Earth as a Planet: Surface and Interior 197
(1–10 years). Recent work on the 100-km-long Carrizozo
flow field in New Mexico, however, suggests that such mas-
sive deposits may have formed at much lower volume effu-
sion rates over much longer periods than previously thought
(10–100 years or more). The same may be true for lava flows
of similar appearance on other planets. [SeePlanetary
Volcanism].
Perhaps the most familiar kind of subaerial volcanism is
the well-behaved, generally nonexplosive, Hawaiian-style
low viscosity eruptions of tholeiitic basalts that form shield
volcanoes, erupting in long sinuous flows. Typically such
flows are either very rough (“aa”) (Fig. 7a) with well-defined
central channels and levees or very smooth, almost glassy
(“pahoehoe”) (Fig. 7b).
FIGURE 7 Aa flow from Mauna Loa Volcano, Hawaii, USA;
Advancing flow of incandescent aa lava. Generally, aa flows are
very rough and meters to tens of meters thick. They form broad
toes and lobes and can advance kilometers per day, as often
happens during eruptions of large a flows on Mauna Loa volcano
in Hawaii (e.g., Mauna Loa 1984 eruption). (Courtesy of the U.S.
Geological Survey). (b) Pahoehoe from Kilauea Volcano, Hawaii,
USA cascading over scarp. Incandescent (∼1400K) fluid
pahoehoe flows near the coast south of Kilauea Volcano, showing
a lava breakout from an upstream lava tube cascading into two
main branches. The cliff is approximately 15m high. Fields of
Pahoehoe lava tend to form in a very complex intertwined
fashion, and old cooled flows are often smooth enough to walk on
in bare feet. (Courtesy of the U.S. Geological Survey).
These lavas are thought to be comparable to lavas ob-
served in remote-sensing images of Martian central vent
volcanoes (e.g., Alba Patera, Olympus Mons). Shield volca-
noes on both planets tend to exhibit very low slopes (i.e.,
∼ 5 ◦). Active submarine basaltic volcanoes tend to occur
along midoceanic ridges. Often the hot sulfide-rich wa-
ters circulating at erupting submarine venting sites pro-
vide habitats for a wide variety of exotic chalcophile (sulfur-
loving) biota found nowhere else on Earth and proposed as
a model for submarine life on Europa.
The transport of water across the land surface also has
a hand in forming constructional landforms. Sediment ero-
sion, transportation, and deposition can set the stage for a
variety of landscapes, especially in concert with continental
scale tectonic (“epirogenic”) uplift. The Colorado Plateau
in the southwestern United States is perhaps the best ex-
ample of this type of landscape. The Grand Canyon of the
Colorado River slices through the heart of the Colorado
Plateau and exposes over 5000 vertical feet of sedimen-
tary layers, the oldest of which date to the beginning of the
Cambrian era (Fig. 8a).
Water itself can form constructive landforms on the
Earth. In its solid form, water can be thought of as an-
other solid component of the Earth’s crust, essentially as
just another rock. Under the present climatic regime, the
Earth’s great ice sheets—Antarctica and Greenland—along
with numerous valley glaciers scattered in mountain ranges
across the world in all climatic zones, compose a distinct
suite of landforms. Massive (up to kilometers thick) deposits
of perennial ice form smooth, crevassed, plastically deform-
ing layers of glacial ice. Continental ice sheets depress the
upper crust upon which they reside and can scour the subja-
cent rocky terrains to bedrock, as during the Wisconsin Era
glaciation in Canada (i.e., last Ice Age in North America).
Valley glaciers, mainly by mechanical and chemical erosion
in concert, tend to carve out large hollows (cirques) in their
source areas and have large outflows of meltwater at their
termini (Fig. 8b).3.2 Destructive Geomorphic Processes
Friction probably represents the largest expenditure of en-
ergy as geologic materials move through the landscape: fric-
tion of water (liquid or solid) on rock, friction of the wind,
friction of rock on rock, or rock on soil. All of these processes
are driven by the relentless force of gravity and generally
express themselves as transport of material from a higher
place to a lower one. Erosion (removal and transport of geo-
logic materials) is the cumulative result, over time reducing
the average altitude of the landscape and often resculpting
or eliminating preexisting landforms of positive relief (e.g.,
mountains) and incising landforms of negative relief (e.g.,
river valleys or canyons). Overall, the source of potential
energy for these processes (e.g., the height of mountain