Geology Reference
In-Depth Information
Figure 3.6. Rocks formed from the cooling of magma are either
volcanic (resulting from eruption of magma to the surface and
forming glass or
fine-grained minerals) or plutonic (resulting from
magma cooling from below the surface and forming large-grained
materials). The classi
cation of both volcanic and plutonic rocks is
based on the mineral content and their percentages. Not all igneous
rocks are indicated in this simpli
ed diagram (courtesy of the US
Geological Survey).
Figure 3.5. A view of the Zagros Mountains, Iran, showing elongate
anticlines that resulted from folding of Earth
s crust by compression.
Most of the mountains mirror the upwarped folded rocks (Apollo 7
image AS7
'
-
1615).
Fig. 3.5
the mountains coincide with anticlines (upwarped
rocks). More commonly, however, there is an inversion of
topography; during the process of folding, rocks along the
axes of the anticlines are subjected to tension, which may
cause them to be jointed, or in effect to
“
open up.
”
Rocks
along the axes of synclines (downwarped rocks) tend to
be compressed, causing pore space and other voids to
close. Consequently, weathering and erosion are enhanced
along anticlines and are retarded along synclines. With
time, net erosion is more rapid along anticlines, leading to
the development of valleys, while synclines often form
ridges.
Global-scale tectonic processes are generally linked
to the thermal evolution and interior characteristics of plan-
ets. The smaller terrestrial planets (Mercury, the Moon,
and Mars) have thick lithospheres that show little evidence
of tectonic deformation over the last 80% of their history.
Sometimes called one-plate planets, they have crusts
that formed early in Solar System history and preserved
the terminal period of accretion. Tectonism on these planets
is expressed primarily by vertical movements, forming
features such as grabens (
Fig. 2.9
), as the crust cooled.
understanding volcanic processes, spurred partly by the
discovery that volcanism is important on other planets.
Volcanism involves the generation of magma (molten
rock) and its eruption onto the surface, providing clues
regarding the thermal evolution and interior characteris-
tics of planets. On Earth, magma forms in the lower crust
and upper mantle as a result of heat generated from differ-
entiation of the mantle and core, friction from tectonic
processes, and radioactive decay.
Most active volcanoes on Earth form on or near tectonic
plate boundaries. As discussed above, crustal spreading
zones are typically marked by ma
c volcanism, involving
iron-rich silicate magmas that commonly produce basaltic
rocks (
Fig. 3.6
). Subduction zones typically involve
silica-rich magmas, which produce andesitic, dacitic,
and rhyolitic rocks.
are also important loca-
tions for volcanism. As crustal plates slide across these
zones, magma erupts through the plates to the surface,
producing chains of volcanoes much like an assembly
line, typi
ed by the Hawaiian volcanoes in the Paci
c
ocean.
Basaltic volcanism dominates the terrestrial planets. It
is the principal rock on Earth
'
s sea
floors, constitutes the
dark areas of Earth
'
s Moon, forms the huge volcanoes of
Mars and Venus, and is the rock constituting many of the
smooth areas of Mercury. In addition, several lines of
“
Hotspots
”
3.3 Volcanic processes
Prior to the middle of the twentieth century, topics on
volcanism were primarily descriptive. Beginning in the
1970s, a more quantitative approach was taken toward
