Geology and ground models - Practical Engineering Geology - page 45

such as rhyolite (the fine-grained equivalent of granite). Acidic

volcanoes erupt in a much more violent way than basaltic volca-
noes. Other rocks associated with volcanoes are formed from
the huge amounts of dust and other airborne debris in volcanic
eruptions. Volcaniclastic sediments ( Table 3.2) are called tuffs
and often exhibit many sedimentary features, especially where
deposited in water. Hot clouds of debris deposited on land
become welded tuffs or ignimbrites. A welded tuff with pumice
inclusions ( (fiamme) that were hot and plastic when deposited and

then flattened by the weight of overlying material, is illustrated in

Figure 3.5.
Dykes are intruded in extensional (tensile) regions and cut across
other geological structures ( Figure 3.6); sills follow existing geological
structures such as bedding and are more concordant. Dykes can be
quite local or very extensive. The Great Dyke in Zimbabwe can be
traced across country for more than 450 km. Tertiary dyke swarms
and associated intrusion complexes in Scotland, Ireland and northern
England are shown in Figure 3.7.

Table 3.2 Simpli ed classi cation of sediments and sedimentary rock (see Tucker, 1982 for more

detail).

mm CLASTIC
SOIL 1, 2

CLASTIC ROCK VOLCANICLASTIC or

PYROCLASTIC ROCK

CHEMICAL &
BIOCHEMICAL
ROCK

BOULDERS LIMESTONE

(examples):
Chalk
Calcarenite (sand and
gravel size)

PYROCLASTIC
BRECCIA or
AGGLOMERATE

>200

CONGLOMERATE
(rounded clasts)
BRECCIA (angular)

COBBLES

60

GRAVEL LAPILLI TUFF

2

SAND SANDSTONE

Greywacke (generally
poorly sorted)
Arkose (feldspathic
sandstone)

Calcilutite (mud size
matrix)

COARSE ASH
TUFF

Oolite

0.06

DOLOMITE
(Mg rich)

SILT SILTSTONE

MUDSTONE
as general
term
SHALE if

EVAPORITE (salts)

COAL

<0.002

FINE ASH TUFF

FLINT & CHERT
(Cryptocrystalline
silica)

ssile

CLAY CLAYSTONE

1 C lastic means derived from fragments of other rocks. The term detrital is sometimes used, essentially
synonymously. Some clay is neither of these, but newly formed, sometimes from solution.
2 C lassi

cation and description of soil, including mixed soils, is dealt with in Chapter 4 and Appendix C.