Page 100 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

3. Subaerial basaltic lava flows
More than twelve 1-3 m thick, subaerial, basaltic lava
flows are exposed in this cliff section. The flow
interiors are massive whereas top and basal parts
comprise red-brown scoriaceous breccia. Very thin
fallout ash layers occur between some flows. The
upper part of the thickest fallout ash deposit (arrow)
has been thermally oxidized (red) close to the contact
with the overlying lava.

Subaerial basaltic lavas, Late Miocene; Porto da
Morena, Porto Santo, Madiera Archipelago.

4. Voluminous subaerial dacitic lava flow
In the distance is the 350-400 m high flow front of the 14.5
km long Chao dacitic lava flow. The flow comprises three
lobes, two of which are visible (Chao I and Chao III; De
Silva and Francis, 1991). The Chao lava has a large volume
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(about 26 km ) and long outflow distance. The flow
occupies the saddle between two composite andesitic
volcanoes, Paniri to the north and Leon (L) to the south.

Chao lava, >100 ka; northern Chile.

5. Subaerial andesitic lava flow
This photograph shows the extremely rough, irregular
surface of a subaerial andesitic lava flow. The top of the
flow comprises blocks and spires up to several metres
across characterized by scoriaceous and spinose surface
textures. The interior of the flow consists of massive,
coherent, evenly porphyritic andesite that is cut by
smoothly curving cooling joints.

Oni-Oshidashi Lava, erupted AD 1783; Asama
volcano, Honshu, Japan.

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