GEO EXAM 3

Volcanoes

an opening in the Earth's crust through which molten lava, ash, and gases are ejected.

Magmas: Characteristics & Generation

principles of volcanism

- volcanic rocks: lava and pyroclasts

-Pyroclasts- fragmented volcanic rock ejected upwards explosively (Ex. Ash)

Magma

molten rock inside the earth

lava/pyroclasts

magma that reaches the surface

Characteristics of Magma

1. Composition

-Silica- minerals with Si and O

A) Mafic rocks ~ 50% or less silica ---> low silica (Mg, Fe)

-Ex: Basalts (dark, dense rocks)

B) Felsic rocks ~ 60-70% silica ---> high silica (quartz, feldspar), high amount of gases

Ex: Rhyolites (light, less dense rocks)

C). Intermediate rocks ~60% of silica

Ex: Andesites---> most abundant

Rhyolite

A brown fine-grained, felsic, igneous rock made up mostly of feldspar and quartz.

Basalt

A dark, dense, igneous rock with a fine texture, found in oceanic crust

Andesite

a dark, fine-grained, brown or grayish volcanic rock that is intermediate in composition between rhyolite and basalt.

Classification and flow characteristics of volcanic rocks

Viscosity

- resistance to flow

- controlled by silica content and temp

A). Mafic magmas- silica poor, hot --> low viscosity (gases moves easily)

B). felsic and intermediate magmas- silica-rich, cooler --> higher viscosity (gases don't move as easily)

Volcanic gasses (volatiles)

Water vapor (H20), carbon dioxide (CO2), SO2, N2, CL2, CH4

-Exsolve--> driving force of eruptions

How will low-viscosity magmas erupt? Why?

The composition must be?

Since they're so hot, they tend to flow easily out of earth.

Composition = Mafic

Eruption Dynamics

1) Mafic Magmas

- Silica - poor - Hot

- Low viscosity - Gases escape

- RESULT: Fluid eruption --> lava flows (Ex: Hawaii)

2) Felsic / Intermediate Magmas (convergent boundary)

- Silica - rich - Cooler

- High viscosity - Gases trapped

- RESULT: Explosive eruption --> Pyroclastics

***Deadly whether erupted or not***

Basaltic eruptions

not as violent and lots of lava flow

divergent boundary

hazards= lava flows

Andesitic/Rhyolitic Eruptions

plate boundary- convergent

hazards= stro volcanoes, continental volcanic arc

Generation of Magmas

1). Mid-Ocean Ridges

-pressure decrease

2). Subduction zones

-water added

3). Hot spots

-heat added

most land volcanoes

pacific ring of fire

convergent boundary

\pyroclastic eruption

Viscosity= High

Temp= Low

Composition= felsic

Explosivity= High

Hazard= very hazardous

Volcanic Edifices

-craters

-calderas- from massive volcanoes, low frequency

sheild volcano

- broad dome, gentle slope, ; largest

- composition--> basaltic lava flows

made out of mafic magmas

Ex: Iceland and Hawaii (Mauana Loa)

composite/strato volcano

- Steep, tall, layered sides

- Pyroclastic flows & lava flows

- Comp --> mostly andesite

- Gas in magma chamber

Ex: Mt. St. Helens, Mt. Vesuvius

strato volcano hazards

- Eruption cloud

- Ash fall

- Acid rain

- Lava domes

- Lava flows

- Gases

- Debris avalanche - Bombs

- Lahar (mudflow)

-PROCLASTIC FLOWS & LAHAR FLOWS

cinder cone

-steep , small; short-lived

- composition --> andesitic basaltic pyroclasts

Types of Eruptions

Volcanic explosivity index, VEI (0 to 8).

