Page 51 - Fundamentals of Geomorphology

34 INTRODUCING LANDFORMS AND LANDSCAPES

100
Quartz z
Quart
(Calcium-rich)
(
alcium-rich)
C
80
(%) 60
content Olivine Pyroxene (Orthoclase)
Olivine
(Orthoclase)
Mineral 40 (Sodium-rich)
Pyroxene
(Sodium-rich)
20 A mphibole Muscovite
Amphibole
Muscovite
iotite
B Biotite
0
40 50 60 70 80
Silica content (%)
ULTRABASIC BASIC INTERMEDIATE ACID
EXTRUSIVE Rhyolite
Fine-grained (rare) Basalt Andesite Trachyte Obsidian
INTRUSIVE (rare) Dolerite Microdiorite Microsyenite Microgranite
Medium-grained
INTRUSIVE
Coarse-grained Peridotite Gabbro Diorite Syenite Granite

Figure 2.1 Igneous rocks and their component minerals. The classification is based on the silica content, which
produces an ultrabasic–acid axis. The terms ‘acid’ and ‘basic’ are not meant to suggest that the rocks are acidic or
alkaline in the customary sense, but merely describe their silica content.

The surface phase, and particularly the land-surface of weathering. Some weathering products revert to a
phase, of the rock cycle is the domain of geomorphol- rock-likestatebyfurthertransformations:undertheright
ogists. The flux of materials across the land surface is, conditions, some chemicals precipitate out from solution
overall, unidirectional and is a cascade rather than a to form hardpans and crusts. And many organisms pro-
cycle. The basics of the land-surface debris cascade are duce resistant organic and inorganic materials to shield
as follows. Weathering agents move into the soil and rock or to support their bodies. The weathered mantle may
along a weathering front, and in doing so fresh rock is remain in place or it may move downhill. It may creep,
brought into the system. Material may be added to the slide, slump, or flow downhill under the influence of
land surface by deposition, having been borne by wind, gravity (mass movements), or moving water may be wash
water, ice, or animals. All the materials in the system or carry it downhill. In addition, the wind may erode it
are subject to transformations by the complex processes and take it elsewhere.