Geology Reference
In-Depth Information
Figure 5.23
Peperite example from intermixed aeolian sand and lava fragments.
Glassy (now weathering orange/red) bulbous and irregular feather like contacts,
with the sand highly indurated (baked), Etendeka, Namibia.
A key sub-type of epiclastic deposit are
lahars
. These are formed by hot-
cold debris flows which can occur with the reworking of hot mixtures during
or very shortly after eruption, or by reworking of cold poorly-sorted material,
commonly by water sourced by rain or melted ice. A simple classification of vol-
caniclastic mass flows is given in Figure 5.25, showing the continuum between
debris avalanche and lahar type deposits. The deposits frequently consist of
large volcanic fragments in a finer ashy matrix which is commonly coarser than
mud-grade (Table 5.1) (Figure 5.26). The Indonesian term
lahar
is preferable
for such water-deposited poorly-sorted deposits of volcanic materials. Lahars
are also variable in terms of the proportion of juvenile and non-juvenile vol-
canic material. Fragmented volcanic materials deposited from a cool water-rich
flow are also sometimes referred to as
mudflows
. Eruptions of volcanic material
through lakes, below ice or during heavy tropical rain may generate lahars com-
posed largely or entirely of juvenile volcanic material. Conversely, non-juvenile
material may be erupted and there may, therefore, be a transition from hot
(pyroclastic) lahars to water-rich
debris flows
formed on the slopes of volcanoes.
These are especially common where deposits form in shallow-water conditions
