OREGON STATE UNIVERSITY
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In this section we will briefly explore various features that commonly associated with hydrovolcanism. Hydrovolcanic landforms are typically comprised of very fine-grained, laminar beds dipping away from the vent region. Due to the high density and explosive nature of hydrovolcanic eruptions, hydrovolcanic eruptions are often associated with base surges (Figure 1). Base surges form from both the gravitational collapse of the dense water-rich plume and from the rapid lateral expansion of steam and gas as material reaches the surfaces. Base surges are typically fine-grained, can range from massive to cross-bedded and laminar, and interestingly can be fairly cool (<100 degrees Celsius) during emplacement, beautiful base surges can be seen in the early nuclear bomb tests at the Bikini Atoll.
Accretionary lapilli are another common feature in hydrovolcanic eruptions. Accretionary lapilli form when wet, sticky ash within the eruptive column plasters itself together. Once these particles start to accumulate, they can continue to incorporate more material, resulting in a round mud ball (Figure 6). The accretion of wet ash can also occur around lithic fragments within the column, and in this case the accreted particle is called an armored mud ball.
Owing to the nature of wet sediments, soft sediment deformation is also common in hydrovolcanic eruption. The most recognizable type of soft sediment deformational features are bomb-sags. Bomb-sags form when chunks of rock ejected from the vent land in wet sediments, because the beds are wet and ductile, the rocks deform the beds without breaking through (Figure 7). If the sediments were dry, the rock would simply break through the ashy layers. Another common deformation feature in hydrovolcanic deposits is slumping. In response to the force of gravity, the heavy, wet sediments simply start to flow downslope.
Figure 6 (left) Accretionary lapilli from Kiauea Volcano. Photo credit-USGS.
Palagonitinization is another common process that can readily be seen in hydrovolcanic eruptions. Following the interaction of water and magma in a hydrovolcanic eruption, warm basaltic magma (the glass) is easily susceptible to alteration. The basaltic glass readily hydrates to yellowish mixture of iron oxides and smectite clay known as palagonite (Figure 8).
Although palagonite is a very common feature in hydrovolcanic eruptions, it can also form during the weathering processes, so although it is useful in helping to identify hydrovolcanic deposits, it should not be used alone as a sure indicator of such processes.
Figure 7 Right - Bomb-sag in deposits from Hole in the Ground, central Oregon. Photo courtesy of Arron Steiner.
Figure 8 left. Palagonized scoria from Fort Rock, central Oregon. Photo courtesy of Arron Steiner.
Figure and Photo Credits