Geology Reference
In-Depth Information
Figure 5.19
Example of welded ignimbrite structure, Bishop Tuff, Owens Valley,
California. Note streaked out (dark) fiamme, with two lithic fragments maintain-
ing their shape.
Eruptions under ice are characterised by the formation of flat-topped volcanic
landforms termed
Tuyas
(table mountains) from examples in Iceland. Here the
ice restricts the eruptive products leading to restricted and chaotic breccias
(Figure 5.21a), and in some cases a meltwater lake will develop resulting in
the formation of more typical bedded hyaloclastites as the volcano emerges and
fills the accommodation space (for example, Figure 5.21b).
5.4.2 Peperites
The dynamic interaction of unconsolidated (commonly wet) sediments and
juvenile magma result in a rock type known as
peperite
. These are relatively
localised occurrences at the base of lava flows have flowed over and/or where
intrusions have occurred into unconsolidated sediments. When sediments are
wet the interaction is fuelled by the explosive fragmentation and
autobreciation
of the magma, and characteristic angular-irregular glassy fragments interspersed
with indurated and fused sediment results (for example, Figure 5.22), commonly
with sediment-filled dyke injections back into the host magma, and chilled
contacts. Care should be taken not to invoke the role of water in all peperite
formation as examples of dynamic interactions between juvenile magma and dry
