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 H 2 O is the most important volatile in controlling volcanic eruptions Dissolved H 2 O Increase in Temperature 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 Chamber Cooling 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

Hyaloclastite

Hydrovolcanic Eruptions Efficient cooling of magma Phreatic EruptionsPhreatomagmatic 0.3

Hydrovolcanic Eruptions Efficient cooling of magma Phreatic EruptionsPhreatomagmatic Phreatic Eruptions: Maars Mainly fragmented country rock, sediments can deform in-situ

Hydrovolcanic Eruptions Efficient cooling of magma Phreatic EruptionsPhreatomagmatic Phreatic Eruptions: Maars Mainly fragmented country rock, sediments can deform in-situ Contact between magma and an acquifer High 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 H 2 O magmas Basaltic magmas Intermediate/acid magmas after explosive activity (i.e. loss of volatiles)

Size and Shape of Lava Flows Andesite lava flow Rhyolite Dome Basaltic pahoehoe flows Rhyolites Andesites, Dacites Basalts

Size and Shape of Lava Flows Rhyolites Andesites, Dacites Basalts Gradient Viscosity Low viscosity High viscosity High temperature Low crystal content High water content Low-Si (basaltic) Low temperature High crystal content Low water content High-Si Effusion Rate High Low

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 lava Higher 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

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 Komatiite Mg-rich Ultrabasic lavas Pyroxenite lavas are also known. Archean lavas

Extrusive Igneous Rocks: Basic Rocks Intermediate Rocks Dacite, latite and Andesite often contain hornblende

Extrusive Igneous Rocks: Basic Rocks Intermediate Rocks Dacite, latite and Andesite often contain hornblende Basalt Anorthite-rich plag + px Often contains olivine

Extrusive Igneous Rocks: Basic Rocks Intermediate Rocks Dacite, latite and Andesite often contain hornblende Basalt Anorthite-rich plag + px Often contains olivine Andesite More Na-rich plag Hornblende

Extrusive Igneous Rocks: Basic Rocks Intermediate Rocks Dacite, latite and Andesite often contain hornblende Basalt Anorthite-rich plag + px Often contains olivine Gabbro Anorthite-rich plag + px Often with olivine Dolerite = medium-grained equivalent

Extrusive Igneous Rocks: Basic Rocks Basalts are split into Tholeiites – these have no olivine 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 no olivine 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