Volcanoes, Magma & Plutons

Volcanoes, Magma & Volcanoes, Magma & PlutonsPlutons

Earth Science and the Environment (4Earth Science and the Environment (4thth ed) ed)

Thompson & TurkThompson & Turk

8.1 Magma8.1 Magma

►Creation of magma –Creation of magma –

rocks melt more easily with:rocks melt more easily with: Increasing temperatureIncreasing temperature Decreasing pressure Decreasing pressure Addition of waterAddition of water

8.1 Magma8.1 Magma

►Environments of magma formationEnvironments of magma formation At the axis of Mid-Ocean RidgesAt the axis of Mid-Ocean Ridges

► e.g. East Pacific Rise, Mid-Atlantic Ridgee.g. East Pacific Rise, Mid-Atlantic Ridge

Above Mantle plumesAbove Mantle plumes► e.g. Hawaii, Iceland, Yellowstonee.g. Hawaii, Iceland, Yellowstone

Above Subduction zonesAbove Subduction zones► e.g. Andes Mountains, Marianas Islandse.g. Andes Mountains, Marianas Islands

At a mid-ocean ridge the crust and lithosphere are thin, which means that hot asthenosphere is nearer to the surface, which is therefore at a lower pressure than normal. The over-lying crust is fractured, which allows water to seep down and come in contact with the hot rock below.

A spreading center, therefore, has all the conditions necessary to melt rock: High heat, low pressure and added water.

The lithosphere above a mantle plume (hot spot) is subjected to high heat, which cause it to bulge upwards and thin. This dynamic force lowers the pressure on the underlying asthenosphere and induces melting.

The crust atop the descending lithosphere in a subduction zone is heated and releases water as it sinks, which eventually leads to melting.

Although most (about 80%) of the volcanism around the world is submarine and occurs along the mid-ocean ridge system, volcanoes forming above subduction zones account for most of the subaerial volcanism. The subduction zones that lie along the margins of the Pacific Ocean produce about 75% of the subaerial volcanoes, which are collectively referred to as the “Ring of Fire”.

8.4 Magma behavior8.4 Magma behavior

►Magma risesMagma rises Cooling solidifiesCooling solidifies Lower pressure keeps it liquidLower pressure keeps it liquid

►Magma composition differencesMagma composition differences Granitic – 70% silica, up to 10% waterGranitic – 70% silica, up to 10% water Basaltic – 50% silica, 1-2% waterBasaltic – 50% silica, 1-2% water

8.4 Magma behavior8.4 Magma behavior

►Effects of silicaEffects of silica High silica content increases magma High silica content increases magma

viscosityviscosity

►Effects of waterEffects of water Increased water lowers solidification Increased water lowers solidification

temperature (dry magma solidifies quicker)temperature (dry magma solidifies quicker) Rising magma loses water, hardens quickerRising magma loses water, hardens quicker

Intrusive StructuresIntrusive Structures

► PlutonPluton – a bulbous mass of magma that – a bulbous mass of magma that intrudes into and solidifies within the crust.intrudes into and solidifies within the crust.

► Intrusive structures:Intrusive structures: BatholithBatholith – a collection of plutons that form a – a collection of plutons that form a

body the size of a mountain range, exposed by body the size of a mountain range, exposed by erosion, typically bigger than 100 kmerosion, typically bigger than 100 km22

StockStock – an irregularly shaped intrusive body that – an irregularly shaped intrusive body that is similar in size to a pluton (1 to 10 kmis similar in size to a pluton (1 to 10 km22))

DikeDike – a discordant tabular intrusive body – a discordant tabular intrusive body SillSill – a concordant tabular intrusive body – a concordant tabular intrusive body

Batholiths form above subduction zones as plutons of magma collect and cool within the crust beneath a volcanic arc. The plutonic rock of a batholith is exposed after subduction and volcanism cease and the overlying volcanic rocks have eroded away. A batholith is presently forming underneath the volcanoes of the Andes Mountains.

Plutons of the Sierra Nevada batholith are exposed by glacial erosion here in Yosemite Valley. The bright granite face of the El Capitan pluton is over 3000 ft high (~1Km).

