Volcanoes

Kilauea

What happens to temperature and pressure as you descend beneath the

Earth’s surface?

• If low pressure areas develop them the solid rock will melt and become magma– This happens when mantle material flows to

areas of lower pressure faster than the temperature can be lowered

Volcanism

• This describes the movement of magma towards or onto the Earth’s surface– Magma is less dense than solid rock so it

pushes towards the surface through cracks and joints

– This upwelling causes more rock to heat and melt, adding to the amount of magma

Volcanism

• Magma often forms at plate boundaries, especially in subduction zones, mid-ocean ridges and “hotspots”

• When magma breaks through the surface it is called lava

• Openings where lava erupts are vents• The accumulation of lava at the surface

around vents is called a volcano

Subduction zones

• Where plates converge oceanic crust and ocean water are subducted and these materials move beneath the lithosphere– Results in deep ocean trenches on the ocean

floor and mountains along the edge of the continent

– The same may occur where two oceanic plates converge and one is subducted except that island arc volcanoes are created instead.

Subduction zones

• The Pacific Ring of Fire is an example of convergent boundary volcano formation– Cascade Mountains of North America– The Aleutian Islands are an example of a

volcanic island arc

Panoramic view of five Cascade volcanoes, looking north from Middle Sister: (from left) Belknap Crater, Mt Washington, Three Fingered Jack, Mt Jefferson, Mt Hood

Mid-Ocean Ridges

• Where plates are diverging and new magma is rising to the surface new volcanoes care created

• Most are not noticed because they are far below the surface

• Iceland is one location where this process is easily observed– Can you think of another, older site?

Hot Spots• Where there are weak spots in the crust,

especially over an area where convection brings a magma plume close to the surface you may find a hot spot volcano– Hawaii is one of the best examples of this

• As time goes by the plate continues to shift moving the lithospheric plate away from the hot spot and a new volcano may form– This is evident in the chain of Hawaiian islands

Volcanic Eruptions

• There are two kinds of lava that result from different chemical combinations– These form the basis for the rock types named

after them• The type of magma can affect the type of

volcanic eruption that occurs• Silica is a main ingredient in all magmas

and determines the viscosity of the magma (and it’s ability to flow)

Volcanic Eruptions

• Magmas high in silica form felsic lava and resist flow– These form andesitic and rhyolitic volcanoes– This also account for their lighter colour

• Magmas low in silica form mafic lava and flow more easily– These form the darker basalts often seen in

mid-ocean volcanoes or hot spots

Rhyolite, Andesite, Basalt

Characteristics of Magma

MaficBasaltic

IntermediateAndesitic

FelsicRhyolitic

Silica content Least (about 50%)

Intermediate (about 60%)

Most (about 70%)

Gas content Least Intermediate Most

Viscosity Least viscous Intermediate Most viscous

Type of eruption

Rarely explosive Sometimes explosive

Usually explosive

Melting temperature

Highest Intermediate Lowest

Location Rifts, oceanic hot spots

Subduction zones Continental hot spots

Volcanic Eruptions

• Magmas also contain gases – mostly water vapour and CO2

• Basaltic or mafic magmas are very hot and thin and release gases easily, very quiet

• When they reach the surface any remaining gases usually form harmless fountains and floods

Mafic lava

• Dark in colour• Form much of the oceanic crust

– mid-ocean ridges, oceanic hot spots• When mafic lava cools rapidly the surface

flow forms a crust– Ropey or wrinkled material is called pahoehoe– If this crust is is further deformed to make

wrinkles that break into jagged pieces it is called aa

Pahoehoe lava

                                                        

                                       

Pahoehoe and aa

Hawaiian aa

Cooling aa from Mt. Etna

Eroded cinder cone

Different colours of sand on Hawaiian all due to volcanism

Eroded basalt

Olivine

Mafic lava

• Lava tubes form when the outer layer cools quickly but the interior remain hot and liquid and flows out of the crust leaning the tube behind it

• Lava that flows into the water cools rapidly forming pillow lava (blobs)

