Geology Reference
In-Depth Information
crust: the continents and oceans,
mountain belts, ridges and trenches;
with the Earth's internal structure,
the
asthenosphere
and
lithosphere
;
and finally with the distribution of
present-day tectonic activity in the
form of earthquakes and volcanoes.
setting, are described first. This is fol-
lowed by a discussion of the condi-
tions under which fracturing, and
thus faulting, takes place, and the
relationship between faulting and
earthquakes. Knowledge of how and
why earthquakes occur is particularly
relevant in today's world - afflicted
as it is by so many recent natural
disasters with a geological origin.
about the deformation process
accompanying the metamorphism.
Chapter 8: Igneous intrusions.
Igneous intrusions are rock bodies
whose shape is controlled by the state
of
stress
(or force field) of the crust
into which they have been emplaced;
the different types of igneous body are
thus of structural significance. Moreo-
ver, the process of emplacement itself
exerts some structural control on the
host rocks and produces structures in
them. There are important differences
between the structures of small-scale
bodies such as
dykes
and
sills
and the
large bodies (
plutons
) such as
batho-
liths, laccoliths
and
lopoliths
. Certain
plutonic bodies are emplaced pas-
sively, by the host rocks moving aside to
accommodate them, while others are
forcefully emplaced by a mechanism
termed
diapirism
, driven by gravita-
tional pressure. Certain igneous com-
plexes representing the roots of large
central volcanoes consist of several
different types of igneous body (
ring
dykes
,
cone sheets
and
radial dykes
)
that are related to each other by the
stress field generated by the central
magma body. Examples of these are
described from western Scotland.
Chapter 3: Plate tectonics.
Modern
plate tectonics starts with the theory
of
continental drift
, which, when
added to the more recent concept
of
ocean-floor spreading
, led in the
1960s to the development of
plate
tectonics
. The concept of the tectonic
plate is explained, together with the
three types of plate boundary - ocean
ridges, ocean trenches and
transform
faults
- and the processes that accom-
pany them. This is followed by a brief
discussion of
hot spots
and
plumes
,
and finally by a consideration of the
driving mechanism of plate motion.
Chapter 6: Folds and folding.
This
chapter deals first with the description
and significance of the various types of
fold structure in terms of their shape
and mode of formation, and examines
the nature and geometry of the strain
accompanying each. This is followed by
a discussion of fold systems, the three-
dimensional geometry of folds, and fold
superimposition.
Shear zones
are dis-
cussed here as a special case of folding,
although they are in reality the deep-
seated equivalent of faults. The chapter
concludes with a discussion of the
conditions under which folding occurs.
Chapter 4: Deformation, stress
and strain.
This chapter deals with the
processes that govern how structures
are formed, and with the methods by
which the amount of deformation (the
strain
) can be measured. The study of
deformation requires an understand-
ing of the relationship between
force
and
stress,
and between stress and
strain
. The various types of strain and
the methods of measuring strain are
discussed, followed by a consideration
of the behaviour of rock material under
varying conditions of temperature
and pressure in response to stress.
Chapter 7: Fabric.
The effects of
higher temperature and pressure are
revealed in metamorphic rocks by
special types of structure as well as
by the changed mineral composition.
These new structures include planar
structures (
foliation
), new rock types,
such as
schist
and
gneiss
, and new
linear structures (
lineation
); these
are included under the general term
'
fabric
'. The effects of deformation
under these conditions are to form
structures that penetrate throughout
the whole rock, unlike folds or faults,
and involve micro-structural changes at
the scale of the single crystal. Study of
the fabric can reveal useful information
Chapter 9: Structural effects of
gravity.
The force of gravity has a key
influence on deformation, and this
chapter explores how gravity controls
structures, giving examples of its effects
in terms of gravity sliding on both
small and large (
orogenic
) scales, and
on the emplacement of
salt domes
.
Chapter 5: Fractures, faults and
earthquakes.
The description and
geometry of faults and fault systems,
and their relationship to their tectonic
Chapter 10: Tectonic interpreta-
tion of orogenic belts.
This chapter
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