A pluton or stock may supply magma to a variety of smaller intrusive structures such as dikes and sills, as well as being the reservoir for magma that erupts at the surface to form a volcano or lava flow.

Dikes and Sills are tabular intrusive bodies, which are easily recognized by their relationship with surrounding rocks. Generally dikes form as walls, and sills as layers, such that the former cuts across layers and the latter lies between layers of sedimentary rock.

Dike

8.6 Volcanoes8.6 Volcanoes

►Volcanic VentsVolcanic Vents are places where magma and are places where magma and related fluids erupt onto the surfacerelated fluids erupt onto the surface

Vents Vents typically occur astypically occur as tubes tubes andand fissures fissures

►VolcanoesVolcanoes form where repeated eruptions occur form where repeated eruptions occur from a central vent.from a central vent. A volcano is a conical pile of erupted materialsA volcano is a conical pile of erupted materials

A volcanic crater is a circular depression formed around a central vent, usually at the peak of a volcano. In the foreground below are two small

craters sitting within a larger crater, which itself sits within an even larger crater whose walls are the background.


► Effusive (Calm) Eruptions Produce LavaEffusive (Calm) Eruptions Produce LavaLavaLava – – magma at the surface, flowing or solidmagma at the surface, flowing or solid

BasaltBasalt – – the most common type of lava has two varieties:the most common type of lava has two varieties: PahoehoePahoehoe – ropy surface, easy flowing lava – ropy surface, easy flowing lava AaAa – rubbly surface, viscous (stiff) lava – rubbly surface, viscous (stiff) lava

► Explosive (Violent) Eruptions Produce Explosive (Violent) Eruptions Produce PyroclasticsPyroclastics

PyroclasticsPyroclastics – – fragmental and glassy materialsfragmental and glassy materials Volcanic AshVolcanic Ash CindersCinders Volcanic Breccia Volcanic Breccia (Dominated by Rock Fragments)(Dominated by Rock Fragments)


►Fissure eruptions – low viscosity lava Fissure eruptions – low viscosity lava exuding from cracksexuding from cracks

►Flood basalt – very large, rapid, fissure Flood basalt – very large, rapid, fissure eruptioneruption

►Lava (basalt) plateau – many cubic Lava (basalt) plateau – many cubic kilometers sized eventkilometers sized event

The Columbia Plateau

The plateau formed around 15 million years ago by repeated eruptions of flood basalt that covers an area of about 200,000 Km2 and in places is up to to 3 Km thick.

Individual flow layers of the Columbia River Basalt are between 15 and 100 meters thick. Good exposures are found where the Columbia River and its tributaries have eroded deep canyons through this jointed rock.

FLOO

D B

ASA

LTS

OF T

HE

CO

LUM

BIA

PLA

TEA

U

COLUMNAR JOINTING IS COMMON IN LAVA FLOWSThe Devil’s Post Pile, a formation located in the Sierra Nevada, dramatically illustrates the phenomenon of columnar jointing, which forms as a lava flow cools and shrinks.

A view from above reveals their hexagonal shape


►Shield volcanoesShield volcanoes Built by effusive eruptions of basaltBuilt by effusive eruptions of basalt Gently slopedGently sloped The largest of volcanoesThe largest of volcanoes

►e.g. Mauna Loa, Mauna Kea, Icelande.g. Mauna Loa, Mauna Kea, Iceland

Shield volcanoes form the largest and tallest volcanoes on earth, despite their gentle slopes. They form from copious amounts of basalt lava erupted from a summit crater as well as from fissures around their flanks. Two common types of basalt lava (Pahoehoe and Aa) erupt from Hot Spot volcanoes like this one (Mt. Skjoldbreidier, Iceland). The two are differentiated by their relative viscosities and surface textures.

Pahoehoe is thin, gas-rich basalt lava. It advances quickly as a narrow stream, often within tubes, and has a smooth, ropey surface.

Aa is stiff, gas-poor basalt lava. It advances slowly as a thick sheet with a steep front and has a rough, blocky surface.


