Geology Reference
In-Depth Information
5
VOLCANICS 2 - PYROCLASTIC ROCKS
The second major category of volcanic rocks are those formed by the explosive
eruption of magma at the surface resulting in pyroclastic rocks (Figure 5.1).
These types of deposits can be found in almost all volcanic settings but are
particularly common in convergent plate margins (Chapter 1). Here we will
look briefly at the processes that form pyroclastic rocks, followed by some of
the specific things that need to be recorded to help classify and quantify them.
We will also consider some of the other types of volcaniclastic rocks which
develop as the result of fragmentation of juvenile magma at the surface. The
reader is reminded of the close association of pyroclastic/volcaniclastic rocks
and rocks formed from lava flows described in Chapter 4.
5.1 Structures, Textures and Classification
Pyroclastic rocks are composed of materials fragmented by explosive volcanic
activity. Three types of fragment may be found in any pyroclastic deposit.
1.
Fragments of new lava
which are termed
juvenile
and which may range
from solid un-vesiculated material to fragments of highly vesiculated lava
(for example, pumice and scoria) and shards of glass.
2.
Individual crystals
, yielded by liberation of phenocrysts present in the juve-
nile lava as a result of fragmentation.
3.
Lithic fragments
which may include any older rock within the deposit, but
often comprising older lavas.
5.1.1 Primary structures
Bedding and associated structures that result from the depositional processes are
wide and varied in pyroclastic rocks. This is due to the range of depositional
environments, flow types and fragment types that make up the rocks/sediments
constituents. It is important to make sure that you are able to recognise pri-
mary deposits from those which are re-worked (these will be dealt with under
epiclastic deposits later in this chapter).
