Maar
In a phreatomagmatic eruption, the country rock is shattered and
collapses into a cone-shaped conduit forming a diatreme. A deep
circular crater, a maar (often containing a lake), is formed on
the land surface that may be 200 m deep and 1 km across. The maar's
steep, interior walls expose the country rock. The crater is often
surrounded by a ring, commonly reaching 10 to 30 m in height,
which is made up of layers composed largely of fragments of country
rock. The initial beds contain coarse shattered country rock because
they were ejected during the main, throat-clearing explosions.
Subsequent beds are thinner and composed of finer fragments which
may include a small amount of new lava. Although maar-forming
explosions are violent, the volume of fragments ejected is usually
much less than the volume of the crater.
Tuff Ring
Tuff rings typically are formed by Surtseyan eruptions generated
when magma rises beneath a shallow body of water or in areas of
abundant groundwater. Explosive expansion of the steam is shallower,
less confined, and thus weaker, but more prolonged, than in phreatomagmatic
eruptions because more water is available for a longer period.
Tuff rings commonly rise between 40 m and 60 m in height and enclose
a wide, shallow crater as much as a kilometer across. The crater's
floor is higher than the surrounding country. They are laid down
by hot muddy surges of steam. More magma reaches the surface so
that its shattered fragments constitute the bulk of the tuff ring.
The eruptions continue for several weeks or a few months until
the magma no longer reaches the surface.
Tuff Cone
Surtseyan eruptions construct tuff cones when continuous supplies
of large quantities of water enter into contact with rising magma.
Their formation requires a more abundant water supply, and explosions
at greater depths than those needed to build tuff rings. The resulting
cones are often 100 m to 300 m high and 1 to 1.5 km across. Tuff
cones have deep, broad craters (narrower and steeper than those
of tuff rings). The eruptions may last for several months.
Lava Lake
During any given eruption, water might be prevented from reaching
the vent by a large upsurge of magma, by the construction of an
impermeable tuff cone, or if the water supply were removed. If
the magma continues to rise the crater of the tuff ring or cone
may fill with lava to form a lava lake.
