Geology Reference
In-Depth Information
used to locate the base of the flow (way-up) and possible directions of final
movement of the lava where bent (for example, Figure 4.10). Withdrawal of
lava from the centre of a solidified lava flow may leave a hollow space within
the flow which is termed a
lava tube
. These are common in pahoehoe flows
and the occurrence of filling material on the sides or base of the tube may give
evidence for the position of the top of the flow. The interiors of most aa flows
are more massive than those described above, but some have a platy structure
defined by sheets of small vesicles. Vesicles may be aligned in complex flow
patterns but these are less well developed than in the case of pahoehoe lavas
described above.
4.3.3 Scales of basaltic flows
One important thing to think about when mapping out lava sequences is their
scale. Individual lava flows can range in scales over several orders of magni-
tude, with the largest flow units being greater than 1000 km
3
. More commonly
you will be dealing with erupted units less than 1 km
3
in size, and it may
be possible to map out lateral changes in thickness and volcanic facies on
scales of a few outcrops. In some of the larger terrains, known as flood basalt
provinces, the scales of flow unit and sheer volume of the whole sequence,
require a good understanding of different scales of heterogeneity ion the system
and may involve a regional and even cross continent appreciation of the dis-
tribution of the volcanic facies (for example, flood basalts that have occurred
during continental break-up such as North Atlantic Province or Parana-Etendeka
province Figure 4.11).
4.4 Felsic/Silicic Flows
The major difference in lava flows formed from more silicic compositions
is the change in viscosity. This leads to lower aspect ratio flows, sometimes
very little flow at all, and the development of joints and additional lava struc-
tures within the flow. A brief outline of some key features and example is
given below.
4.4.1 Internal structures and morphology
Whereas aa surfaces occur on both basalt and basaltic andesite lavas, most
andesite, dacite and rhyolite lavas are characterised by
blocky lava
surfaces.
These surfaces have detached polyhedral blocks with planar or slightly curved
faces and conspicuous dihedral angles. Intermediate and acid lavas sometimes
show columnar jointing, but more commonly develop
platy
or
slabby joints
.
The lower massive parts of andesite and dacite flows may be characterised
by abundant close-spaced platy joints, ranging in dimensions from 1 to 50 cm,
