Geology and ground models - Practical Engineering Geology - page 43

3.3.3 Igneous rocks and their associations
Igneous rocks were once molten. As magma cools, minerals grow
with an interlocking texture. As a result, most igneous rocks are
strong and sometimes extremely strong in their fresh state several

times the strength of concrete. They are primarily split into intrusive
rocks that solidify below the Earth

-

' s surface and extrusive rocks that

s surface. They are then differentiated according to
chemistry. Rocks with high silica content, either directly as quartz
(SiO 2 ) or tied up in the structure of other silicate minerals, are termed
acidic. Basic rocks have low silica content and ultrabasic rocks even
lower. A simpli

form on the Earth '

ed classi

cation of igneous rocks is presented in

Table 3.1.
Intrusive rocks, solidi ed at depth, include extensive igneous

bodies now exposed at the Earth s surface, such as granite that

makes up Dartmoor in the UK. This rock solidi

'

ed very slowly

Table 3.1 Classi cation of igneous rocks.

IGNEOUS ROCKS: generally have massive structure and crystalline texture. Typically high strength in fresh
state.

Volcanic rocks deposited as sediments are dealt with in Table 3.2: Sedimentary rocks. More details are given
in Streckeisen (1974, 1980) and Thorpe & Brown (1985).

BASIC 1
Little or no quartz

ACID
Much quartz

INTERMEDIATE
Little quartz

Pale colour Dark

Grain size

Relatively light in weight Heavy

GRANIT E 2 Coarse

>2mm

DIORITE GABBRO

GRANODIORITE SYENITE

MICRO-GRANIT E 3
MICRO-GRANODIORITE

DOLERIT E 4 Medium

0.06

MICRO-DIORITE
MICRO-SYENITE - 2mm

RHYOLIT E 5

Fine
<0.06 mm

ANDESITE BASALT

DACITE TRACHYTE

Glassy OBSIDIAN VOLCANIC GLASS

1 Rocks with even less silica and higher content of Fe and Mg are termed ultrabasic. Identify using standard
geological terminology. Examples are peridotite, pyroxenite and norite, and these can have distinctive
engineering characteristics (e.g. Dobie, 1987).
2 Rock types are often grouped together for engineering purposes, e.g. all coarse rock with free quartz is called
granite. This can be an oversimpli

cation as distinctions can be signi cant. Engineering geologists should use

cations, where possible.
3 Micro-granite is sometimes termed

full geological classi

fine-grained granite (see GeoGuide 3: GEO, 1988).

4 Diabase in US practice.
5 Where porphyritic, then often called quartz porphyry or feldspar porphyry, etc.