Geology Reference
In-Depth Information
and well thought out interpretations of the geology. The study of igneous rocks
might also include petrological and mineralogical investigation, geochemical
and isotopic analysis to determine the age and origin of the rocks and the
use of geophysical measurements in the field to determine the distribution of
rock-types below the ground. Also, many igneous rocks are associated with
distinctive types of economic mineralisation and these are generally discovered
and evaluated by fieldwork. Key to the success of these approaches will be the
detailed understanding of the rocks in the field.
In this handbook we explain how to observe igneous rocks in the field, from
the scale of outcrops down to hand specimens and to tie observations into basic
interpretations of how the igneous rocks formed. Before embarking on the details
of igneous rocks in the field it is valuable to consider the role of igneous rocks
in a global framework and to consider the main occurrence of igneous rocks.
1.2 The Global Picture - Igneous Rocks in Relation
to Regional Tectonics
Igneous rocks are materials that have solidified from molten or partially molten
material, termed magma . Such rocks may be classified as extrusive rocks ,which
were erupted at the surface of the Earth, and intrusive rocks that crystallised
beneath the surface. Igneous rocks of different compositions and field relation-
ships exist at specific regions on the Earth associated with the plate tectonics in
general. This reflects the mode of formation and emplacement of igneous rocks
in the context of regional tectonic patterns. Below we briefly describe the key
plate margin and within plate associations where patterns of igneous activity
can be recognised.
The Earth's crust forms the uppermost part of the outer rigid shell, or litho-
sphere, of the Earth and is divided into large coherent 'plates' that move in
relation to one another. This process termed plate tectonics (continental drift)
reflects our cooling planet and the convection of the mantle beneath. The plates
themselves are split into two types of crust which are defined by their composi-
tion and thickness, oceanic and continental and the configuration of the plates
leads to different types of plate margins (boundaries) where specific igneous
associations exist.
The boundaries between plates are of four types (summarised Table 1.1, see
also Figure 1.7).
1. Constructive plate margins or ocean ridges, where two plates are moving
apart and the upwelling and solidification of magma forms new oceanic
crust.
2. Destructive plate margins, where two plates are converging so that one
plate sinks below the other and is eventually resorbed into the mantle
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