Volcanic Rocks: Eruption Style & Lava Flows

Lava Fountains

The Causes of Volcanic Eruptions

How do magmas rise?

If only due to bouyancy most basaltic lavas could not erupt through continental crust

The Causes of Volcanic Eruptions: Volatiles

Decrease in Pressure

H2O is the most important volatile in controlling volcanic eruptions

DissolvedH2O

Increase inTemperature

Crystallisation(decrease in temp)

Depolymerisation + confining pressure

Depolymerisation

Fractionation

The Closed Magma Chamber

Magma rises until it is trapped within a closed magma chamber

The Closed Magma ChamberCooling and crystallisation causes the exsolution of volatiles

Magma chamber expands due to an over-pressure

The Closed Magma Chamber

Expansion of the magma chamber causes tensile failure of country rocks

Magma chamber expands due to an over-pressure

Rise of less dense vesicular magma

Conduit formed due to fracturing

An Open Conduit

Exsolution of volatiles due to decreasing pressure

Increasing acceleration

An Open Conduit

Exsolution of volatiles due to decreasing pressure

Break up of magma

Foam reaches 77 vol% vesicles

Magma/gas aerosol 600 m/s

Interactions with Water

Hydrovolcanic Eruptions

Hydrovolcanic Eruptions

Hyaloclastite

Hydrovolcanic Eruptions

Efficient cooling of magma

Phreatic EruptionsPhreatomagmatic

0.3

Hydrovolcanic Eruptions

Efficient cooling of magma

Phreatic EruptionsPhreatomagmaticPhreatic Eruptions: Maars

Mainly fragmented country rock, sediments can deform in-situ

Hydrovolcanic Eruptions

Efficient cooling of magma

Phreatic EruptionsPhreatomagmaticPhreatic Eruptions: Maars

Mainly fragmented country rock, sediments can deform in-situ

Contact between magma and an acquiferHigh abundance of lithic materials

Hydrovolcanic Eruptions

Surtseyan

Phreatomagmatic Eruptions

Hydrovolcanic Eruptions

Kilauea

Explains the presence of ashes on kilauea

Magma-Mixing

Acid Magma

Basaltic Magma

Extra volatiles Extra heat

Expansion due to vesicle formation, temp increase and/or volume of the injected magma

Extrusive Activity

Loss of volatiles allows extrusive activity

• Low viscosity magmas• Low H2O magmas

Basaltic magmas

Intermediate/acid magmas after explosive activity (i.e. loss of volatiles)

Size and Shape of Lava Flows

Andesite lava flow

Rhyolite DomeBasaltic pahoehoe flows

Rhyolites

Andesites, Dacites

Basalts

Size and Shape of Lava Flows

Rhyolites

Andesites, Dacites

Basalts

Gradient

ViscosityLow viscosity High viscosity

High temperatureLow crystal contentHigh water content

Low-Si (basaltic)

Low temperatureHigh crystal contentLow water contentHigh-Si

Effusion RateHighLow

Shield Volcanoes

Mauna Kea

Mauna Loa

Rift Zone

Low viscosity basaltic magmas usually lead to shield volcanoes. Eruptions occur mainly from vents.

Fissure Eruptions

Splatter Ramparts and Splatter Cones

Lava Fountains

Mauna Loa

Fissure Eruptions

Splatter Ramparts and Splatter Cones

Lava Fountains

Mauna Loa

Splatter forms agglutinate scoria

Low viscosity, high extrusion rate lavas expand as single flow units

Basaltic Flows: Pahoehoe & Aa

Pahoehoe (ropey) surface formed by folding of the crust

Low viscosity

Basaltic Flows: Pahoehoe & Aa

Flow expands by extrusion of tongues and toes of lavaHigher viscosity/lower extrusion rates

Basaltic Flows: Pahoehoe & Aa

Lava blisters and hornitos can occur on the upper surface of the flow

Basaltic Flows: Pahoehoe & Aa

Insulated flow

Basaltic Flows: Pahoehoe & Aa

Aa flows roll forwards over their breccia

Higher viscosity (lower T)

Basaltic Flows: Channels

Flows tend to form channels

Basaltic Flows: Channels

Flood Basalts

Snake River Flood Basalts

Flood Basalts

Flood Basalts

Giant’s Causeway Shrinkage cracks due to contraction on cooling.Intermediate cooling rates.

Pillow Lavas

Submarine Volcanics

Intermediate Lavas

• Channelled by block levees• Ridges due to compression of magma• Extensive basal and flow top breccias.• Associated with stratovolcanoes or calderas

Dacite & Rhyolite Lavas

Mono Coulee, California

Can form lava flows where viscosity and extrusion rate are sufficiently high

Dacite & Rhyolite Lavas

Mono Coulee, California

Obsidian

Banding

Dacite & Rhyolite Domes

More commonly form lava domes, often within summit crater.

Collapse of lava domes is a major cause of pyroclastic eruptions.

Extrusive Igneous Rocks: Ultrabasic

KomatiiteMg-rich Ultrabasic lavas

Pyroxenite lavas are also known.

Archean lavas

Extrusive Igneous Rocks: Basic Rocks

Intermediate Rocks

Dacite, latite and Andesite often containhornblende

Extrusive Igneous Rocks: Basic Rocks

Intermediate Rocks

Dacite, latite and Andesite often containhornblende

BasaltAnorthite-rich plag + pxOften contains olivine

Extrusive Igneous Rocks: Basic Rocks

Intermediate Rocks

Dacite, latite and Andesite often containhornblende

BasaltAnorthite-rich plag + pxOften contains olivine

Andesite

More Na-rich plagHornblende

Extrusive Igneous Rocks: Basic Rocks

Intermediate Rocks

Dacite, latite and Andesite often containhornblende

BasaltAnorthite-rich plag + pxOften contains olivine

GabbroAnorthite-rich plag + pxOften with olivine

Dolerite = medium-grained equivalent

Extrusive Igneous Rocks: Basic Rocks

Basalts are split into Tholeiites – these have noolivine in their groundmass and can contain quartz.Alkali Basalts – these have olivine in their groundmass.

Olivine-rich basalts (i.e. Mg-rich) are known as picrites.

Tholeiite – can contain phenocrysts of olivine but not in the groundmass

Extrusive Igneous Rocks: Basic Rocks

Basalts are split into Tholeiites – these have noolivine in their groundmass and can contain quartz.Alkali Basalts – these have olivine in their groundmass.

Alkaline basalts are the trachybasalts and include varieties such as hawaiite, mugearite and benmoreite.

Trachyte – alkali feldspar + px, often has aligned phenocrysts = trachytic texture.

Extrusive Igneous Rocks: Alkaline Rocks

Rocks rich in alkali metals (Na and K) within feldspathoids

Phonolites: sanidine, anorthoclase (alkali feld) phenocrysts, nepheline, px in groundmass.

Tephrite: plagioclase, px and nepheline. If olivine is present it is a basanite.

Nephelinite: nepheline and cpx.

Leucitite: leucite and cpx. If olivine and pyroxene dominate it is a ugandite.

Feldspathoids