Tuff rings are small, low-profile, circular volcanic cones with broad, shallow craters and gentle slopes.
These volcanic landforms form from steam explosions that result from lava or magma interaction with groundwater on the Earth’s surface.
Such explosions are known as hydrovolcanic or phreatomagmatic. They are what mostly fragment lava and some country rocks, forming ash, lapilli, or blocks.
These fragments are collectively known as pyroclasts. They will be deposited and later consolidated to form a tuff ring.
Tuff rings are one of the hydrovolcanic landforms on Earth. The others are maars, tuff cones, and littoral cones.
Other related terms you will find helpful in this discussion are juvenile and edifice. Juvenile means derived directly from magma involved in an eruption.
On the other hand, an edifice is part of the main volcano portion that forms lava, tephra, pyroclastic flows, lahars, or any other volcanic deposits.
Usually, tuff rings occur together, form similarly, and are often compared to maars or tuff cones. It is good to give you a hint of what these two are.
Maars are shallow or low-relief craters formed during phreatomagmatic eruption. These craters are usually filled with water, forming crater lakes called maar.
On the other hand, tuff or ash cones are small, nearly rounded, steep-sided volcanic cones. They have a bowl-shaped summit crater.
Description and appearance
Tuff rings are small, low-profile, circular cones with wide, bowl-shaped craters and gentle-sloping rims made of pyroclast.
Their shape resembles doughnuts, and their craters are dry and above the surrounding ground.
Tuff rings are 0.2 to 3 km wide, and their pyroclastic rims are usually less than 50 meters thick. However, some, like the Capelinho tuff ring, are about 200 meters thick. Still, their diameter-to-height ratio is large.
Usually, these volcanic edifices have gentle external slopes of 25° or less. However, most rims dip at 2-10° with a comparable inward and outward slope. Their ring-like rims may be intact or breached.
Tuff ring rims are made of distinctive, relatively fresh, and unaltered pyroclasts, often ignimbrite. These consolidated pyroclast have thin beds and are deposited by volcanic surges or ash falls.
Pyroclasts that form tuff rings are mostly well-stratified juvenile clasts or fragments with a small amount of accidental. Accidental clasts are those shattered from country rock.
Most of these fragments are ash to non-vesicular lapilli. However, they may have larger blocks, some as large as 3 meters or more.
However, these hydrovolcanic landforms don’t have spatter or agglutinates. These are welded pyroclasts deposited while magma is still plastic.
Lastly, while most are circular and somewhat asymmetric, eruptions during strong winds can make these landforms less symmetrical.
Composition
Most tuff rings form from highly fluid basaltic, other mafic, or even ultramafic magmas.
Mafic magmas are low in silica and relatively high in iron and magnesium or minerals high in these elements. Examples are magmas that form basalt and basanite.
On the other hand, ultramafic magmas have at least 90% mafic minerals. Examples include magmas that form kimberlite, komatiite, and picrite basalt.
How do tuff cones form?
Tuff rings form phreatomagmatic eruptions, usually Surtseyan or Taalian types. These hydrovolcanic eruptions are driven by steam formed when rising lava or magma interacts with less abundant groundwater on the surface.
Usually, water will flash into steam when it encounters magma or lava, producing an explosion or phreatomagmatic eruption.
This explosion will blast lava and sometimes country rock into ash, lapilli, volcanic blocks, and volcanic bombs into the air. Also, it causes a sideways, lateral, or base surge rather than upwards.
The lateral surge and fall of ash or fragment air will deposit pyroclasts. Over time, these materials will consolidate, creating a tuff.
Most tuff rings usually form from single or monogenetic phreatomagmatic eruptions. However, this might involve phases or pulses of repeated explosions.
Also, they occur under the same conditions and will grow rapidly within a few days, months, or, at most, a year. This results in simple stratigraphy compared to polygenetic shield volcanoes and stratovolcanoes.
However, some can evolve into other volcanic landforms or have other phases like cinder cones, lava flows, etc.
Note: Unlike maars that cut into the Earth’s surface, tuff cones and rings are constructive. They add material onto the Earth’s surface. This happens because tuff rings occur on or very close to the surface. This explains why they don’t cut into the Earth’s surface.
Tuff ring examples
Tuff cones, rings, and maars are the second most common volcanic landforms after cinder cones. These three may occur together in some volcanic fields.
In the US, Hopi Buttes volcanic fields in Petrified Forest National Park have about 300 tuff rings, maars, cinder cones, and diatremes. Others are Mud Hill Tuff Ring Raton-Clayton Volcanic Field in Volcano National Monument and Fort Rock Volcanic Field in Oregon.
Outside the US, the Manyara Rift Escarpment has 350 maars, tuff rings, and cones, especially the Hanang and Kwahara volcanoes in North Tanzania.
Other notable examples are:
- Songaksan and Suwolbong tuff ring South Korea
- Puketarata tuff ring in Taupo Volcanic Zone in New Zealand
- Hverfjall and Sæfjall tuff ring volcano in Iceland,
- El Cuchillo tuff ring, Lanzarote, Canary Islands.
- Capelinhos tuff ring, Faial Island, Azores in Portugal
Hazards
Hazards associated with the formation of tuff rings include near-vent ejected ballistics and lateral surges. However, their small size makes them less dangerous unless you are near the vent or where surges will reach.
Also, existing ones have hazards like falling rocks.
References
- 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.