Unit 2: Volcanoes

Volcano

Place where hot material contained at a specific depth is expelled to the surface

Volcano definition 2

Elevated part of the ground composed of material previously expelled

Original Volcano

on an island named Vulcano in the Mediterranean (offshore Italy; Sicily on subduction zones, so very active!)

Boundaries where volcanoes form

Divergent Boundaries, Convergent Boundaries

Boundaries where earthquakes form

Divergent, Convergent, and Transform boundaries

Ring of Fire

Subduction in the Pacific

Magma components

melt = liquid
solid = solidified mineral crystals (stay in magma chambers)
volatiles = gas dissolved in melt

Volatiles and their effect on volcanoes

The more volatiles, the more violent eruption
- Increase v, lower temp, less melt, high silica

Fewer volatiles

Higher temp, more melt, low silica

Main types of volcanic gas

CO2, SO2, H2O (others too, but they usually combine with h2o to produce toxic gas)
- volcanoes steam off mostly water

High silica

Lower temp, more volatile, high viscosity, explosive

Low silica

Higher temp, less volatile, low viscosity, not too explosive

Monogenetic volcanoes

smallest volcano, not much height with steep slopes. usually one use. they usually have straight sides and very large summit craters
- high silica, high viscosity

Stratovolcano

when a monogenetic volcano has more eruptions. it has alternating layers of lava flows, volcanic ash, cinders, blocks, bombs. usually very steep sides.
- high silica, high viscosity

Shield volcano

broad, low-profile features with basalt? diameter from a few kilometers to over 100 kilometers.
- common on hot spots
- low silica, low viscosity

caldera

large volcanic depression formed by the collapse of a summit into a massive magma chamber

fissure volcano

linear opening (like a crack or a gap) in the surface of the earth through which volcanic products are expelled
- all mid-ocean ridges are fissure volcanoesfl

flux melting

add water (explosive!), high volatiles, high explosivity (SUBDUCTION!!!)

decompression melting

change in pressure, high temp, low volatiles

Contact melting

hot spots

Eruptions

expelling material from volcano

Effusive eruptions

liquid comes out and flows (high temp, low silica, low viscosity); produce lava, or liquid rock
- common to midocean ridges

Explosive eruptions

Liquid and solid come out in small pieces and fly (low temp, high silica, high viscosity)

Hawai'i eruptions

all effusive, on a hot spot!

pahoehoe

smooth, flowing streams of lava

A'a

lava flows with spiky, cindery surface

blocky

Lava flows with surfaces broken into angular fragments

Explosive eruptions produce

tephra

tephra

general term that describes all rocks that fly out of the volcano

bombs

fly liquid, fall solid

blocks

fly and fall solid

Spatter

flying fragments of lava that fall while still molten. they get enriched with air bubbles while in the air (pumice!)

Largest volcanic feature

mid-ocean ridge

Ridge push (mid-ocean ridge mechanics)

new material coming out at MOR on pressure to push plates

Slab pull

subduction of all material (stronger)`

Mid-ocean ridge

spreading center, divergent boundary between 2 tectonic plates

Depth "formula"

√ age (smaller age, shallower depth) (hotter and less dense, so higher point)

MOR Anatomy

volcanic vents along axis; ridges make oceanic crust, which consists of 3 layers (pillow basalts, sheeted dikes, gabbro cumulates); lower 2 layers are dense, heavy, and richer in Fe and Mg than basalt

volcanic vents

areas of gas and material erupting from the MOR; referred to as "black smokers"; provide evidence for where life originated

pillow basalts

products of effusive eruptions underwater

sheeted dikes

intrusive features that cut across previously present material, basaltic in nature

gabborro cumulates

dark

rift to ridge

before a spreading ridge can form and a new ocean can open, you need to break a continent (called rifting)

Hot Spot Volcanism: hot spot

region of rising mantle that is less dense than surrounding rock (it plumes)
- stable vertical column that rises from the deep mantle for an extended period of time
- stay stationary as plates plume
- as long as plume is active, a volcano will keep growing

hotspot tracks

lines of volcanoes generated as the crust moves over a hotspot over many millions of years
- plume stays stationary
- only leading few volcanoes are active

Subduction Zone Volcanism: convergent boundaries

subducting plate (always oceanic, microcracks full of water, aka flux melting)
overriding plate (oceanic or continental)
trench (convergence point, deepest part of ocean)

volcanic arc/front

not a plate boundary; far from plate and inland, bc of trapped water.

oceanic overriding plate

overriding lithosphere is oceanic; get volcanic islands in the ocean (Marina Trench)

continental overriding plate

ocean over continental; overriding lithosphere is continental; get a volcanic chain along edge of the continent (Andes)

In the presence of ____ at around ____ km, rocks will start melting

water, 100 km
- melted rock, less buoyant, easier to melt
-angle of plate determines volcano location (ex: steeper, closer inland)

_____ doesn't melt, overriding ____ melts!

plate, mantle

once mantle has melted

partially molten rock ascends until it reaches the lithosphere, where is accumulates and becomes explosive

Strombolian Eruptions

short lived, loud, explosive
-pasty lava ejected high into the air; never develop sustained column, eject scoria bombs
-ex: Mt. Etna

Vulcanian eruptions

-discrete, canon-like, short lived
-no molten lava, high velocity ejections
-quiet and substained subsequent eruptions
-lava fragments don't take on aerodynamic shapes

Plinian eruptions

powerful convecting plumes of ash ascending to 45 km
- volatile rich

Phreatic eruptions

In cold areas, just add water to magma
-fine grained ash

remote sensing

visual observation, satellite; find changes in behavior
-ex) looking
-difficult because volcano can be obscured

ground deformation

changes in volcano shape preceding eruptions
-as magma body grows, so does volcano

volcano breathing

changes in the height and shape of volcano reflect movements of magma inside

gas

volcanic gases present a hazard independent of eruptions and need to be monitored continuously (bc of gases like sulfur and chlorine)
-direct or remote sampling

seismicity

dominant form of sensing
-Seismic activity (e.g. earthquakes, tremors) beneath volcanic areas is primarily caused by the dynamic interaction of molten rock and hydrothermal fluids with the solid host rock, by fracturing and fragmentation of the magma itself, and by tectonic processes interacting with the volcano.

volcanoes and climate

release greenhouse gases (CO2), but when ash covers the sun, it causes earth cooling

Weather

Day to day changes (local and short-term)

climate

long-term, changes over time