VIII. Structures Built Around Volcanic Vents

Maar Volcanoes

Maar volcanoes are low volcanic cones with bowl-shaped craters that are wide relative to rim height. They were originally recognized as small subcircular crater lakes, the term being derived from the Latin "mare" for sea. The various kinds of maar volcanoes are:

1.   Maar (sensu stricto): Volcanic crater cut into country rock below general ground level and possessing a low rim composed of coarse- to fine-grained tephra. They range from about 100 to 3000 m wide, about 10 to more than 50 m deep and have a rim height of from a few meters to nearly l00 m above general ground level.

2.   Tuff ring: Large volcanic crater at or above general ground level surround by a rim of pyroclastic debris (tuff or lapilli tuff), similar in diameter to maars.

3. Tuff cone: These cones have higher rims, attaining heights of up to 300 m, and are essentially tuff rings where volcanic activity was of longer duration.

The distinction between tuff cones and tuff rings becomes arbitrary where one side of a crater stands high and another side low.
Most maars result from hydroclastic eruptions; wide craters develop from shallow explosions, subsidence or a combination of both. In groups of nearly synchronous eruptive centers, those erupting on high ground form spatter or cinder cone whereas associated eruption centers in valleys, depressions, or alluvial gravels in coastal regions form maars, tuff rings or tuff cones. Juvenile clasts within the deposits are glassy, non-vesiculated, and have blocky shapes, suggesting that magma was quenched prior to exsolution of volatiles, that breakage of glass resulted from thermal shock and (steam) explosions, and that the vapor and steam phase in the eruption column was partly or largely vapor from external water.
Tuff cones and tuff rings are distinct landforms that result from slightly different types of hydroclastic activity an represent a "continuum" of landforms from cinder cones to pillow lava related to environments of eruption and mechanical energy of eruptions. Tuff rings evolve through a stage of explosion breccia emplacement to a stage dominated by base surges which deposit thinly bedded layers. Tuff cones may be built when continuing activity evolves into a third stage, characterized by rock emplaced by poorly inflated base surges and ballistic fallout. The differences are related to water: melt ratios; fragmentation of melt attains maximum explosive energy when the water:melt ratio is about 0.5 for basaltic compositions, whereas initial ("vent-coring") eruptions with small ratios result in the formation of breccia with abundant cognate and accidental fragments. Increasing ratios cause development of expanded dilute surges which deposit thin-bedded layers as tuff rings. Higher ratios produce "wetter" and denser eruption columns giving rise to poorly expanded surges, hence dominantly massive beds and tuff cones. The rates of magma and water influx controls the process, and such "cycles" may be interrupted, reversed or alternate. Most commonly, tuff cones may have craters filled or partly filled with lava, and agglutinated spatter and cinders. In some volcanic fields, scoria cones contain deposits of phreatomagmatic origin commonly developed during their initial eruptive stages.