INTRODUCTION AND OCCURRENCE - The Field Description of Igneous Rocks - page 4

of magmatic and metamorphic activity, uplift, erosion and sedimentation, and
hence consists largely of metamorphic and igneous rocks with a thin veneer of
sedimentary rocks. Because of it's greater age and complex geological history,
the structure of the continental crust is much more varied than that of the oceanic
crust. The continental crust is therefore considered to have a complex structure
characterised by rapid lateral and vertical variation, and uplifted sections from
which the sedimentary veneer has been eroded expose sections of a wide variety
of igneous rocks emplaced at great depth within the crust.
Igneous rocks formed at locations distant from plate margins (locations
within-plate, Table 1.1) may have distinctive modes of occurrence, for example
as flat-lying sheets of plateau lavas, as discordant plutonic magmatic bodies
within continental rifts and as concordant or discordant gabbroic intrusions.
Such igneous rocks may have characteristic compositions; indeed, many

magmas emplaced at locations within a plate have distinctive alkali-rich

chemical compositions which may be reflected in their mineralogy.

1.3 Mode of Occurrence of Igneous Bodies
In general we can split the different types of igneous body into three main
sections relating generally to their relative positions within the Earth. Volcanic
rocks erupt at and onto the Earth's surface, minor intrusions tend to occur at
shallow depths within the Earth's crust and plutonic rocks form larger intrusive
bodies at a variety of depths. In the context of this field guide we will gener-
ally look at these three main modes of occurrence and different subsections of
these (for example, Volcanic - lava, Volcanic - pyroclastic, Plutonic - Granite,
Plutonic mafic/ultramafic). Most field areas will be dominated by rocks from
one of these general levels, though many examples will exist where different
styles of occurrence are found (for example, shallow intrusions with lavas) so
these basic subdivisions are used as a guide to the main features of each type.
Below we briefly describe these major subdivisions.

1.3.1 Volcanic rock units
Volcanic rocks are classified as lavas and pyroclastic rocks (volcaniclastic). Lava
is the term for molten extrusive rock and its solidified product, and pyroclastic
rocks are composed of a mix of materials (lava fragments, pumice and crystals)
fragmented by explosive volcanic activity. Within the guide we will deal with
lava flows in detail in Chapter 4 and pyroclastic rocks in Chapter 5. Below is a
brief summary of the main occurrence of volcanic rock units.
Often lavas and pyroclastics associated with individual volcanoes are concen-
trated within valleys and depressions around the volcano. The most extensive
pyroclastic deposits may form large-scale stratigraphic units that blanket the
topography and may form plateau-like features around volcanoes, additionally