Welded Tuff

Welded tuff forms during violent volcanic eruptions when an eruption column collapses forming a pyroclastic flow. Pyroclastic flows are sometimes described as a stone wind that can travel great distances from a volcano's vent.

Welded tuff, Photo by Myrna Martin

Dark tuff welded together with a  piece
deformed pumice in the rock
, Myrna Martin

Ignimbrite forms during violent volcanic eruptions when an eruption column collapses forming a pyroclastic flow. The pyroclastic flow carries hot gases and other pyroclastic material down the slopes of a volcano at hurricane speeds. The super hot gases and debris in the pyroclastic flow cools in three layers called ignimbrite. 

Tuff layers
The top and bottom layers form a soft rock called tuff. Tuff forms a top and bottom layer of soft rock because the air and ground beneath the pyroclastic material is much cooler than the inside of the flow deposit.

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Center layer of ignimbrite
The center layer remains hot enough to "weld" the ash, pumice and other debris into a hard rock. Glassy rock fragments that make up much of the material deposited by pyroclastic flow deforms readily from the heat.

Deforming pumice
Pieces of pumice is flattened and deformed during the welding process forming flattened shapes. You can see pieces of pumice in the rocks because it is a slightly different color and usually has a curved or oval shape.

Layers of tuff at Yellowstone
Most tuff forms during eruptions of rhyolite lava. Rhyolite lava is associated with super volcanoes and their eruptions. Welded tuff can be seen at Yellowstone Park on the hillsides where the Lava Creek Tuff is exposed.

Lava Creek Tuff
The Lava Creek Tuff is 1000 times larger than the deposits formed during the eruption of Mount Saint Helens on May 18, 1980. Large deposits of these rocks formed during the Alaskan eruption of Novarupta Volcano in 1912. The rocks can be found in the Valley of Ten Thousand Smokes where Knife Creek cuts through the pyroclastic flow deposits.

More Rock Cycle Links

Intrusive Rocks Learn more about intrusive rocks that cool underground forming batholiths like the Sierra Nevada Mountains in California.

Clastic Rocks Find out how bits and pieces of older rocks form shale, sandstone and conglomerate rocks.

Pyroclastic Material This page gives all kinds of information about volcanic material that forms during volcanic eruptions.

Organic Rocks Learn how animals extract calcite out of ocean water to form shells that are organic rocks.

Limestone Caverns Learn how limestone caverns form and where you can visit some of them.

Welded Tuff Find out how tuff is produced during violent eruptions and by supervolcanoes.

Rock Cycle Learn more about igneous, sedimentary and metamorphic rocks and how they form.

Kids Fun Science The links on our home page include information about volcanoes, science activities, plate tectonics, the rock cycle and much more.

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