Unit 7: Volcanos

how volcanoes form

the layering of pyroclastic materials and lava

volcanism

when magma rises to surface

• divergent

• convergent O-O

• convergent O-C

volcano

landform created by magma becoming lava at the surface

two lava textures

• pahoehoe

• a’a

what causes pahoehoe lava

• hot lava (high temperature)

• low silica content

• low viscosity (runny and fast)

what causes a’a lava

• cooling lava (low temperature)

• high silica content

• high viscosity (thick and slow)

pahoehoe

a’a

viscosity of thick liquid

high viscosity

viscosity of thin liquid

low viscosity

viscosity

measures a liquid’s resistance to flow

A’a

lava with rough, jagged surface composed of broke lava blocks called clinkers

Rhyolitic magma

• felsic

• highest silica content

• highest viscosity and lots of dissolved gas

  • tends to erupt explosively

Andesitic magma

• intermediate to felsic

• high viscosity and lots of dissolved gas

  • tends to erupt explosively

Basaltic magma

• mafic

• low viscosity and less dissolved gas trapped

  • tends to erupt gently

Batholith

large magma chamber that solidifies underground

Caldera

Large depressions that are caused by the collapse of the ground above an emptied magma chamber

Clinkers

• subtype of a’a lava

• broken lava blocks

• that sounds like clinking glass as it breaks

Phaneritic

• Coarse-grained

• rock that has large mineral crystals

• cooled slowly on interior of planet (intrusive)

• opposite of aphanitic

Felsic

magma/rock composition that has high silica and a light color

aphanitic

• Fine-grained

• rock that has very small mineral crystals

• cooled quickly on the exterior of the planet (extrusive)

• rhyolite, andesite, basalt

• opposite of phaneritic

pegmatitic

• Very-coarse grained

• rock that has very large mineral crystals

• cooled slowly on the interior of the planet (intrusive)

• pegmatite

plutons

igneous rock formations that cool and solidify below the surface. they are only seen at the surface when the ground above them is weathered/eroded away

Concordant

• plutons that are parallel to layers of surrounding rock

• horizontal

types of concordant plutons

sill, laccolith,

Discordant

• plutons that cut across layers of surrounding rocks

• vertical

types of discordant plutons

• dike

  • volcanic pipe

  • volcanic neck

• batholith

• stock

Crystallization

the cooling of magma/lava at different speeds to form, different sized of mineral crystals in the rock

Dike

a discordant pluton that is anything that goes straight up

Extrusive (Volcanic)

rock that cooled (quickly) on the surface of earth

Glassy

used to describe rock with mineral crystals so small you cannot see them (b/c the rock cooled fast)

Hornito

small splatter cone that forms on surface of lava flow that is usually steep sided

Igneous rock

rocks that formed from cooled magma/lava that have different mineral grain sized based on how fast/slow they cooled

Intermediate

magma/rock composition that is between mafic and felsic

• has a medium silica quantity

• has a medium color

Intrusive (Plutonic)

rock that cooled (slowly) on the interior of the planet

Kipuka

small piece of land completely surrounded by lava flows

Laccolith

concordant pluton that forms when magma goes between layers and forms a semi circle-like shape between them

Lapilli

the least deadly pyroclastic that is 2mm to 64mm in diameter

Lava

molten rock that is above ground

Lava Bench

Platform of new lava that extends the old shoreline and makes new land. It is unstable because it is underlain by loose sand and wavy eroded rock (collapse).

Lava Block

pyroclastic that is bigger than 64mm comes from parts of volcanic cone getting blasted in air

Lava Bomb

large pyroclastic still molten as it flies and often has a streamlined shape

Lava Lake

large volume of molten lava (basaltic) contained in a volcanic vent, crater, or broad depression

Lava Mold

Cylindrical hollows left behind in lava flow if lava has incinerates a tree that it once flowed around. It can mold other things too.

Lava Tree

created when pahoehoe flows around a tree and cools to form crust around trunk

Lava Tube

natural tube that forms when lava travels beneath surface of lava flow

Mafic

lava/magma that is

• dark in color

• low in silica

• high in Fe + Mg

Magma

molten rock located underground

Magma Chamber

the underground space usually deep below the earth's surface occupied by magma that feeds the volcanic activity of the surface

• creates plutons when it cools

• creates craters when it empties

Pahoehoe

lava that has a smooth but wrinkled surface because it had a low viscosity when it was flowing

Pele’s Hair

Gold in color and very fine naturally-spun volcanic glass that is blown away from lava fountains, cascading lava falls, or turbulent lava flows

Pele’s Tears

Small bits of molten lava in fountains that cool quickly and solidify into glass particles shaped like spheres or tear drops

Pillow Basalt

when there is divergent volcanism underwater and basaltic lava erupts and cools quickly to form round bulges

Pluton

igneous rock formations that cool and solidify below the surface

Pyroclastics (Tephra)

fragments of volcanic rock and lava blasted into the air by a volcanic eruption that are classified by size

• ash (smallest)

• lapilli

• blocks & bombs (biggest)

Rift Eruption (Curtain of Fire)

divergent volcanism that occurs on land

Ring of Fire

the region surrounding the Pacific plate where an increased amount of volcanism takes place due to subduction

Silica

a mineral that is found in magma/lava in different quantities and can effect its viscosity which then effects how a volcano erupts

