Page 138 - Fundamentals of Geomorphology

SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 121

Runny (low viscosity) lava spreads out over a large area, ( ) Scoria cone
a
while sticky (high viscosity) lava oozes out and spreads
very little. Mixed-eruption volcanoes combine explosive
phases with phases of lava production. Pyroclastic rocks
that fall to the ground from eruption clouds are called
tephra (from the Greek for ashes), while both lavas and
b
pyroclastic rocks that have a fragmented, cindery texture () Tuff ring
are called scoria (from the Greek for refuse).

Pyroclastic volcanoes
50 m
Explosive or pyroclastic volcanoes produce fragments () Tuff cone
c
of lava that accumulate around the volcanic vent to
produce scoria mounds and other topographic forms
(Figure 5.4; Plate 5.3). Pyroclastic flows and the deposits
they produce are varied. Tephra is a term covering three
types of pyroclastic material of differing grain size. Ashes
are particles less than 4 mm in diameter, lapilli (from the Figure 5.4 Pyroclastic volcanoes. (a) Scoria cone. (b) Tuff
Italian for ‘little stones’) are between 4 and 32 mm in ring. (c) Tuff cone.
diameter, and blocks are larger than 32 mm. The main Source: Adapted from Wohletz and Sheridan (1983)
types of pyroclastic flow and their related deposits are
shown in Table 5.1. Notice that two chief mechanisms material that stream upwards into the atmosphere from
trigger pyroclastic flows: (1) column collapse and (2) lava volcanic vents.The second involves the explosive or grav-
flow and dome collapse. The first of these involves the itationalcollapseoflavaflowsordomes. Pumicecontains
catastrophic collapse of convecting columns of erupted the most vesicles (empty spaces) and blocks the least.

Plate 5.3 Cinder cone, Mono Craters, California, USA. Cinder cones are the simplest of volcanoes. They are built of
cinder that falls around a vent to form a circular or oval cone, no more than 300 m or so high, usually with a bowl-shaped
crater sitting at the top.
(Photograph by Kate Holden)