►Cinder cones Cinder cones Formed of pyroclastic fragments (ash & Formed of pyroclastic fragments (ash &

cinders)cinders) Often steep, small and symmetricalOften steep, small and symmetrical May from abruptly (hours or days) May from abruptly (hours or days)

►Eruptions are driven by escaping Eruptions are driven by escaping gasesgases

This cinder cone in Bolivia has a symmetrical shape and a well formed crater (circular depression) at its peak.


►Composite conesComposite cones aka: stratovolcanoesaka: stratovolcanoes Layers of lava and pyroclastics Layers of lava and pyroclastics

accumulate from both effusive and accumulate from both effusive and explosive eruptionsexplosive eruptions

Relatively steep-sidedRelatively steep-sided Associated with subduction zonesAssociated with subduction zones

Mount Rainier is a classic example of a composite volcano. Its presence is a constant reminder to the residents of Seattle of their precarious location above a subduction zone where tsunamis, earthquakes, volcanic eruptions and mudslides have occurred in the past and are surely to occur at anytime in the near future.

Ash flows and CalderasAsh flows and Calderas

►Ash Flow – a cloud of pyroclastics that flows Ash Flow – a cloud of pyroclastics that flows along and buries the surfacealong and buries the surface Nueè ardente – “glowing cloud”Nueè ardente – “glowing cloud” Hot, Fast (200 km/h), Far-reaching (100 km)Hot, Fast (200 km/h), Far-reaching (100 km)

►Caldera – collapsed roof of magma chamberCaldera – collapsed roof of magma chamber Large (10+ km) circular depression, steep sidesLarge (10+ km) circular depression, steep sides Associated with catastrophic eruptions of AshAssociated with catastrophic eruptions of Ash

CALDERA: The sequence of events leading to the formation of a caldera involves creation of “ring” fractures in the crust that lies above a rising pluton of magma. The circular depression of the caldera is formed during the eruption as magma is ejected through the fractures and the unsupported crust sinks. This catastrophic eruption is driven by escaping gases and produces a huge cloud of pyroclastic debris.

Crater Lake in southern Oregon fills a caldera that formed about 7000 years ago by the eruption of Mt. Mazama. The caldera is

about 10 km across.

8.7 Volcanic explosions:8.7 Volcanic explosions:

► Yellowstone – 3 calderasYellowstone – 3 calderas Last eruptions (1.9 mya) – 2500 kmLast eruptions (1.9 mya) – 2500 km3 3 of of

pyroclastic materials (in 1980 Mt St Helens pyroclastic materials (in 1980 Mt St Helens erupted only 1 kmerupted only 1 km33))

(0.6 mya) – 1000 km(0.6 mya) – 1000 km3 3 of ash & debrisof ash & debris

► Long Valley Caldera (Bishop Tuff ~ 0.8 mya)Long Valley Caldera (Bishop Tuff ~ 0.8 mya) 170x larger than 1980 Mt St Helens eruption170x larger than 1980 Mt St Helens eruption Hot Springs and COHot Springs and CO22 releases denote continued releases denote continued

magma activity todaymagma activity today

Fig. 8.30, p.199

8.7 Volcanic explosions:8.7 Volcanic explosions:

►Mt Vesuvius – a composite volcano Mt Vesuvius – a composite volcano near Naples, Italynear Naples, Italy Buried the cities of Pompeii & Buried the cities of Pompeii &

Herculaneum during its 79 A.D. eruptionHerculaneum during its 79 A.D. eruption 5-8 meter thick ash flow deposits5-8 meter thick ash flow deposits Intermittent activity in early – mid 1900sIntermittent activity in early – mid 1900s Magma still underlies VesuviusMagma still underlies Vesuvius

Excavations at Pompeii revealed casts of humans buried in the ash flow deposits M

oun

t Vesu

viu

s eru

pte

d in

79

A

D

Plateaus and Calderas are not volcanoes. But they do represent end members of the spectrum of eruption styles from most gentle (effusive) to most violent (explosive), respectively. Although the magma that erupts during formation of a caldera can be called “granitic” it is more correct to refer to the erupted material as Rhyolite. The true volcanoes are accumulations of materials erupted from a central vent. From largest to smallest they are the shield volcano, composite volcano, and cinder cone.

Documents

Volcanoes, Magma & Plutons