INTRODUCTION AND OCCURRENCE - The Field Description of Igneous Rocks - page 5

pyroclastic deposits may be underestimated in volume due to a large proportion
of fine ash material which can be carried great distances from the volcano.
The largest lavas flows build up thick sequences (plateaus), and construct large
igneous provinces of immense volume.
Volcanoes show a wide variety of forms, depending largely upon the com-
position of the erupted material and hence the style of eruption (cf. Chapters 4
and 5). Basaltic volcanoes such as Hawaii erupt dominantly (over 80%) lava,
the dominantly andesitic products of many volcanoes in island arcs and active
continental margins have less than 10% lava and over 90% pyroclastic rocks.
Further, the erupted proportion of pyroclastic rocks is often underestimated from
subsequent field studies because these materials are often rapidly dispersed by
the wind, or are eroded after deposition more rapidly than the equivalent volume
of solid lava. Hence, bear in mind that lavas might be over-represented in many
island arc and continental margin volcanoes.
Many volcanoes, particularly of andesite composition are composite in the
sense that they comprise both lava and pyroclastic materials and have a steep
irregular conical form (for example, Figure 1.1). Such volcanoes are built by flow
of lava down depressions around the volcanoes and the eruption of pyroclastic
materials; they commonly have diameters of 10 - 40 km. Volcanoes associated
with mainly basaltic eruptions form relatively low relief shallow sided volcanoes
Figure 1.1 Colima volcano with lava flows and pyroclastic flows which have
destroyed parts of the woodland around the mountain flanks.