1. Plinian

-felsic

- powerful vertical ejection of pyroclasts, gas

- most powerful

- VEI = 3 - 8

2. Hawaiian

- mafic

- fluid basaltic lava flows

- least explosive

- VEI = 0 - 1

3. Strombolian

-strato volcano

- small eruptions of pyroclasts

- VEI = 1 - 3

Yellowstone

- Giant collapse caldera: Hot Spot

- A volcanic province

- RI = about 600,000 years

- caldara is 80km

- rhyolites

- hot springs

- mantle plume ~ 2,900 km

Melting of continental crust produces rocks that are...

felsic

Catastrophic Summit Collapse

Formation of a composite -volcano caldera

Anak Krakatau

type of volcano:

type of eruption:

some examples of calderas

Crater Lake, Krakatau, and Santorini

Hawaii

The Big Island

Shield Volcanos: Only 2 active

-Kilauea- RI is high, VEI is low

-Mauna Loa

oceanic crust, mafic, low viscosity, basalt rock

Why? the hotspot is no longer feeding the magma chamber

Deadliest Volcanic Hazards

1. pyroclastic flows- Nuee Ardente

- Scalding avalanche of ash and toxic expanding gases

- fast speed down volcano flank

causes:

- column collapse - Pilinian style of eruption follows explosive eruption moves down the sides of volcano, destroying everything in its path

- dome collapse- hot viscous plug that builds overtime and follows collapse of dome

2. volcanic mudflows- Lahar

- rapid flood of pyroclasts and water

causes:

-rainfall on loose ash

- melting of snow and ice

- breakout of a crater lake

- rivers and lakes

Mt. St. Helens

Mt. St. Helens:

- strato-volcano

- cascadia subduction zone

- plate tectonics is the Juan De Fuca and North American plates

precursor events in 1980:

- swarms of EQ's

- stream/ash explosions

main eruption (VEI 5)

- M=5 EQ

- triggered a gigantic landslide

- lateral steam blast

- mudflows

-Plinian column with pyroclastic flows

-57 deaths- mudflows and pyroclastic flows

COLUMN COLLAPSE

Mt. Pelee, 1902, Caribbean

tectonics:

-Atlantic seafloor under the Caribbean- North American and Caribbean plates

precursors:

- EQ's, ash eruptions

St. Pierre - elections

3 days before: invasion of red ants, snakes, centipedes

- eruption--> pyroclastic flows

-death toll: 30,000

- 2 survivors

DOME COLLAPSE

What's Mt. St. Helens producing and how are they formed?

-Plinian column collapse

-formed by pyroclastic flows

Mass Wasting

downslope movement under the influence of Gravity of rock/soil/sediment

Forces of Mass Wasting

-gravity

-friction (slope-resisting force)

slope failure-->gravity > friction

Factors in Mass Wasting

1. Water content

-excess water increases slope failures

2. slope angle

-steep slopes increase slope failures

3. Geological

-tilt of rock layers, different types of rocks, clays

4. Human

-overloading of slopes, artificial fill, excavation of highways, deforestation

5. Vegetation

-decreases slope failures

Types of Mass Wasting

-Falls, Slides, Flows

1. Rockfall--> free fall of rocks

-very rapid. Common in mountainous regions

2. Slides--> movement of rock layers down a failure surface

A)Slump- Rotational Slide: Curved surface, weak sediments, small

B) Rockslide- Translational slide: Planar surface, more coherent, deadlier

3. Flows--> movement as a viscous fluid. Critical component?

A) Mudflow- most fluid; heavy rainstorms

B) Debris flow/ avalanche- largest, fastest; very deadly; rare

C) Lehar- volcanic mud/debris flow

D) Creep- slowest, most widespread

Triggering events of mass wasting

rainstorms, earthquakes, volcanoes

what are some examples?

What is the role of rain? reduces the friction

retaining wall

a wall built up from low ground to hold the higher ground in place.

slump

A type of mass movement that occurs when a mass of material moves down a curved slope

debris flow

A downslope movement of mud mixed with larger rock fragments.

Southern California

costal cliffs with soft, loose sediment:

--> slope failures

Solana Beach

Seawalls

walls built parallel to the coast to block waves before they hit the shore

Shoreline without seawalls

waves move sand on and off the beach, and erosion from the cliffs helps replenish what's lost

Shoreline with seawall

-the barrier helps dissipate wave energy and fend off surging water

-the seawall disrupts the natural replenishment of sand

gradual loss of beach

-the sea continues to rise, squeezing out the beach

-the seawall limits sand replenishment and locks the shoreline in place

Cliff armoring on the beaches

reduces natural creation of sand supply

creep

the slow downhill movement of weathered rock material; bending trees are caused my creeps

wave energy

as waves come in it undercuts and cliff starts to recede