have got Asiatic fame in constructional qualities.Pakur was earlier a Sub-Division of Santhal Parganas district of Jharkhand.

Major part of the district is characterized by undulating topography covered by basaltic flows of Rajmahal Trap. The main geomorphological features of the district are the rolling peneplain in the south with numerous remnants of ancient ridges and resistant lava plateau of Rajmahal. These plateaus rise above the general level and occupy major part of the district. The general elevation of the hills and plateau varies from 70 to 371 m above MSL.

Rajmahal Trap is the major rock type in the district. The other geological formations of the district are alluvium, laterite and gondwana. In the eastern part of the district, recent alluvium occurs in patches, which is mainly composed of sand and sub ordinate clay. Laterites are mainly of in situ origin and have been formed by sub-aerial erosion of underlying basalts under favorable climatic conditions. Laterites provide a productive ground water  reservoir due to their very porous and permeable nature. The most significant  Barakar coal measures of Gondwana formation occurs in western part of the district.

The district of Santhal Paraganas is mainly a dissected upland of ancient crystalline rocks which are covered with thick flows of volcanic lava. The ancient crystalline rocks, collectively called the Archaean gneisses, cover the greater part of the district. The principal rock is a granitoid gneiss earlier known as the Bengal gneiss.

Fuller’s earth ( commonly known as Multani soil) mining is limited up to a shallow depth only and it is done manually. Most of the Fuller’s earth is send to local market of Pakur and neighboring state of West Bengal. Local villagers have the ownership of the mining and the profit is distributed among them.

Fuller’s earth is a variety of clay, so named because it has been used by fullers to full, or remove grease from cloth. Its marked absorptive powers, however, have caused it to be more widely used to filter and decolor oils, fats, and greases.

The dominant use (about 70 percent) of fuller’s earth is in petroleum refining for filtering and clarifying petroleum products, mainly lubricants, and about 10 percent is utilized in the refining of vegetable oils and animal fats. Its use is growing in domestic water purification. It also removes putrescence, odors, and even coliform bacteria from oily waste waters.

In oil refining, colored oils passed through fuller’s earth come out colorless. In addition, it removes naphtha gum, and improves the sludge and carbon content, acidity, and viscosity of lubricating oils. It removes color, odor, and taste from vegetable and animal oils. Fuller’s earth cannot be reused after filtering vegetable oils.

Minor uses include printing, abrasives, detection of coloring agents in food products, filler, and cosmetics. Fuller’s earth is a clay substance that is hugely popular for its healing property against acne and blemishes.  It is very rich in magnesium chloride which helps to reduce acne.  In oil refining fuller’s earth is being displaced by activated bentonite.

Fuller’s earth is mainly of sedimentary origin. It occurs interstratified with geologically late sands and clays. It is thought that most fuller’s earth originated from volcanic ash. Montmorillonite and attapulgite are the principal constituents, and montmorillonite is characteristic of bentonite. There is further suggestion that it was derived from bentonite by natural leaching in surface water, assisted perhaps by plant acids and bacteria. Ash deposited in situ becomes bentonite; that which is gradually washed in, with leaching of grain surfaces, become fullers earth. Extended weathering of fuller’s earth may yield Kaolin.

Reference:

Bateman, A.M. 1955. Economic mineral deposit. John Wiley & Sons, Inc. New York.

http://cgwb.gov.in/District_Profile/Jharkhand/Pakur.pdf