(By D. Burns, 2010)
Hydrovolcanic eruptions are volcanic eruptions that are generated from the interaction of magma and/or lava with water. Magma rising through the Earth’s crust carries with it a great deal of heat, or thermal energy. If this hot magma comes in contact with water (or ice), either on the surface or in the subsurface, the water can be quickly converted to the gaseous state (steam) via the transfer of thermal energy. This conversion of liquid water to steam is typically associated with a significant increase in the volume of the water molecules (depends on pressure). Because the magma and surrounding country rock can be viewed as a finite space, the rapidly expanding water can cause the surrounding rock/magma to mechanically fail (the rock breaks into fragments). The efficiency of the fragmentation process (i.e. to what degree the surrounding rocks are broken apart) is a function of the confining pressure and the water to magma ratio. The optimum range of water to magma ratios to fuel an explosive event is 0.1-0.3 (Sheridan and Wohlerz, 1983). When discussing hydrovolcanism it is important to be familiar with some basic nomenclature.
Phreatomagmatic refers to the interaction of water with juvenile magma, whereas phreatic or steam explosions simply involving hot rocks and water, and although magmatic heat is likely the source of the thermal energy in the reaction, the deposits do not contain juvenile magmatic material. The pages in this section will discuss various hydrovolcanic structures, processes, and features in an attempt to illustrate and provide an introduction to hydrovolcanic eruptions.
Figure 1. Base surge and eruptive column at Calelinhos in the Azores. Photo credit-R.V. Fisher.
The main factors that control the type of landform created from interaction of water and magma are the ratio of water to magma and the depth within the crust the interaction takes place. As discussed in the introduction to hydrovolcanism, deposits from hydrovolcanic eruptions are typically highly fragmented due to the explosive nature of the interaction. Along with being highly fragmented hydrovolcanic eruptions are typically dispersed over smaller areas, likely due to the increased density of a wet eruption relative to a similar dry eruption. In this section we will discuss various volcanic landforms that form from the interaction of magma and water.
Figure 2 (right). Diagram showing various hydrovolcanic landforms and the type of water reservoir, water to magma ratios, and mechanical energy where the different constructs are likely to form. Figure from Francis and Oppenheimer (2004).
Maar volcanoes are simple circular depressions surrounded by gently sloping beds of highly fragmented pyroclastic material (Figures 3 and 4). Maars form when rising magma comes into contact with subsurface water (an aquifer for example) and subsequent phreatic explosions excavate a hole in the country rock. Maars are typically easy to distinguish from other hydrovolcanic features because they excavate the subsurface and leave craters in the ground.
Figure 3. Image of one of the Inyo Craters (small maar), eastern California. Photo courtesy of Arron Steiner.
Tuff rings are another landform commonly associated with hydrovolcanism. Tuff rings differ from maars in that the magma-water interactions that form tuff rings occur on the Earth's surface. This interaction can simply occur when rising basaltic magma encounters groundwater at the surface (Figures 2 and 5). Tuff rings also typically contain more juvenile material than maars. Tuff cones are smaller, steeper versions of tuff rings, resembling cinder cones (Figure 2). The factors that dictate rather an eruption will create a tuff ring or tuff cone are the relative amounts of water and magma and the duration of the eruption. If abundant water is present (water to magma >0.3) at the time of eruption the fragmentation process will be less efficient and eruptive products will be water saturated allowing the structure around the vent to steepen beyond the natural angle of repose. Tuff cones commonly form when rising magma is emplaced into a shallow body of water.
In discussing hydrovolcanic phenomenon it is also important to discuss lava-water interactions. Examples of these interactions are commonly seen at ocean islands like Hawaii. When lava flows into a body of water rapid fragmentation can occur and small cones of fragmented ejecta can form. These littoral cones may look similar to other tuff rings or cones, but the lack of a vent makes them unique.
Submarine volcanism is another type of hydrovolcanic eruption, but because submarine volcanism is discussed in great detail elsewhere on the Volcano World website this discussion will be brief. Due to incredibly high water-magma ratios and a large amount of confining pressure from the water column above, explosive interactions in deep bodies of water are limited. Some local fragmentation can occur on the surface of submarine lava flows, but erupting lava stays mostly intact and commonly form pillows due to rapid cooling. In the case that an island is building vertically from the sea floor there will be a point as the island reaches the surface when the confining pressure is no longer a factor and the magma-water ratio reaches a point where explosive activity can commence. This phenomenon has been seen at numerous ocean islands, including the island of Surtsey off of the southern coast of Iceland (these type of explosive eruptions are often referred to as Surtseyan eruptions).
Figure 4. Image of the Wabah maar in Saudi Arabia. Photo courtesy of Vic Camp.
Figure 5. Image of the Jabal Bayda tuff ring in Saudi Arabia. Photo courtesy of Vic Camp
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