Welded tuff can be found here in the Jodhpur Fort hill area.
The site has been identified by the Geological Survey of India as being of significance and is listed by them as one of 26 national geographical monuments in the country.
Welded tuff is rock produced by emissions from volcanic vents consisting mainly of ash and other larger particles composing of glass, quartz and feldspar which were carried away on streams of air before settling down on the earth. Tuff that is not welded will contain smaller particles and be loosely consolidated - and will often contain holes where the gas has been present.
Welded tuff is a pyroclastic rock, (rock formed from volcanic materials) that was sufficiently hot at the time of expulsion to weld together. During welding, the glass shards and pumice fragments stick together, deform and compact.
Pyroclastic rocks are made up of a large range of particle sizes; from the largest rocky pieces, to very fine ashes and tuffs. Ash is considered to be pyroclastic because it is a fine dust made up of volcanic rock. One of the most spectacular forms of pyroclastic deposit are the ignimbrites, deposits formed by the high-temperature gas and ash mix of a pyroclastic flow event.
Ignimbrite is the result of explosive eruptions from volcanos or faults caused by the rapid explusion of magmatic gases. The escaping gas carries the magma with it up through the hole in the earth, and in doing so will create fragments like pumice and ash, which are carried in the surrounding gas and will flow downhill and spread to wherever the atmosphere around it is lighter than the material it carries. This is known as a pyroclastic density current, sometimes as a pyroclastic flow.
Ignimbrites form sheets that can cover as much as thousands of square kilometers. Sometimes they can form a landscape covering carpet that is thicker in valleys and other depressions in the surface of the earth.
Many igimbrites are loosely held together particles of rock, but some show welding through extreme temperatures, as is the case here, which leaves the ignimbrite with the texture of solid rock, and so these two types are called either welded tuff and welded ashflow, depending on the size of the particles, with tuff being larger and ash smaller.
Upon cooling they develop fractures or cooling joints which result in near vertical columns and horizontal terraces. The columns that result are rectangular to hexagonal and are getting up to lengths of 30m or more at places. The joints are covered by deep purple coloured rhyolite, which is common in welded tuff formations. This covering over of the columns gives the rocks a uniform and colourful deep purple appearance. This covering does mask the underlying make up of the columns of welded tuff, which would show more particles to the naked eye if they were still exposed rather than being covered as they are. The rhyolite covered the columns later towards the end of the eruption as the magma flowed out more slowly and cooled slower over a wider area.
Columns of Welded Tuff
To log this cache, please got to the location given by the coordinates and please answer the questions below by emailing me using the contact details through the site. Please tick the box that allows me to repond directly to your email - it makes the process simpler. I will aim to respond within 48 hours.
1. How high are the columns immediately in front of you?
2. Can you see any signs of tuff that is not welded in the rocks around you? If not, why would you suppose that is?
3. Look across the valley from where you are standing, and you should be able to see a very different type of landscape which is made up of a terrace of horizontal layers of rock again with cracks and fractures in it, but no columns. How do you think that landscape came to be formed and what does that suggest to you in terms of the proximity to you of the fault or volcano that emitted the tuff that welded to form the columns?
The nearby fort is built on another national geological monument - the Malani Igneous Suite Contact - this is where the igneous multicoloured rock that emerged from the volcano or fault has come into contact with the lighter coloured Jodhpur sandstone that occurs widely in the area and is what the fort itself is built of. Look up from your current location and you will see the two types of rock meeting. You will be able to see this even more clearly if you do visit the fort itself and stand at N26 17.943 E073 01.111 and look up to the base of the foundation of the fort.
There is a zip ride locally that will give you excellent aerial views of the local geology and the fort. Not needed for the cache but to add interest to your log, please feel free to post any photos of zip riders you see while finding the cache - they will be almost overhead!
Terrace of Tuff Across the Valley from the Cache Site
References:
1. The Geological Survey of India website.
2. "Fractures in Welded Tuft" - The Geological Society of America Special Paper 408, 2006 Kenneth Wohletz.
3. Wikipedia - Welded tuff, rhyolite, pyroclastic, etc.