South Korea Thurston Lava tube, Hawaii

Active lava tube, Kilauea

Pillow lava

Volcanic Eruptions

• The gases in felsic magmas can’t escape as easily and when these gases rise they expand and are propel the magma quickly to the surface, these magmas are cooler and thicker (why?)– This results in explosive eruptions of gas and

molten and solid debris called pyroclastic material

– For example – Mt. St. Helens

Mt. St. Helen’s before May 18, 1980

Mt. St. Helen’s after May 18, 1980

Ash from the eruption 19 km away

Empty bus caught in the mud flow

Lahar

Downed trees

Felsic magmas

• Lighter colour• Forms much of the continental crust• These lavas tend to be cooler and thicker

with a high dissolved gas content• When an opening forms they tend to boil

out explosively sending out molten and solid pyroclastic material

Pyroclastic Material

• Volcanic dust – less than 0.25 mm• Pumice – very frothy, formed by gas inclusions• Ash– less than 2 mm in diameter• Lapilli – less than 64 mm in diameter• Bombs – larger than 64 mm (fist sized)• Blocks – can be very large pieces• Note: tephra refers to materials as small as ash or

as large as bombs

Mt. Pinatubo dust cloud

Pumice

Volcanic ash

Lapilli

Volcanic bombs

Tephra that falls from the eruption column produces a tephra fall deposit.

Mt. Etna

Pyroclastic Material

• Pyroclastic flow– Combination of volcanic gases and particulate

matter that forms a superheated cloud that travelers downhill very quickly (more than 100 km/h)• Pompeii was buried under pumice and ash in 79 AD

• Lahar– Pyroclastic flow mixed with snow and ice that

rapidly melts and form a fast flowing mud flow• Mt. St. Helen’s the lahar was 46 m deep

Pyroclastic flow on the island of Monserrat

Flow of materials from the Twin Towers

Types of Volcanoes

• Lava and pyroclastic material that build up around a vent are known as volcanic cones

• There are three main types of cones– Shield volcanoes– Cinder cones– Composite cones or stratovolcanoes

Shield Volcanoes

• Cones that have a broad base and gently sloping sides build up from (usually) mafic flows over long periods of time– Hawaiian islands – Mauna Loa is 4170 m

above sea level and rises 5000 m from the ocean floor (total – 9170 m total)

• These tend to be less explosive but have frequent flows

Hawaiian shield volcanoes

Cinder Cones• Simplest type of volcano• Made from molten lava and solid material

ejected by more explosive eruptions• Tend to be smaller volcanoes

– rarely more than a few hundred meters

• Often have very steep slopes– more than 40o

• Typically from in groups and on sides of larger volcanoes– Eg. Capulin in Mexico

Paricutin, Mexico

Paricutin seen from the air

Composite Cones

• Also known as stratovolcanoes due to their height– E.g. Mt. Fuji, Mt. St. Helens

• Formed by alternating layers of lava (quiet eruptions) and pyroclastic material (explosive eruptions)– Lava adds to the sides and pyroclastic material

adds around the vent

Mt. Fuji

Mt. St. Helen’s shortly after the eruption of May 18, 1980

Craters and Calderas

• These form at the top of the volcano, a funnel shaped pit– Sometimes the sides of the crater erode and

sometimes block the vent allowing pressure to build behind it

– Sometimes smaller cones build inside a crater from materials erupting from the vent

– If the magma chamber empties the crater falls inwards and forms a caldera

Craters and Calderas

• Crater Kale in Oregon is a caldera that filled with water and now has a new cone rising in it

• Krakatau (1883) is a caldera 6 km in diameter – there is a new volcano rising in the caldera now

• Yellowstone is one of the most famous caldera

Crater Lake

http://1.bp.blogspot.com/_bNlrg2siv38/R6PRv3r0aZI/AAAAAAAAAzs/MiGeyBxlUUA/s1600-h/Krakatoa.JPG

Yellowstone

• This area is still geologically active – it’s over a hot spot formed about 640 000 yrs ago

• Result of several overlapping calderas starting 2 M yrs ago measuring 55 km by 72 km

• The magma below this area generates the heat that drives the geysers and hot springs in the park

• The last eruption was about 1000 times more powerful than Mt. St. Helens

                                                            

                       

Echinus Geyser

Mammoth hot springs

Exploding mud pot

Predicting Eruptions

• Seismic activity• Temperature changes• Bulging on the surface• Changes in gas composition• Knowledge of previous eruptions

Extra-terrestrial volcanism

• Moon – Maria are basaltic flows• Mars – Olympus Mons is the light known

volcano in the solar system (shield volcano)• Venus – radar evidence of thousands of

volcanoes many of which are active– May contribute to the greenhouse effect and the

high temperatures

Radar image of Venus’ surface

Olympus Mons, Mars

Extra-terrestrial volcanism

• Io – a moon of Jupiter – gravitational tugs between two moons and the planet may mean surface is constantly being shifted out of shape which provides energy for volcanic acitivity

• Titan – a moon of Saturn – recent evidence of activity but the flows are of methane and other gases, not lava