Volcanoes & Deformation

Volcano Formation

1. Magma/molten rock rise from asthenosphere

2. form magma chambers in crust → find paths to Earth’s surface

3. create vent where lava, ash, & gas can escape

magma rises to surface @different parts of curst under different positions

Mid-Ocean Ridges

• MOR generated oceanic crust covers 70% of Earth

• Basalt erupts from fissures → quenches as pillows → pulled apart via plate motion

Convergent Boundaries

• most volcanoes from @convergent boundaries

• voltiles from sediments initiate melting

Ring Of Fire

• most volcanoes in world

Arc volcanoes

develop on overriding plates

Oceanic Hot spots

• decompressional melting → creates large volumes of lava

• lava buildes upward & outward

• creates shield volcanoes

Crater

• bowl shaped depression atop volcano

• form when erupted lava piles around vent

caldera

• giant volcanic depression

• much larger than crater → magma chamber empties and volcano collapses into evacuated space

Shield Volcanoes

• broad and slightly dome shaped (inverted shield)

• ex: mauna loa

Cinder Cones

• steep, conical hills formed by the accumulation of ejected lava fragments

Stratovolcanoes

• cone shaped

• composite volcanoes

• symmetric & formed by layers of types of lava

Volcanic Deposites

• pyroclastic debris from andesitic & rhyolitic eruptions (felsic and intermediate)

• tephra & tuff

Tephra

• pyroclastic debris of any size

  • wetted debris → forms volcanic debris flow that moves downhill (wet concrete)

Tuff

• lithified ash → accumulates like snow

Lahars

• mudflows

• water rich debris carrying ash & large blocks

• move fast and far

VEI

• volcanic explosivity index

• 1 (smallest) → 8 (largest)

• each # increases *10

Explosive Eruption

• violent disruption of magma into gas & ash

Effusive Eruption

• gentle and passive emission of magma as lava & ash

INCREASE in Gas Solubility (H₂O and other gasses dissolved)

INCREASE in pressure

DECREASE in Gas Solubility (more bubbles)

DECREASE in pressure

magma viscosity

→ opposes motion

INCREASE VISCOSITY → more fluid

DECREASE VISCOSITY → less fluid

Mafic lava

• low viscosity (thin)

• low water & silica content

• effusive

• mainly basaltic

• flow long distances

Felsic lava

• high viscosity (thick)

• high water & silica content

• explosive

• mainly rhyolitic

• piles up

Intermediate lava (andesite)

• medium viscosity

• crust fractures into rubble called blocky lava

Magma steps

1. bubble nucleate when magma saturates with gas

2. bubble grow more and more as gas exsolves into bubble

3. → depends on type of lava

Low viscosity magma

• bubble escape and blow out magma

high viscosity magma

• bubbles trapped & fragment magma explosively when pressure inside is too great

orogenesis

• mountain building

• applies force to rocks causing deformation

deformation

bending, breaking, shortening, stretching, & shearing

strain

• changes in shape via deformation

folds

• bends in layered rock formed via shearing or slow plastic flow

joints

• fractures with no offset (displacement)

faults

• fractures with offset (displacement)

foliation

• planar metamorphic fabric

Undeformed

• unstrained

• rocks display horizontal beds and spherical sand

• NO FOLDS OR FAULTS

deformed

• strained

• rocks display tilted beds and metamorphic alteration

• FOLDS & FAULTS

Displacement

• change in location

• moving one piece of rock relative to another

rotation

• change in spatial orientation

• tilting of rock face

distortion

• change in shape

• bending/folding of rocks

Anticline

• oldest rocks @center

• arch (A shape)

Syncline

• youngest rocks @center

• U shape

Ductile Deformation

• high temp

• high pressure

• SLOW rate

• bend/stretch/fold

• deep crust

Brittle Deformation

• low temp

• low pressure

• FAST rate

• break/fracture

• shallow crust

Ductile → stress

Fold

Brittle → stress

• fold

compression stress

• squeezes rock

• shortens

tension stress

• stretches rock

• lengthens

shear stress

• slides past

pressure stress

• uniform in all directions

• no deformation but can lead to metamorphism

normal fault

• hanging wall moves DOWN

• tension

• ex: basin

reverse fault

• hanging wall moves UP

• compression

• ex: himalayas

thrust fault

• hanging and foot wall

• compression

• ex: appalachian mts

strike-strip fault

• horizontal motion

• shear

• ex: san andreas fault

Ocean-Continent Convergence

• subduction & volcanic arcs

• ex: andes mts

Continent-continent convergence

• thickening of crust & large mountain ranges

• ex: himalayas

Divergent boundaries

• rifting & lithosphere thinning

• ex: East African Rift