Geology Reference
In-Depth Information
4
VOLCANICS 1 - LAVA FLOWS
When magma reaches the surface it erupts as volcanic material, either as effusive
lava flows (see Figure 4.1) or explosively to form fragmental pyroclastic rocks.
In the next two chapters we will focus on the products and deposits of volcanic
activity. In this chapter we will look at the key structures and observations
that can be made from lava flows. In Chapter 5, Volcanics 2; pyroclastic rocks
will be the focus. It is important to remember that a mixture of lava flows and
pyroclastics rocks are often found together in most volcanic settings, and there
are some gradations between these two major divisions which we will touch on
in both chapters.
4.1 Lava Flow Emplacement Mechanisms
Before we focus on the key surface and internal structures found in lava flows,
it is important to briefly consider the mechanisms that are involved in their
emplacement as this will help to put some of your observations into context.
Also it is important to realise that our understanding of how lava flows are
emplaced has recently been much enhanced (in the late 1990s to early 2000s).
The largest lava flows are predominantly basaltic in composition and form vast
outpourings of lava known as flood basalts. These have similar compositions
to modern lavas as on Iceland and Hawaii and it was observations on Hawaii
that resulted in our understanding of the emplacement of lavas by inflation
(Figure 4.2). Flows start as thin sheets of lava which quickly form an insulating
crust. Further development of the lava involves inflation from inside of the flow
core, which is protected from cooling by this insulating crust. By this mech-
anism lavas can travel long distances from source without significant cooling
and crystallisation.
Other mechanisms for producing lava flows include the development of flows
from fire fountains, where the initial fountain of hot lava droplets coalesce back
into a coherent lava and flow away from source. Examples of this have been
recorded from Hawaii and from Japan, but such a process may be difficult to
identify in the field as the result will look like a normal lava flow. This is also
true of examples of rheomorphic ignimbrites (see Chapter 5).
