196 Encyclopedia of the Solar System
gravitational and radiogenic thermal energy of the Earth
(endogenic processes) powers the construction of terrestrial
landscapes. Thus, the Earth’s main constructional landscape
processes, plate tectonics, and resulting volcanism, are en-
dogenic processes.
Destructional processes, such as rainfall-driven runoff
and stream flow, are essentially exogenic processes. That is,
the energy that drives the evaporation of water that even-
tually results in precipitation, and the winds that transport
water vapor, comes from an exterior source—the Sun (with
the possible exception of very local, but often hazardous,
weather effects near explosive volcanic eruptions, and en-
dogenic energy source). In familiar ways, such destruc-
tional geomorphic processes work to reduce the “gravita-
tional disequilibria” that constructive landscapes represent.
For instance, the relatively low and ancient Appalachian
Mountains, pushed up during one of the collisions between
the North American and European continental landmasses,
were probably once as tall as the current Himalayan chain.
Their formerly steep slopes and high altitudes represented
a great deal of gravitational disequilibria, and thus a great
deal of potential energy that was subsequently expended as
kinetic energy by erosive downhill transport processes (e.g.,
rainfall runoff and stream flow). Once the processes of con-
tinental collision ebbed and tectonic uplift ceased, contin-
uing erosion and surface transport processes (such as rain-
fall, associated runoff, snowfall, and glaciation) over only a
few tens of millions of years reduced the proto-Appalachian
Mountains to their present gently sloping and relatively low-
relief state.
Volcanic landforms provide myriad illustrations of the
competition between destructive and constructive pro-
cesses in the landscape. For example, Mt. Fuji, the most
sacred of Japanese mountains, is actually an active volcano
that erupts on the order of every 100–150 years. Its perfectly
symmetrical conical shape is the result of volcanic erup-
tions that deposit material faster than it can be transported
away, on average. If Fuji stopped erupting, it would become
deeply incised by stream erosion and it would lose its classic
profile over a geologically short time interval (Fig. 6).
3.1 Constructive Processes in the Landscape
Over the geologic history of the Earth, volcanism has been
one of the most ubiquitous processes shaping its surface.
Molten rock (lava) erupts at the Earth’s surface as a result
of the upward movement of slightly less dense magma. Its
melting and upward migration are triggered by convective
instabilities within the upper mantle. Volcanic processes
very likely dominated the earliest terrestrial landscapes and
competed with meteorite impacts as the dominant surface
process during the first billion years of Earth’s history. With
the advent of plate tectonics, multiphase melting of ultra-
mafic rocks tended to distill more silicic lavas. Because
silicate-rich rocks tend to be less dense than more mafic
FIGURE 6 Mt. Fuji, Japan, at sunrise from Lake Kawaguchi.
Perhaps the world’s quintessential volcano, the perfect conical
shape of Mt. Fuji has inspired Japanese landscape artists for
centuries. It is considered a sacred mountain in Japanese
tradition and thousands of people hike to its summit every year.
Volcanologically, Mt. Fuji is termed a “strato-volcano” and rises
to an altitude of 3776 m above sea level. It erupts approximately
every 150 years, on average.varieties, they tend to “float” and resist subduction, thus
continental cores (cratons) were generally created and en-
larged by island-arc accretion.
Most volcanism tends to occur on plate boundaries. Sub-
aerial plate boundary volcanism tends to produce island
arcs (e.g., Aleutian Islands; Indonesian archipelago) when
oceanic plates override one another or subaerial volcanic
mountain chains (e.g., Andes) under-ride more buoyant
continental plates. Such volcanism tends to be relatively
silica-rich (e.g., andesites), producing lavas with higher vis-
cosities, thus tending to produce steeper slopes. Rough lava
flows on these volcanoes tend to be classified as aa or blocky
lavas. High interior gas pressures contained by higher vis-
cosity magmas can produce very explosive eruptions, some
of which can send substantial amounts of dust, volcanic gas,
and water vapor into the stratosphere.
Another kind of volcanic activity tends to occur within
continental plates. As is thought to have been widespread
on the Moon, Mars, and Venus and to a lesser degree
within impact basins on Mercury, continental flood erup-
tions have erupted thousands of cubic kilometers of lay-
ered basalts (e.g., Deccan and Siberian Traps in India and
Russia; Columbia River Basalt Group in the USA). These
are among the largest single subcontinental landforms on
the Earth. Such lavas were mafic, of relatively low viscos-
ity, and are thought to have erupted from extended fissure
vents at very high eruption rates over relatively short periods