Sill

pluton that is concordant

Skylight

hole in the top of lava flow that lets in light

Stock

small batholith, smaller solidified magma chamber

difference between stock and batholith

• batholith has an exposed surface area greater than 100 km2

• stock has an exposed surface area less than 100 km

Texture

mineral grain size in a rock

what are the different textures of rocks

• pyroclastic

• glassy

• vesicular

• aphanitic

• phaneritic

• pegmatitic

Vesicular

gassy

non-crystaline

when the mineral grains are so small that they are undetectable to the naked eye

Volcanic Explosivity Index (VEI)

• measures quantity of ash and debris thrown into atmosphere during an eruption

• scale of 1 (low) to 8 (high)

  • eruptions with high number can greatly effect climate for years

Volcanic fissure

an elongate fracture or crack at the surface from which lava erupts

Volcanic Neck

discordant pluton, vertical structure left exposed after a volcano erodes away

Volcanic Pipe

discordant pluton, how magma goes from chamber too surface

Volcanism

activity if magma forming and coming toward the surface

Volcano

land form created when magma reaches the surface through an opening on earth’s crust

Xenolith (Inclusion)

solid rocks picked up on the way out by magma

how is viscosity effected by temperature

the cooler the magma the more viscous it is

how is viscosity effected by silica

the more silica, the more viscous the lava

how viscosity of magma effects the explosivity of a volcano and why

• the pressure from viscous lava builds up and cause an explosive eruption

• low viscous lava can flow and the gas can escape the eruption is mild

three tectonic plate settings where volcanism takes place

• divergent

• convergent O-O

• convergent O-C

convergent O-C/O-O (subduction) plate boundary volcanism

• felsic to intermediate (andesitic)

• partial melting

divergent plate boundary volcanism

• mafic (basaltic)

• decompression melting

type of inactive, former volcano

caldera

three types of (active) volcanos

• stratovolcano/composite volcano

• shield volcano

• cinder cone

stratovolcano/composite volcano

• shape = basic triangle that gets steeper at the top

• what it is made of = alternating layers of lava and pyroclastics

• type of eruption it tends to have = explosive

• type of magma/lava = andesitic (felsic to intermediate), with lots of silica (partial melting) and lots of gas

• where it is found = convergent subduction volcanism

• example on earth = Cotopaxi, Ecuador

shield volcano

• shape = tall and very very wide (width>>height)

• what it is made of = lava flows (pahoehoe) piling on top of each other

• type of eruption it tends to have = effusive

• type of magma/lava = mafic and basaltic magma with low viscosity and low silica

• where it is found = oceanic hotspots

• Mauna Loa

cinder cone

• shape = (small)anthills with steep sides and an abnormally large crater

• what it is made of = the lava and pyroclastics that it spews

• type of eruption it tends to have = Short, small scale eruptions

• type of magma/lava = andesitic to basaltic gas-rich magma made up of pyroclastics and very little lava

• where it is found = anywhere

• example on earth = Paricutín, Mexico

how cone shape relates to eruptive materials

lots of silica, high viscosity, gassy, explosive

• shield volcanos never have steep slopes because the not silica rich + not viscous lava can flow forever and doesn’t build up.

• composite cones have high silica + high viscosity lava that builds up and doesn’t flow which leads to a steeply sloped top

↑ silica =

↑ viscosity = gassy = explosive

↓ silica =

↓ viscosity = not gassy = mild

6 deadly/destructive volcanic hazards

• lava

• volcanic gas

• ash

• pyroclasic flow

• tsunami

• lahar

lava (as a hazard)

• Eldfell volcano in Heimaey, Iceland

• very hot and can seriously burn you

• can destroy buildings/towns

• moves slowly (low death count)

volcanic gas (as a hazard)

• Lake Nyos on Mt. Cameroon, Cameroon

• When a volcano erupts, large quantities of hot, somewhat poisonous gas are ejected out and can build up and travel faster than lava

  1. H2O  (steam burns you)

  2. CO  (suffocation)

  3. CO2  (suffocation)

  4. SO2 (stinky, corrosive to lungs)

ash (as a hazard)

• Tambora, Indonesia

• fine powdery pyroclastic made of volcanic glass and minerals that stays in the air for a while after an eruption

  • can ruin lungs

pyroclastic flow (as a hazard)

• Mt. Pelee, Martinique

• ground hugging avalanche of hot ash, rock fragments, and volcanic gas rushing down the side of a volcano (100-140 mph)

tsunami (as a hazard)

• Krakatoa, Indonesia

• harbor wave that is generated when an eruption displaces lots of water from an explosion or landslide

lahar (as a hazard)

• Mt. St. Helens

• rapidly flowing mixture of rock, debris, and water that originates in slopes of volcano

  • requires a steep slope and can end up very far from where it started

three ways magma is formed

• decompression melting

• introduction of water

• heating of crustal rocks

where decompression melting occurs

low pressure regions in the (upper) mantle that hot mantle from lower down moved into

the factors that lead to decompression melting

when confining pressure drops sufficiently because hot solid mantle moves (up) into regions of lower pressure

where melting by the introduction of water occurs

O-O and O-C convergent boundaries b/c oceanic crust is soaked with seawater

where melting by the heating of crustal rocks occurs

in the mantle when basaltic magma is less dense than surrounding rocks and rises to the surface

• ponds beneath crustal rocks in continental settings

• erupts on ocean floor in oceanic settings to form volcanic islands

Mt. Pelee

• the town was packed due to an election and the entire town was buried except for the one guy that was in an underground jail cell

• 1815

• volcano type = composite

• eruption type = explosive

• hazard = pyroclastic flow

• in Martinique (Caribbean)