Igneous intrusions - Introducing Tectonics, Rock Structures and Mountain Belts - page 67

dykes

plug

vent

8

lopolith

laccolith

sill

sill

V

V

V

V
V
V
V

V

V

66 67

V

V

A B C D

Figure 8.1 Types of igneous intrusion. A. Dykes are discordant with the host-
rock structure and steeply dipping; sills are concordant with the host-rock
structure and horizontal (or gently inclined). B. Plugs are discordant and
forcefully intruded, usually into vents formed by explosive volcanic activity.
C. Laccoliths are concordant intrusions formed by arching up of overlying
strata. D. Lopoliths are also concordant, but formed by depression of the floor
of the intrusion. E. Stocks are discordant, broadly cylindrical bodies, either
forceful or permitted. F. Diapirs are forcefully-intruded stocks with a broadly
circular cross-section and an inverted tear-drop shape. Scale: dykes and sills
are typically in the range 1-100 m thick; plugs 100 m-1 km in diameter; both
laccoliths and lopoliths are several kilometres thick.

stock

F

E

host-rock bridge

regional
least stress
direction
(gravitational
load)

intrusion
plane

1

bridge broken

intrusion
plane

2

magma
pressure

regional
least stress
direction

bayonet structure

magma
pressure

A

B

3

xenoliths

4

500m

E

C

500m

Figure 8.2 Methods of emplacement. Dilational emplacement of a dyke ( A ) and ( B ) a
sill; intrusion can occur only if the magma pressure exceeds the host-rock stress acting
on the intrusion wall; the intrusion plane will therefore normally be perpendicular to the
regional least stress direction, which, in the case of a sill, will correspond to gravitational
pressure. C. A sill that is concordant with dipping strata may step up along faults in
order to maintain the same crustal level (based on a cross-section through part of the
Stirling Castle sill, central Scotland, based on McGregor & McGregor, 1948). D. The
extension direction is given by matching irregularities on each side of a dyke or sill. E.
Bayonet structure (E3) is formed by breaking of a bridge of host rock between the ends
of two overlapping dykes (E1-2), allowing the dyke to expand; pieces of the bridge may
subsequently break off to form xenoliths (E4). Based on Nicholson & Pollard (1985).

extension

D