INTRODUCTION AND OCCURRENCE - The Field Description of Igneous Rocks - page 7

cones, flows and domes may occur within any of the three zones. Therefore,
even for young volcanoes, it may be difficult to correlate individual lava flows,
pyroclastic fall and flow deposits with a single eruption. Debris flows (as
distinct from pyroclastic flows) formed when material collapses from volcano
sides and unconsolidated deposits, may travel several kilometres around source,
and can carry great masses of lava as blocks (for example, around Mount
Egmont/Taranaki, in North Island New Zealand).
1.3.2 Intrusive rock units
Intrusions vary widely in size and relationship to the country rock and are
generally grouped according to size into
1. minor intrusions , which have mean minimum dimensions measured in tens
of metres (or less) and were emplaced relatively near to the Earth's sur-
face and
2. plutonic intrusions , which are commonly emplaced at greater depth and
have sizes measured in terms of kilometres.
Within the guide we deal with shallow/minor intrusions in Chapter 6,
granitic plutonic rocks (granitic complexes) in Chapter 7 and mafic/ultramafic
complexes in Chapter 8. A brief description of the main modes of occurrence
is introduced below.
1.3.3 Minor intrusions
The most common forms of minor intrusions are shown schematically in
Figure 1.3 Dykes are sheet-like intrusions which were approximately vertical at
the time of emplacement and are hence discordant to host rocks such as shallow-
dipping sedimentary rocks. As a consequence of their attitude, the outcrops
of dykes are little affected by the topography of the countryside in which they
occur and often appear as nearly straight lines on geological maps. The width
of dykes ranges from centimetre size to sizes measured in hundreds of metres,
but in general the average width is probably in the range 1 - 5 m. Since this is
too small to portray accurately on large-scale geological maps, dykes are often
shown with a uniform or 'conventional' width (like roads on geographic maps).
Sheet intrusions that were approximately horizontal at the time of emplace-
ment are termed sills (for example, Figure 1.4). These are often emplaced into
horizontal or shallow dipping sedimentary rocks, in which case they may be
broadly concordant, with the stratification, and appear on a geological map as
part of the sedimentary succession. Other forms of intrusion which are broadly
concordant with the surrounding strata are blister-shaped masses with a sub-
horizontal base and elevated upper surface, termed laccoliths , and saucer-shaped
intrusions, termed lopoliths (Figure 1.3); both forms range in size from small