Environmental Engineering Reference
In-Depth Information
INTRUSIVE ACTIVITY
Intrusions
create a plumbing system of underground magma reservoirs and pipework
which connects asthenos-phere/lithosphere melts with diapirs and surface extru-sions.
Two-thirds of new magma is thought to be intrusive. Large igneous intrusions are formed
mostly by the viscous flow of low-temperature, silicate-rich granitic magma or more
'solid' lithosphere under relatively thick continental crust by subduction or thermal
diapirism. The material permeates the
country
(surrounding) rock at depth through
existing voids, either as magmatized continental granitic crust or as the residual part of a
less silicic melt. Sustained flow, magma buoyancy and
anatexis
(melting of country
rock) enlarge them into substantial plutons or underground reservoirs. Batholiths are the
largest plutons, found in the cores of island arc complexes, subduction zones and some
extension orogens. Individual Cenozoic batholiths of 10
2-5
km
3
contribute to 10
6
km
3
of
intrusive assemblages in the Andes. The largest North American Mesozoic batholiths
extend over 1600 km in the Yukon-British Columbia-Washington coast range. Magma
pipes which feed batholiths or surface extrusions intrude country rocks
accordantly
as
sills
, thickening locally into sizeable
laccoliths
, and
discordantly
as
dykes
or fault-
directed
cone sheets
. The latter intrude rocks above the rising diapir and may feed
volcanoes. Intermediate or basic magmas develop as the original ground surface is
approached. Solidified plugs of volcanoes beneath extrusive lava flows mark the
intrusion/extrusion boundary (Figure 12.4a).
Figure 12.4
(a) Principal forms of igneous intrusion and
extrusion and (b) modern annual rates of global magma
production.
Source: Partly after Smith (1982).
The full nature of intrusion becomes apparent when it is exhumed as erosion removes
the
overburden
rocks. The intrusion usually forms prominent surface relief by virtue of
its high mechanical strength. Its mineral species and radioactive isotopes also provide
