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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