Hydrovolcanic landforms form from the explosive steam eruption created by the interaction between hot rising magma and water. These landforms are maars, tuff rings, and tuff cones.
Usually, water will flash into steam when it encounters hot, molten magma or lava. However, since steam takes more space than water, it will rapidly expand.
This rapid steam expansion will fragment and eject magma, lava, and country rock into the air. Also, it causes lateral or horizontal surges.
We collectively call the ejected material, including ash, lapilli, volcanic blocks, or bombs that later fall to the ground, pyroclasts.
The deposition of these pyroclasts and, in some cases, surface excavations create various hydrovolcanic landforms, which we will discuss.
Usually, most hydrovolcanic landforms form from single subaerial eruptions, i.e., monogenetic. However, some of such eruptions may create hybrid landforms.
Factors that affect the landform created include water content and where the explosion occurs.
Let us briefly define or describe the various landforms and tell you how they form. Later, we will compare them.
1. Maars
Maars are small, shallow, nearly circular to oval low volcanic craters surrounded by low profile rims of pyroclasts. They lie below the pre-eruptive surface and are often filled with water, forming lakes known as maars.
Their bottom surface is nearly horizontal with rims sloping at about 20° and are associated with diatremes. Diatremes are breccia-filled, carrot-shaped, or downward-tapering volcanic pipes.
Usually, maars measure 0.3 to 3 kilometers in diameter, and their rims are less than 30 meters high. These rims have mostly accidental clasts but can have juvenile clasts or both.
Juvenile indicates that the pyroclasts originated from magma that was erupting. In contrast, accidental clasts are those from country rock, i.e., not erupting magma.
Maars form when magma interacts with groundwater just below the Earth’s surface. The resultant steam explosion will fragment and emplace bedrock and magma.
Examples of maars include Espenberg and Ukinrek in Alaska and Hopi Buttes in Arizona, USA. Others are Meke and Narköy in Turkey and Kilbourne and Hunts Hole in Mexico. Also, several maars occur in the Eifel region of Germany.
2. Tuff rings
Tuff rings are small, nearly round, low-profile volcanic cones with a broad, shallow crater. These volcanic cones form on the Earth’s surface from the interaction of rising magma or lava and surface water.
Tuff rings are 0.2-3 km in diameter, with gently sloping, less than 25° rims. However, most have slopes dipping at 2-10°.
These pyroclastic rims are usually less than 50 meters thick.
Tuff ring rims are made of mostly fresh, unaltered, or less altered pyroclasts. These pyroclasts have thin bedding with a small number of accidental clasts.
Examples of tuff rings in the US occur in Hopi Buttes volcanic fields in Arizona, Mud Hill, and Fort Rock Volcanic Field in Oregon.
Others are Puketarata in New Zealand, El Cuchillo in the Canary Islands, Capelinhos Tuff Ring in Faial Island, and the Azores.
3. Tuff cones
Tuff cones or ash cones are small conical volcanoes with steep slopes and a broad bowl-shaped crater. They are built on the Earth’s surface during moderately explosive phreatomagmatic eruptions that involve abundant water.
Tuff cones are about 0.1-1.5 km wide and 50-400 meters high, with slopes over 25° at rims. They have thick beds of predominantly ash but may have other pyroclasts, often poorly sorted.
Their steeper slopes, palagonized basaltic volcanic glass, and accretionary or armored lapilli indicate they form where what is abundant.
Other indicators are mudflows during the eruption, vesiculation of tuff after it forms, and vertical bed plastering.
Famous tuff cones include Sinker Butt in Idaho and Punchbowl and Diamond Head on Oahu Island in Hawaii, USA.
Others are in volcanic fields like Auckland in New Zealand, East Eifel in Germany, and Western Snake River plane in the US.
Tuff cones vs. tuff rings vs. maars
Maars, tuff rings, and tuff cones are the second most abundant landforms after scoria or cinder cones. They form from phreatomagmatic activities and share some similarities.
However, they also have differences in appearance, i.e., morphology, shape, and size. Also, the explosion energies involved and the amount of water vary.
Appearance and description
Tuff rings are broader, low-profile landforms with a gentle slope, while tuff cones are smaller, higher-profile, and have steeper slopes.
These two are constructive volcanic landforms since they form on the pre-eruptive surface. Therefore, they are higher than the surrounding surface.
On the other hand, maar craters resemble tuff rings. However, they lie below the pre-eruptive surface.
These three hydrovolcanic landforms may have comparable diameters, although maars and tuff rings tend to be larger.
However, tuff cones have a higher elevation, reaching 400 meters. Tuff rings are usually less than 50 meters, and maars are less than 30 meters.
Lastly, maars have diatremes, while tuff cones and rings don’t since eruption occurs on the surface.
Pyroclasts and bedding
Tuff rings have mostly fresh and unaltered juvenile pyroclasts, while tuff cones are altered and have palagonite from altered basaltic glass. Maars, on the other hand, have mostly accidental clasts.
Also, tuff rings have thin, well-beaded layers, while in tuff cones, the bedding is massive, poorly sorted, and mostly ash.
On the other hand, maars have unbedded diatremes, followed by layered bedding, some showing cross-bedding structures seen in sedimentary rocks. The upper parts may have sediments.
Water abundance and eruption power
Tuff cones occur in abundant water, resulting in a weaker phreatomagmatic activity. The excess water suppresses eruptions, resulting in ejecta falling near the vent. This, together with abundant water, creates steep slopes.
Tuff rings, on the other hand, occur in intermediate eruption power less abundant water. An example is an ephemeral lake in a desert. This results in ejecta spreading wide and forming a lower profile.
Lastly, maars have the most powerful or energetic explosion because aquifer properties limit water supply.
A single eruption can form hybrid landforms
A single or monogenic eruption may form a hybrid volcanic landform. For instance, the Motukorea volcano in New Zealand has an intra-tuff ring scoria cone.
However, the tuff cone was breached by a later stage lava flow. All these happened from a single eruption.
References
- Brand, B. D. & Brož, P. (2015) Tuff Cone. In Hargitai, H. & Kereszturi, Á. (ed) Encyclopedia of planetary landforms (2nd ed. pp. 2197-2203). Springer.
- de Silva, S. & Lindsay, J. M. (2015). Primary Volcanic Landforms. In Sigurdsson, H. (ed.) The encyclopedia of volcanoes (2nd ed. pp 273-292). Elsevier Science Publishing Co. Inc
- De Hon, R. (2015). Maars. In Hargitai, H. & Kereszturi, Á. (ed) Encyclopedia of planetary landforms (2nd ed. pp. 2197-2203). Springer.
- Winter, J. D. (2014). Principles of igneous and metamorphic petrology. Pearson Education