INTRODUCTION AND OCCURRENCE - The Field Description of Igneous Rocks - page 3

or 'destroyed'. This process is accompanied by formation of a range
of magmas. Such plate margins may occur on oceanic (island arcs) or
continental lithosphere (active continental margins).
3. Conservative plate margins, are faults where two plates slide past each other
(transverse faults), so that lithosphere is neither created nor destroyed, and
igneous activity is minor.
4. Collision zones, where two island arcs and/or continents have collided so that
subduction of oceanic material has ceased. Such areas are characterised by
widespread extrusive and intrusive igneous activity which commonly contin-
ues for a considerable time after collision.
Over 99% by volume of igneous activity occurs at constructive and destruc-
tive plate margins and at collision zones and some occurs at locations within
the plates, for example volcanoes such as those of Hawaii and those associated
with the East African rift system.
Igneous activity at constructive plate margins is responsible for the forma-
tion of the oceanic crust. The composition and structure of the oceanic crust
is known from the study of rocks dredged from the ocean floor, from seismic
studies and from studies of onshore exposures of older rocks that are believed to
be fragments of the oceanic crust ( ophiolites ). These lines of evidence indicate
that the oceanic crust consists of layers of basalt lavas, basalt/dolerite dykes,
gabbro and peridotite. These rocks form a distinctive association which may be
recognised in ancient orogenic belts, where it is termed the ophiolite associa-
tion . The recognition of such associations is clearly of great palaeogeographic
significance and the ophiolite associations are described in detail in Chapter 8.
The oceanic lithosphere moves away from the oceanic ridge by the process of
sea-floor spreading and is generally returned to the mantle at a destructive plate
margin within circa 200 Ma. The descent of oceanic lithosphere into the mantle
is accompanied by partial melting above the descending plate where water is
driven off at depth. This melting in the overlying mantle forms magmas rang-
ing in composition from basalt, through andesite to rhyolite in composition.
These intrude the crust and may be erupted at the surface or emplaced at depth
as gabbro, diorite and granite. In some places, the emplacement of such rocks
causes melting of the lower crust and this results in the emplacement of intru-
sions dominantly of diorite, granodiorite and granite composition at destructive
continental margins, accompanied by eruption of andesite, dacite and rhyolite.
The intrusive rocks emplaced at active continental margins form linear belts of
intrusive complexes of diorite-granite composition, often termed batholiths.
The composition of the continental crust broadly resembles that of the igneous
rocks of andesite composition. Much of continental crust is thought to have
formed as a result of igneous activity of the type seen today at island arcs and
at destructive continental margins. The crust has evolved continuously as a result