Agriculture Reference
In-Depth Information
Figure 1.3
Granite boulders exposed by weathering in northeast Victoria. Photograph by the author.
See color insert.
noes (volcanic ash and pyroclastic flows, collectively called
tephra
), or it flows over
the land as streams of lava. Volcanic magma that does not reach the surface ac-
cumulates in magma chambers where it solidifies as a pluton or batholith. Sub-
sequent weathering and erosion of the overlying weathered material exposes the
harder, more resistant rock as boulders, which remain as dominant features in the
landscape (fig. 1.3).
Igneous rocks are broadly subdivided into two categories based on their min-
eral composition: (1)
acidic
rocks (e.g., granite, rhyolite), which are relatively rich
in quartz and the light-colored calcium, potassium, and sodium feldspar miner-
als, and (2)
basic
rocks (e.g., basalt, gabbro), which are low in quartz, but high in
the dark-colored ferromagnesian minerals (hornblende, olivine, pyroxene) (section
2.2.3.2).
Quartz is a very resistant form of crystalline silica (SiO
2
). The higher the ra-
tio of Si to (Ca
Mg
K) in a rock, the more likely it is that some of the Si is
not bound in silicate minerals such as feldspars, but remains as residual quartz
when the rock weathers. Thus, soils formed on granite have more quartz grains
than those formed on basalt, and they are usually well drained and deeply weath-
ered (fig. 1.4). Soils formed on basalt are usually high in silicate clay particles and
less well drained than soils formed on granite, unless they are very old and hence
highly weathered.
Sedimentary rocks
are composed of weathered rock products deposited by wind
and water. Cycles of geologic uplift, weathering, erosion, and subsequent deposi-
tion in rivers, lakes, and seas have produced thick sequences of sediments. Under
the weight of overlying sediments (hundreds of meters thick), the deposits grad-
ually consolidate and harden to form rocks. Faulting, folding, and tilting move-
