Physical Geology 101
Volcanic Rocks


Volcanic Landforms

Landforms which develop from the eruption of magma and are built of volcanic materials (lava, ash, etc.)
Volcanic landforms include more than just volcanoes

Volcanic Materials

Volcanic materials can be divided into two main categories:

Pyroclastic Material

Pyroclasts (tephra) are fragments of lava generally produced during violent, explosive eruptions
Tephra is classified by size:
Ash = <2mm
Lapilli = 2-64 mm
bombs/blocks = >64 mm

Pyroclastic Deposits

Pyroclastic deposits - welded, cemented or unconsolidated pyroclasts
Tuff, ignimbrite, ash fall, ash flow deposits are all terms used to describe pyroclastic deposits
Lahars and Mudflows (Example 1, Example 2)

Lava Flows

Non-violent, effusive eruptions of lava which flows along the surface
Basalt is the most common lava rock type
The type of flow is controlled by the composition of the lava

Lava Flow Types

Pahoehoe - very fluid flows - low viscosity; ropy surface appearance; basalt.
Aa - higher viscosity than pahoehoe; surface consists of spiny, angular clinkers; basalt, andesite.
Block flows - surface consists of angular blocks; andesite, rhyolite, obsidian
Pillow lava: