Maars are shallow, nearly rounded to oval bowl-shaped volcanic craters surrounded by low rims of fragmental debris or pyroclasts.
These landforms are hydrovolcanic and usually monogenetic. Monogenetic means they result from a single eruption phase, and hydrovolcanic indicates they form when rising magna interacts with water.
Maars have sub-horizontal bottoms and gently sloping low rims unless eroded. Their craters lie below the pre-eruptive surface.
Also, they are often filled with water to form lakes called maars and often extend to the water table.
Usually, maars are associated with diatremes. Diatremes are downward tapering, funnel, or carrot-shaped vertical pipes filled with breccia.
Breccia are angular clasts, gravel to boulder-sized, cemented in a matrix.
Compared with scoria cones, maars have a lower profile and a larger central crater. Also, their rims are less steep and thick.
Also, maars differ from tuff rings and tuff cones since they lie lower than the pre-eruptive surface. These are the other two hydrovolcanic landforms.
Lastly, the word maar is of Rhineland or Franconian dialect of Germany, derived from the Latin word mare, meaning sea. It was applied to describe these volcanic explosion craters in Eifel, Germany.
Description and appearance
Maars are shallow, circular, oval to elliptical volcanic craters with low-profile pyroclastic rims. Pyroclasts are fragmental material ejected during an explosive volcanic eruption.
These craters occur below the pre-eruptive surface. Also, they are filled with water and/or alluvium and are underlain by diatremes and a feeder dike on their base.
Some basaltic maars have downward growing diatreme from hydromagnetic processes. Also, some are merely a surface expression of diatremes.
Lastly, these diatremes are visible where erosion exposes the root of maars.
Size and shape
Maars are 0.2 to 3 km in size, 10-500 meters deep, with rims often less than 30 meters. However, Joya Honda maar has rims that are more than 100 m thick.
These hydrovolcanic lands are sub-horizontal, about 20° at rims, with all of them having slopes generally less than 25°
Although maars are generally circular to oval and asymmetrical around a vent, those that form during strong winds may be asymmetrical.
For example, the Cerro Colorado maar in the Pinacate Volcanic Field in the USA is uneven and asymmetrical.
Lastly, the shift of the explosion center may result in other shapes, including lobate. Also, later eruptions may modify the original maar crater. Such may build a cinder cone on its floor or fill it with basaltic lava flows.
What does it have?
The low profile, circular to rounded rims in maars may be entirely accidental clasts, mainly magmatic (cinder and scoria), or a mixture of juvenile and accidental.
On the other hand, diatremes have breccia, while the crater floor has fragmental materials that fell back and sediments that fell from crater walls.
Also, it may have lake sediments if the crater receives water from the surface ruff-off or it intersects with the water table below. Most extend below the water table.
Composition
Maar craters form mostly from basaltic but can occur in other rock types. These hydrovolcanic landforms are dominated by accidental or country rock with subordinate to no magmatic debris.
Therefore, its composition may include deep crustal basement rock, near-surface sediments, volcanic ash, lapilli, scoria cinders, volcanic bombs, and blocks.
Magmatic fragments may be glassy or have altered basaltic glass called sideromelane. Alteration occurs through the interaction of hot magma and water or steam during explosive eruptions.
A few may have mantle-derived xenoliths of peridotite, kimberlites, or lamproites.
Lastly, maars may have lake deposits, including clay, evaporative materials, and carbonates.
How are maars formed?
Maar craters form from Surtseyan-style phreatomagmatic or hydromagmatic eruptions. These are explosive eruptions that occur when magma interacts with groundwater.
Usually, rising magma will encounter near-surface groundwater or an aquifer. Aquifer refers to sediments or porous rock saturated with groundwater.
This interaction makes water flash into steam, causing an explosion. This explosion occurs below the surface.
Therefore, it will excavate, fragment, and eject pyroclasts, mostly surrounding country rock and sometimes some magma, into the air and laterally, i.e., horizontal ground-hugging surges.
Lateral surges will further erode materials near the vent and deposit far away as velocities decrease. They help widen the crater.
Also, some debris ejected into the air material will fall back further away and heavier into the eruption vent and crater. Those that fall into the vent may undergo subsequent fragmentation and ejection.
Therefore, the explosive eruption excavates subsurface rock and forms a crater with rims of fragmented and ejected debris.
Please note that gases from volatiles don’t drive the eruption in magma but by steam formed from magma-water interaction.
Also, most phreatomagmatic explosions that form maars start at shallow depths and go deeper as they occur on the water table.
That is not all. These craters may form from a series of explosions. Such explosions may shift, affecting the near-circular shape of the crater, some becoming lobate.
Also, ejected volume materials from the crater are comparable with materials deposited on rims.
However, since some maars and diatremes are associated with lamproites, peridotites, and kimberlites, it may suggest a deeper eruptive mechanism than groundwater.
Lastly, eruptions forming maars are more explosive than those forming tuff rings or cones and involve less water. Also, they are more powerful than those of scoria cones as they gain heat energy via vaporization.
Occurrence and distribution
Maars are common in many monogenetic volcanic fields. Sometimes, they are associated with cinder cones in the same fissure and basaltic lava, such as in Eifel, Germany.
However, since lava flows may cover and fill some of these maars, surviving ones occur at the edges.
Examples include:
- Eifel region of Germany with 30 craters, each about a kilometer across
- Espenberg and Ukinrek Maars, Alaska
- Rotomahana maar in New Zealand
- Meke Maar in and Narköy maar Turkey
- Laguna de Armenia near San Miguel Volcano in El Salvado
- Marteles maar on Gran Canaria, Canary Islands
- Asososca in Managua, Nicaragua
- Kilbourne, Hunts Hole, and Zuni Salt Lake in New Mexico, USA
- Hopi Buttes in Arizona, USA
- and Pinacate craters in Mexico
Maar erosion and degradation
Erosion can fill the crater and reduce rim heights. Also, slumping can widen the crater wall via backwasting or parallel erosion. This results in a shallow depression.
In some cases, erosion will remove the crater. However, diatreme and pipe breccia lithologies will remain evidence it exists.
Lastly, maars filled with lava may erode, forming buttes. Buttes are steep-sided hills with relatively flat tops. A good example is Hopi Buttes in Arizona, USA.
References
- De Hon, R. (2015). Maars. 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.
- Winter, J. D. (2014). Principles of igneous and metamorphic petrology. Pearson Education.