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CHAPTER 44

Planetary Volcanism

Lionel Wilson

Lancaster University

Lancaster, United Kingdom

1. Summary of Planetary Volcanic Features 4. Explosive Eruptions


2. Classification of Eruptive Processes 5. Inferences about Planetary Interiors


3. Effusive Eruptions and Lava Flows Bibliography

V

olcanism is one of the major processes whereby a
planet transfers heat produced in its interior outward
to the surface. Volcanic activity has been directly responsi-
ble for forming at least three quarters of the surface rocks
of Earth and Venus, all of the surface materials of Jupiter’s
satellite Io, and extensive parts of the surfaces of Mars,
Earth’s Moon, and probably Mercury. Investigations of the
styles of volcanic activity (e.g., explosive or effusive) on a
planet’s surface, when viewed in the light of environmental
factors such as atmospheric pressure and acceleration due
to gravity, provide clues to the composition of the erupted
magma and hence, indirectly, to the chemical composition
of the interior of the planet and its thermal state and his-
tory. Investigations of volcanic features on other planets
have been an important spur to the development of an un-
derstanding of volcanic processes on Earth.

1. Summary of Planetary Volcanic Features

1.1 Earth

Only in the middle part of the 20th century did it become
entirely clear that the∼70% of Earth’s surface represented
by the crust forming the floors of the oceans consists of ge-
ologically very young volcanic rocks. These erupted from
long lines of volcanoes, generally located along ridges near
the centers of ocean basins, within the last 300 Ma (million

years). Along with this realization came the development of

the theory of plate tectonics, which explained the location

and distribution of volcanoes over Earth’s surface. Volca-

noes erupting relatively metal-rich, silica- and volatile-poor

magmas (called basalts) tend to concentrate along the mi-

docean ridges, which mark the constructional margins of

Earth’s rigid crust plates. These magmas represent the prod-

ucts of the partial melting of the mantle at the tops ofcon-

vectioncells in which temperature variations cause the

solid mantle to deform and flow on very long timescales.

Magma compositions are very closely related to the bulk

composition of the mantle, which makes up most of Earth’s

volume outside of the iron-dominated core. The volcanic

edifices produced by ocean-floor volcanism consist mainly

of relatively fluid (low-viscosity) lava flows with lengths

from a few kilometers to a few tens of kilometers. Lava

flows erupted along the midocean ridges simply add to

the topography of the edges of the growing plates as they

move slowly (∼10 mm/year) away from the ridge crest. [See

Earth as aPlanet:Atmosphere andOceans;Earth

as aPlanet:Surface andInterior.]

Lavas erupted from vents located some distance away

from the ridge crest build up roughly symmetrical edi-

fices that generally have convex-upward shapes and are de-

scribed, depending on their height-to-width ratio, as shields

(having relatively shallow flank slopes) or domes (having rel-

atively steeper flanks). Some of these vent systems are not

related to the spreading ridges at all, but instead mark the

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