Geology Reference
In-Depth Information
Rocche Rosse obsidian coulee, Lipari, Italy
Ogive
Surface breccia
Foliated obsidian
Basal breccia
(a)
Southwestern Idaho rhyolite flows, USA
Large gas
cavities
Upper vitrophyre
Shrinkage
joints
Medial slope
Upper
zone
Flow lobe
Central
zone
Crumble
breccia
Flow
base
Basal flow
breccia
Basal vitrophyre
Near vent
pyroclastics
Feeder dyke
(b)
Figure 4.12
Examples of internal felsic flow structures.
4.4.2 Domes
Few dacite or rhyolite flows travel more than a few kilometres from the vent;
these may reach thicknesses of up to several hundred metres. Extrusions of
dacite or rhyolite lava that are circular in plan and have a height: diameter
(
h/d
) ratio of 0.5
0.3 are termed
domes
(Figure 4.13). Domes commonly have
heights of 50 - 150 m and diameters of 150 - 500 m. Domes form commonly in
volcanoes in arc settings (for example, Chapter 1), and can collapse to form
pyroclastic flows when they get over steepened during inflation. Dome growth
can occur within the top cone of the volcano (for example Figure 4.14), or as
a dome growth in the centre of a recently erupted and destroyed volcano as a
resurgent dome
, which starts the process of rebuilding the volcano (as in Mt
St Helens, USA; Figure 4.15). Domes are most easily recognised in modern
volcanoes and the recent geologic record.
−
4.5 Pillow Lavas and Hyaloclastites
Pillow lavas
(Figure 4.16) are basalts that erupted below water and, since the
upper part of the ocean crust is basaltic, such lavas may be the most abundant
volcanic rocks of the Earth's surface (see also Chapter 8). However, it should be
