Tuffs form an unusual group of rocks that combine the characteristics of two major rock groups, sedimentary rocks and igneous rocks. Like some sedimentary rocks such as sandstone, tuffs consist of deposits of separate particles that are later bonded together to form a solid rock. Like extrusive igneous rocks such as basalt, a tuff deposit is the direct result of a volcanic eruption. The resulting rock type is called a pyroclastic rock, from pyro for “fire” and clastic for “broken piece.” A welded tuff is a one of the pyroclastic rocks that forms under specific conditions.
A tuff is a rock type that results from the hardening or “lithification” of volcanic ash, solidified crystals, and small fragments of volcanic rock; all of which are lumped together under the term “tephra.” Tuffs may form from ash and other bits of volcanic detritus that settle from the air, a type of tuff that is considered an ash-fall tuff. Ash falls and resulting tuff deposits of large eruptions can cover thousands of square miles; such as the blanket of ash that settled over the Pacific Northwest following the 1980 Mount St. Helens eruption.
While the deposition of ash-fall tuffs might resemble a gritty snowstorm, many tuffs are deposited in a far more dramatic setting, the pyroclastic flow. In a pyroclastic flow, fragments of tephra are kept in fluid suspension created by the included gases and water vapor; much like mudflows consist of super-dirty water. The chief difference is that a pyroclastic flow’s internal temperature can reach hundreds of degrees.
Pyroclastic flows are catastrophic events that occur during a volcanic eruption when a portion of the erupting material separates from the main eruption. Gas-infused tephra bursts from the volcano, creating a boiling cloud of tephra and semi-liquid rock particles that on steep slopes can attain speeds of more than 60 miles per hour. Although such flows usually cover relatively small areas, some have been known to travel more than thirty miles. If dominated by fragments of pumice and ash, the flow is called an ignimbrite. A different type of flow that contains large chunks of partially-cooled lava is called a “nuée ardente,” or glowing cloud. Welded tuffs form from the ignimbrite flows that contain mostly ash and fragments of pumice.
As the hot gases escape from the cloud of tephra, the flow slows and begins to deposit its still-hot contents. The top and bottom of the flow more rapidly cool because they are in contact with the air and the ground. This quicker cooling creates soft, poorly-lithified deposits that resemble ash-fall tuffs. The interior of the flow, however, retains its volcanic heat. The weight of the overlying deposits compresses the interior of the flow while the latent heat allows particles, particularly shards of volcanic glass, to deform and “weld” together; creating the distinctive texture known as welded tuff. Semi-liquid particles of volcanic rock such as pumice also flatten; which often gives welded tuff the appearance of layering like a foliated metamorphic rock or some sedimentary rocks. The texture of a welded tuff, with its flattened chunks of volcanic rock and bits of volcanic glass, is known as a eutaxitic texture.
Welded tuffs are defined by this eutaxitic texture and by the size of the grains, rather than by the minerals or rock types present. The grains may be ash, glass and rock fragments less than 4mm (1/6 inch) in size; and lapilli, rock and glass fragments from 4 to 32mm (about 1-1/3 inch) in size. A tuff that is dominated by fragments that are larger than ash size is called a lapilli tuff or a welded lapilli tuff, depending on whether it’s an ash-fall tuff or has been welded.
Welded tuffs are known from around the world wherever there has been volcanic activity. In North America, many deposits can be found from the Rocky Mountains westward, including the pacific Northwest and the southwestern deserts. Welded tuffs are have been created by the eruption of many well-known volcanoes, such as Mt. Pinatubo in Indonesia and Pompeii in Italy.