Geology Reference
In-Depth Information
Figure 7.10. Repeated imaging of the same areas reveals newly
formed features; this small (54.5m) impact crater formed between
February and July 2005; the dark ejecta is fragmented basalt
excavated frombeneath a lighter mantle of dust (NASAHiRISE image
ESP_011425_1775).
Figure 7.11. The Olympus Mons shield volcano in the Tharsis
province is more than 600 km across, making it one of the largest
volcanoes in the Solar System (NASA Viking Orbiter 646A28).
built-in uncertainties, especially for the very early and the
very young ages. For Mars, various astrophysical models
are used to account for factors such as the proximity to the
asteroid belt and potential contributions from comets. In
addition, the discovery of recently formed small impact
craters
(Fig. 7.10)
helps establish the current
flux of
incoming objects on Mars. Despite the uncertainties and
assumptions, in the absence of any other dates for Mars,
crater counts remain as the sole technique for assessing
surface dates.
basaltic composition, commonly emplaced by lava tubes
and channels erupted from central vents, or calderas, or
from parasitic
flank eruptions that built gently sloping
structures. The martian shields are remarkable on account
of their great size; Olympus Mons is more than 550 km
across, stands 21 km above the 0 km reference datum, and
contains a summit caldera some 80 km across. Some vol-
canoes, such as Hecates Tholus in the Elysium province,
appear to be blanketed with fine-grained basaltic material
inferred to be pyroclastics. The steeper slopes of some
shields suggest a more evolved magma and a transition
from effusive to mild explosive activity.
Tharsis Tholus exempli
es dome volcanoes
(Fig. 7.12)
,
which are characterized by flank slopes that are steeper than
the shields. They are inferred to represent either more silicic
lavas or lower rates of effusion that would produce short
flows that piled on top of each other, rather than spreading
great distances from their sources. Some dome volcanoes
could simply be the steeper summit regions of shield vol-
canoes for which more gentle
flanks have been buried.
Unfortunately, many of the central-vent volcanoes on
Mars are covered with a mantle of windblown dust that
precludes obtaining compositional information by remote
sensing. TES data, however, suggest the presence of silica-
rich materials in limited areas, and it is reasonable to expect
evolved magmas to be present, given the extensive volcan-
ism. For example, the Syrtis Major volcano has remote
7.5.3 Volcanic features
Mars has some of the most impressive volcanoes in the
Solar System, with the Olympus Mons shield
(Fig. 7.11)
and similar features in the Tharsis region. Mars
'
volcanoes
can be classi
ed into constructs formed by central vents
(i.e.,
“
point-source
”
eruptions), such as Olympus Mons,
and volcanic plains emplaced from
fissures or inferred
vents lacking discernible surface features
(Table 7.2)
.
The total areal coverage of martian volcanic materials is
more than half the planet
'
s surface. While most attention
has been focused on the shield volcanoes, various plains
units of volcanic origin represent most of the volcanism
on Mars.
The central volcanoes include shields, domes, highland
patera, and a unique structure, Alba Patera. The shield
volcanoes have all the attributes of classic Hawaiian
shields in that they are composed of countless
flows of
