Chapter 8 - Igneous Activity

Nature of Volcanic Activity

Primary factors include - All affect the magma's

viscosity - (resistance to flow) High viscosity means low flow

composition of the magma
the higher the silica (SiO₂) the higher the viscosity - basalt flows better than granite
magma temperature
increase temp you decrease viscosity
amount of dissolved gases
increase gases decrease viscosity
also a pressure function - KABOOOMMMM!!!

Materials extruded during an eruption

Lava Flows

Basaltic flows
low in silica - very fluid
flow in thin, broad sheets or stream like ribbons
average rate of flow - 10-300 m/hr (30 kph - top)

Pahoehoe

relatively smooth skinned
resemble twisted braids of rope

aa

surface is rough, jagged blocks with sharp edges
looks like rubble
produced by escaping gases
relatively cool, thick flows 5-50 m/hr

Gases

1. 1 - 5% total weight

most is water vapor
Hawaiian eruptions 70% - water, 15% CO₂, 5% each of N &S also Cl, H and Ar
% is small but overall mass is large

Pyroclastic materials

from highly viscous materials,
magma becomes superheated,
1,000x expansion
ejected lava fragments - fine dust and sand sized ash to volcanic bombs and blocks

ash - fine particles caused by lots of gas

welded tuff - if many ash particles stick together

pumice - if the particles are big enough and have enough voids to allow it to float

cinders - pea size

lapilli - walnut sized, if they are bigger than this

blocks - if they a made of hardened lava

bombs - if the are ejected as incandescent (semi molten) lava streamlined shape
 

Volcanoes and Volcanic Eruptions

volcano

- when successive eruptions from a central vent result in a mountainous accumulation of material

craters

- steep walled depression located at the summit

calderas

are over 1 km in diameter

parasitic cones

- if lava leaves from a side vent

fumaroles

- secondary vent emitting only gases

Types of volcanoes

Shield Volcanoes

primarily basaltic
small % of pyroclastics
slopes low on flanks
15% at the summit
big - Mauna Loa - our biggest mountain

Cinder Cones

ejected lava fragments (pyroclastics)
steep slopes 30-40^o
small < 300 m
often in groups
last phase of volcanism in an area

Composite Cones

large
symmetrical with a central vent
alternating layers (lava-pyroclastics)
andesitic (granitic) lava - viscous

nuée ardente - a pyroclastic flow

steam and gas cloud
speed up to 100 kph
moves up to 100 km from origin

lahar

mud flow

Volcanic Features

pipes    and  vents

- connect to magma source

volcanic necks

- leftovers after erosion removed the cone

calderas

- summit collapses into a partially empty magma chamber

fissure eruptions

cracks
largest amount of lava
flood basalts - build lava plains

pyroclastic flows

- can resemble lava on hardening, but it is actually welded tuff

Intrusive Igneous Activity - Plutons

1. SHAPE

tabular - sheet like

massive - blob(?) like

2. ORIENTATION to the host rock

Concordant - along

sills

tabular
very fluid magma
shallow b/c they have to lift overlying layers
often develop columnar joints due to contraction upon cooling

laccoliths

magma more viscous than sills
produces an arch in overlying strata

Discordant - across

dikes

moves into fractures that cut across layers
usually more resistant - The Flume (?)

Batholiths

Big - >100 km2 surface exposure
Sierra Nevada batholith

Origin of Magma

most comes from solid rock that melts in the crust and upper mantle
750^oC for granitic magmas, 1000^o C for basaltic magmas

Role of Heat

the deeper you go the hotter it gets
20-30^o C / km - the geothermal gradient

Role of Pressure

the higher the pressure  (deeper in the ground - confining pressure) the higher the melting temperature
therefore if pressure is reduced melting is induced

Role of Volatiles

water is the big one
it migrates upward and lowers melting temp of the rock above it

Partial Melting and Magma Composition

Remember Bowen's reaction series - high silicate minerals melt first
this enriches magmas toward the granitic end

this is a review of chp 7 as it relates to igneous activity
Study Figure 8.26 carefully