Extrusive – lava or ash onto the Earth surface
pyroclastic
pyro – fire
clastic – small pieces of rock
(same root word will be used to describe sediments)
Table 4.1 Summary of textures in volcanic rocks *** Know these ***
Fine-grained
Porphyritic
Obsidian
Vesicular
Pumice
Tuff
Volcanic breccia
Volcanic glass – obsidian
Figure 4.9
Andesite porphyry
Figure 4.10
Porphyry thin section
Figure 4.10
Vesicular basalt Figure 4.11
Bubbles produced by expanding gas in the magma
Pumice Figure 4.12
A
volcanic glass full of bubbles
Usually rhyolitic (lots of silica and very viscous)
Volcanic
bombs Figure
4.13
Volcanic
tuff Figure
4.14
Volcanic breccia
Big chunks of broken rock in a matrix of ash
Table 4.2
Another word for composite volcano:
stratovolcano
Shield volcano – created by basalt flows
Cinder
cone – steep pile of coarse ash
Composite volcano – layered lava and ash
andesite
or rhyolite
typically explosive eruptions
Basaltic
lava flows easily Figure 4.1
Rhyolitic
and andesitic lavas tend to explode
water & volatiles under pressure,
viscous magma
Cascades
volcanoes of the Pacific Northwest Figure 4.5
produced by subduction
of the Juan de Fuca plate
Juan de Fuca plate is young, hot, low
density, buoyant
Mt. Shasta, northern
California
Mt. Fuji, Japan
Internal structure Figure 4.21
Mount St. Helens before erupting in 1980
extensive ash deposition and pyroclastic flows
Mt. St. Helens eruption sequence Box 4.1
Forming a dome – because of viscous lava Figure 4.26
Pyroclastic flow
– a cloud of hot gas and ash collapses and flows down the sideResponsible for many of the volcano-related disasters in history
Mt. Pelée 1902
Crater Lake, Oregon
– a caldera formed by collapse
Figure 4.4 forming a caldera
produced by repeated basalt flows Figure 4.16 Low-relief slopes (because
basaltic lava flows easily) Hawaii – Mauna Loa and Mauna Kea Kilauea caldera
Cinder cones
Shield volcanoes
Basaltic lava – Pahoehoe “pah hoy hoy”
aa “ah-ah” partially cooled, crumbled surface texture
Crater and lava pool
spattercone
firefountain
Lava pool tectonics – a “rifting zone”
a “triple
junction”
Lava tube – flowing lava drains out of cooled outer tube
Basalt – columnar jointing Figure 4.28
Note: based on triple junctions (120° angles)