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Tuff Rock

Tuff is a kind of rock that is made of volcanic ashes. The volcanic ashes get ejected from a vent during the volcanic eruption. This is then followed by ejection and deposition. The deposited ash is then lithified into the form of solid rock. This rock contains ash (75% or more) which is considered tuff, while those rocks containing 25% to 75% ash are described as tuffaceous. 


We will know more about this interesting rock in our prevailing section. Below contains information about volcanic tuff as well and some cool facts about this rock. Let us launch into it already. 


More on Tuffs

Tuff is a soft, porous rock that is usually formed by compaction and by the process of cementation of volcanic ash or dust. While, in extensive deposits, tuff varies greatly not only in the context of texture but also in the context of chemical and mineralogical composition. 


The presence of any geological period that was entirely free from volcanic eruptions was never the case hence tuffs range in age from Precambrian to the Most Recent. The older ones among them have lost all their original textures and they are recrystallized. Old basaltic tuffs are represented by the green chlorite and hornblende schists and many other rhyolitic tuffs by sericite schists.


In some eruptions, the foaming of the magma wells to the surface as an emulsifier of hot gases and by incandescent particles. The shredded pomaceous material spreads quickly, over the gentle gradients, like a glowing avalanche which may move many kilometers at speeds more than 160 km (100 miles) in one hour.


After this comes to rest, the ejecta or the erupted matter is firmly compacted by adhesion of the hot glass fragments to form streaky, and welded tuffs which are covering New Zealand, Peru, Guatemala, and even in Yellowstone National Park. When explosions explode underground, these fragmented materials are forced to be forced out violently into the surrounding rocks, which form into intrusive tuffs.


Tuffs are classified as either igneous or sedimentary rocks. These rocks are usually studied on the basis of igneous petrology. They are quite at times described being used as sedimentological terms. 


Volcanic Tuff

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Volcanic tuff is a form of igneous rock, which is made up of the material ejected during an explosive volcanic eruption. Due to these eruptions, fragments of volcanic material are blasted and erupted from the volcano, propelled through the air, and then get deposited in the surrounding area. In the open air they get compacted and cemented into rock, the process can be instant if the material is still hot. Tuffs consist primarily of volcanic ashes. They also can contain lapilli (which is 2- 64mm volcanic fragments). Further the volcanic bombs (which are >64mm lumps of lava that cool into solid fragments before they reach the ground). Closer to the vent, a tuff is likely to contain bigger blocks of material that are catapulted from the volcano in a matrix of ashes. Further away from this vent, are the tuff deposits. The tuff deposits are more likely to be exclusively made from the fine volcanic ash particles that are carried by the wind. This specific kind of tuff comes from the Borrowdale Volcanic Group in the Cumbria and it is associated with volcanic activity in the late Ordovician period (which is approximately 450 million years ago). The greenish colour present in them is due to the presence of the mineral chlorite.


Facts on Tuff

Where are Tuffs Found?

Tuff is usually the thickest near the volcanic vent and gradually decreases its thickness with distance from the volcano. Rather than being a "layer," a tuff is usually a "lens-shaped" structure of the deposit. Tuff is the thickest on the downwind side of the vent or on the side of the vent where the blast occurred.


What is Tuff Used for?

Tuff is a soft rock hence it has been used for construction purposes. They are quite common in Italy. The Romans uses it often for construction purposes. The Rapa Nui people also use this rock to make most of the Moai statues on their Easter Island. 


Properties of Tuff Rock:

  • Colour – Brown, Grey, Yellow.

  • Durability – Scratch and Water Resistant.

  • Resistance - Heat Resistant, Impact Resistant, Pressure Resistant, Wear Resistant.

  • Types of Weathering - Biological Weathering, Chemical Weathering, Mechanical Weathering.

How Do You Identify Tuff?

Tuff is not in a "layer," form they are generally in the form of a "lens-shaped" deposit. One can also identify tuff by checking the thickness on the downwind side of the vent or on the side of the vent where the blast was actually directed towards, the thickness is the thickest that side.  Some tuff deposits are hundreds of meters thick and they have a total eruptive volume of many cubic miles.


Which Type of Rock is the Volcanic Tuff?

Volcanic tuff is a type of igneous rock which are formed from the material that is ejected during an explosive volcanic eruption.

FAQs (Frequently Asked Questions)

1. What are Volcanic Ashes?

Ans. Volcanic ash is a kind of mixture of rocks, minerals, and glass particles that are expelled from the volcanic eruption from the volcano vent. These particles are very small are then 2 millimeters in diameter. The volcanic ashes are full of holes, this gives them quite a low density.


Volcanic ash is when the igneous rock material is quite blown into the air after the volcano erupts and this consists of the fragments of the jagged rock, minerals, and volcanic glass, which are created during the eruption. The volcanic ashes are hard and abrasive.

2. What are Igneous and Sedimentary Rocks?

Ans. 

Igneous rocks are those rocks that are formed from the melted rock which are present deep inside the Earth’s crust. Sedimentary rocks are formed from the layers of sand, silt, dead plants, and from animal skeletons. The Metamorphic rocks are formed from the other type of rocks which changes by heat and pressure-formed underground.


There are primarily three kinds of rock: igneous, sedimentary, and metamorphic. Igneous rocks are formed when the molten rock (magma or the lava) cools and solidifies. The sedimentary rocks originate when the particles settle out of water or through the air, or by the process of precipitation of minerals from the water. They accumulate forming layers.

3. What is the Texture Type of Tuff Rock?

Ans. Tuff rocks are coarse-grained textures that indicate the intrusive rocks whereas the fine-grained textures generally indicate the extrusive type rocks. The extrusive rocks are marked by their texture and are discussed under the rock textures: obsidian, pumice, scoria, tuff, and volcanic breccia.

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