Planetary morphologic processes - Introduction to Planetary Geomorphology - page 40

Table 3.1. (cont.)

Physical
nature of
the magma

Character of
explosive activity

Nature of effusive
activity

Nature of domi-
nant ejecta

Structures built
around vent

Eruption type

Phreotoplinian Viscous Water plus

magma;
strongly
explosive

Little or none Accretionary

lapilli

Widespread ash
deposits

GAS ONLY
Gas eruption No magma Continuous or

rhythmic gas
release at vent

None None; or very

minor
amounts of
ash

None

Fumarolic No magma Essentially

nonexplosive;
weak to
moderately
strong long-
continued gas
discharge

None None; or rarely

very minor
amounts of
ash

Generally none;
rarely very
small ash cones

Table 3.2. Factors governing the morphology of volcanic landforms (from Whitford-Stark, 1982 )

Planetary variables Magma properties controlling rheology Properties of eruption

Gravity Viscosity Eruption rate

Lithostatic pressure Temperature Eruption volume

Atmospheric properties Density Eruption duration

Surface/subsurface liquids Composition Vent characteristics

Planetary radius Volatiles Topography

Planetary composition Amount of solids Ejection velocity

Temperature Yield strength, shear strength

Tuff, a volcanic ash deposit covering some 2,200 km 2 .
Much of the ash was so hot when it was emplaced that it
fused together, forming a volcanic rock called ignimbrite.
High-viscosity lavas flow short distances from the vent
( Fig. 3.9 ) in comparison to low-viscosity lavas and tend to
accumulate as steep-sided masses, such as domes
( Fig. 3.10 ). Low-viscosity lavas are very fluid and can
flow long distances, forming broad edi ces such as shield
volcanoes ( Fig. 3.11 ), which are often

3.3.3 Volcanic craters

Volcanic craters of diameter larger than about 2 kmare termed
calderas, which can form by collapse, explosion, erosion, or
a combination of these processes. Most calderas involve
multiple eruptions, leading to nested or overlapping multiple
vents ( Fig. 3.14 ). Smaller (<1 km) collapse features, termed
pit craters, commonly form in basaltic lavas and can form
without associated eruptions. Still smaller craters include
collapse depressions ( Fig. 3.15 )thatformonbasalt ows
and the explosion pits seen on some silicic ows ( Fig. 3.16 ).
Phreatomagmatic explosions involve rising magma
that encounters water (either surface or subsurface water
or ice), forming maar craters ( Fig. 3.17 ). Lava ows that
cross water-soaked ground, swamps, or lakes can lead to the
formation of pseudocraters, which are small cones of lava
and cinders with summit craters ( Fig. 3.18 ). These volcanic

“ fed ” by lava

channels ( Fig. 3.12 ) and lava tubes ( Fig. 3.13 ).
Eruption characteristics constitute the third group of
factors influencing volcano morphology. For example,
low rates of effusion (small volumes of lava erupted per
second) produce relatively short ows that tend to accu-
mulate close to the vent (where magma reaches the sur-
face), forming lava cones, whereas high rates of effusion
form long flows that often produce lava plains.