Tuff cones or ash cones are small, nearly circular, low-profile, steep-sided volcanic landforms with a cone-like shape and a broad, bowl-shaped crater at their summits.
They are one of the hydrovolcanic landforms; others are maars and tuff rings. Hydrovolcanic means they form from the interaction of water and hot magma.
Tuff rings are also rounded or ring-like. However, they are usually slightly larger and have much lower profiles with broader craters and rims with gentler slopes.
Like ash cones, tuff cones form on the Earth’s surface. Thus, they occur at a point higher than the surrounding surface.
On the other hand, maars refer to low-relief craters usually filled with water. They lie deeper than the pre-eruption surface.
How do they form? Tuff rings from rising magma encounter abundant shallow external groundwater on the Earth’s surface. This encounter causes explosive eruptions known as phreatomagmatic eruptions.
Phreatomagmatic eruptions fragment and emplace pyroclasts that later consolidate. These pyroclasts are mostly ash and sometimes lapilli, volcanic bombs, and blocks.
Lastly, the tuff ring shape and size are intermediate between tuff and cinder cones. Also, their crater is wider than spatter cones and forms from a single or monogenetic eruption.
Description and appearance
Tuff cones are small, steep-sloped, low volcanic cones with wide craters at their summit that occur above the surrounding surface.
These volcanic landforms are made of consolidated, often highly altered, thickly bedded pyroclasts.
1. Size and shape
Tuff cones are the smaller, taller versions of tuff rings. Usually, they are more than 100 meters in height but can be 50-330 or 400 meters, with typical diameters of 0.1 to 1.5 km (or up to 2.5km).
Their slopes are 20-30°, with most more than 25° near the rim. However, these slopes can be as steep as 40° near the rim in case of wet ash emplacement.
Lastly, these hydrovolcanic edifices may be symmetrical or asymmetrical depending on wind direction during the eruption. Also, vent migration or multiple vents with different production rates may affect their shape.
2. Bedding
Tuff cones have thick beds. These beds have mainly ash and are poorly sorted. However, they have other pyroclasts like lapilli, volcanic bombs, and blocks. Most of these pyroclasts are juvenile, with a few accidental.
Variation in grain size reflects fragmentation efficiency during eruption. This, in turn, shows water-magma interaction throughout the eruption.
Also, these cones may show repeated normal grading and massive, crudely stratified layers capped with finer, laminated deposits. Also, ash may have minor swellings, pinches, and crossbedding.
Massive beds on rims indicate lateral turbulent surges laden with debris and remobilization process.
Composition
Tuff cones can have any composition. Most are made of highly fluid mafic magmas like basaltic or basanite. However, some may be intermediate, including andesite, basalt andesite, or trachybasalt.
The hotter, lower viscosity of mafic magma allows more magma or lava to mix with external water. This provides more thermal energy that drives the explosive eruption.
How do tuff cones form?
Tuff cones form from phreatomagmatic eruptions, usually Surtseyan. These explosive eruptions result from rising magma or lava interacting with water.
Usually, the phreatomagmatic eruptions that form tuff cones occur with abundant external groundwater on the Earth’s surface. It may be on a shallow sea or lake.
When intruding hot molten magma meets water, it will cause it to flash to steam, which expands violently.
This violent expansion will blast lava and magma fragments – ash, lapilli, or blocks into the air and cause debris-laden basal or lateral surges. These surges are like low-grade pyroclastic flows.
The deposition of fragments ejected into the air and from lateral surges will form this cone. Over time, the cone consolidates, forming a tuff.
Also, some of the expanding gases from volatiles in the lava or magma may help in the fragmentation and ejection of pyroclasts. However, this occurs only to a small extent.
1. Eruption magnitude
Eruptions that form tuff rings are prolonged and moderately explosive. Such eruptions will deposit pyroclasts near the vents.
Remember that external water abundance and lava or magma volumetric flow rate influence the type of landform formed.
For tuff cones, water abundance suppresses eruption strength. Hence, you will have weaker phreatomagmatic explosion activity. Such eruptions will eject and deposit pyroclasts near the vent.
It happens due to a wetter, denser eruption column. Also, there is poor expansion and fewer lateral surges and fallout deposits. All these will form steep-sided, coarse-grained, weakly bedded tuff characteristic of tuff cones.
On the other hand, tuff rings erupt in less abundant water. This results in powerful explosions that spread pyroclasts over a larger area.
Lastly, maars form from the most energetic eruptions with limited water.
How eruption may proceed
Tuff cones form mostly from single or monogenetic eruptions. Such eruptions can occur in pulses and last a few days, months, or even up to a year.
These eruptions may show the drying-out sequence where the formed cone isolates magma from external water.
Evidence for such drying out includes beds becoming coarser and thicker. Also, they will have higher juvenile fragments or scoria. This indicates less external water interacts with magma.
However, cone collapse may allow water to flood the crater, interrupting drying outs.
Also, complete isolation of erupting magma from the water will transition phreatomagmatic eruption to strombolian or effusive. Which one occurs depends on the number of external volatiles.
Lastly, this isolation may form cinder cones or solidified lava lakes in tuff cone craters.
Evidence of abundant water
We mentioned that tuff cones occur in the abundance of water. Evidence includes the following:
- Presence of accretionary and armored lapilli
- Vesicles in tuff formed after deposition.
- Vertical plastering or accretion of beds
- Slurries or mudflows during eruption
- Palagonitization or alteration of basaltic glass into palagonite, a yellowish-orange or brownish material.
- Sagging bedding planes from ballistics
Usually, palagonitization occurs during or shortly after eruption due to the presence of heat and water vapor.
Lastly, note that eruptions occurring under too much water, such as in deeper oceans or seas, will form pillow lava.
Degradation
Tuff cones erode easily. However, those that have undergone palagonitization strongly or are covered with solidified lava lakes will remain well preserved for longer.
Tuff cones examples and occurrence
Tuff cones occur worldwide in various volcanic fields. Examples include Auckland volcanic field in New Zealand, East Eifel in Germany, and Western Snake River Plain in Idaho, USA.
Others are Sabatini in Italy, Pali Aike in Argentina-Chile, and Lamongan volcanic fields in Indonesia.
Some of the famous tuff cones in the US include Punchbowl and Diamond Head on Oahu Island in Hawaii and Sinker Butte in Idaho. Others are Fort Rock-Christmas Valley, Table Rock in Oregon, Koko Crater in Hawaii, and North and South Menan Buttes in Idaho.
North and South Menan Buttes represent some of the largest tuff cones in the world.
Examples elsewhere include El Caldera in Cerro Xico, Mexico, Capelinhos in the Azores, Fajal Island, and Surtsey in Iceland. Another one is Udo Tuff Cone in Cheju Island, Korea.
Lastly, tuff cones also occur on Mars.
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.