Geology Reference
In-Depth Information
Figure 4.4
Photo of spherules in a glassy pitchstone from the Etendeka,
Namibia.
100 Ma in age. However, many ancient volcanic rocks must have originally
contained natural glasses. Crystallisation occurs by spontaneous growth of tiny
crystals or
crystallites
, commonly of quartz and feldspar. Usually, the individual
crystals are not visible in hand specimens but devitrification crystals, in radiating
clusters forming small circular structures resembling cod roe or oolites (termed
spherulites
), may be scattered throughout the devitrified rock (for example,
Figure 4.4). Complete devitrification yields an aphyric microcrystalline rock,
which may be light-coloured lava. (Such rocks may be termed
felsite
, but the
term is also applied to any light-coloured fine-grained rock composed chiefly of
quartz and feldspar.)
4.3 Mafic/Basaltic Lava Flows
As the most common occurrences of lava flows are mainly basaltic in compo-
sition we will concentrate on these in detail, before introducing examples from
felsic/silicic flows, and eruptions of lava in water.
4.3.1 Surface textures
The surfaces of subaerial mafic lava flows belong to
two
major types:
Pahoehoe
and
Aa
(Figure 4.5).
Pahoehoe flows
(Figure 4.5a) have smooth, billowing
ropy or entrail-like (
entrail-pahoehoe
) crusts of quenched glass. Ropy crusts
