Natural Hazards Exam 2 Flashcards

Hot spots

Localized areas of volcanism and shallow earthquakes

Source of hot spots

mantle plumes

Mantle plume

hot, buoyant bodies of mantle rock that rise and partially melt in the upper mantle

The movement of mantle plumes and tectonic plates results in

linear chains of volcanic islands and seamounts (examples of this occur in Hawaii called the Hawaii-Emperor chain)

magma

molten material beneath Earth’s surface

lava

molten material at Earth’s surface

Igneous rock

a rock that forms when magma/lava cools and hardens

what are rocks made of?

minerals

Characteristics of minerals

• naturally occurring

• inorganic (lack C-H bonds)

• solid

• defined by a specific chemical composition

• made up of atoms arranged in an orderly way (=crystal structure)

How is magma generated?

Partial melting of mantle and/or crust

Magma originates from

the asthenosphere

*this layer is prone to melting b/c its pressure/temperatures conditions cause it to remain close to its melting point

Minerals with the _____ melt first

lowest melting points

Raising the _____ and/or decreasing the _____ leads to melting

temperature; pressure

Three primary processes of partial melting

1. decompression melting: reducing the pressure exerted on mantle material

2. addition of volatiles: adding water (or CO2, SO2) to warm but solid mantle

3. addition of heat: transferring heat from rising magma to the surrounding rock

Geotherm

A line that tracks temperature as pressure/depth changes along the geothermal gradient

*Oceanic geotherm is the geothermal gradient below an ocean basin

What happens when we add volatiles?

• Causes the asthenosphere beneath the overriding plate to melt

• Volatiles become incorporated into the new magma and influence the style (explosivity) of eruptions.

• It shifts the solidus line! (reduces the melting point of mantle materials)

Moho

crust-mantle boundary

What does adding heat do?

• The heat from the magma pool is transferred ot the crust above (via conduction), causing partial melting of the crust (acts as a burner on a stove)

• It shifts the temperature (geotherm) into the zone of partial melting.

Earth’s elemental composition (largest to smallest)

• Iron

• Oxygen

• Silicon

• Magnesium

• Other

Primary component of all magma/lava

silica (SiO2); most common form is quartz

bonding b/t silica tetrahedra in magma causes resistance to flow (more bonds = higher viscosity)

Types of Magma/Lava

(three major types)

1. Felsic, aka rhyolithic (high silica content)

2. Intermediate, aka andesitic (intermediate silica content)

3. Mafic, aka basaltic (low silica content)

Magmas/lavas are ____ than solid igneous rocks

less dense (but not by much)

Viscosity

resistance to flow

Adding H2O to magma _____ viscosity

reduces

Lowest viscosity (mafic) leads to.

laterally-extensive lava flows that are thin

moderate viscosity (intermediate) leads to

lava flows of limited extent; thicker deposits

highest viscosity (felsic) leads to

very thick accumulations of lava close to the vent

Bowen Reaction (Crystallization) Series

• different minerals crystallize (and are stable) at different temperatures

• crystallization happens within magma chambers as they cool, and cooling leads to changes in the composition of the magma - becomes more felsic as it cools

*look at slides

Mafic lava cools ____, felsic lava cools _____

first; last

fractional crystallization

the gradual solidification and separation of mineral crystals from a body of liquid magma as it cools

• crystals settle to the bottom and/or stick to the walls of a magma chamber

• remaining magma rises

The longer fractional crystallization proceeds, the more _________ the remaining magma becomes

silica-rich (felsic)

Volatiles in magma contribute to

explosivity of eruptions

if a magma rises past the saturation point, what happens to the H20?

H20 within it will begin to separate from the magma (exsolve)

Magma fragmentation occurs when

bubbles become highly concentrated in a body of magma;

the magma transforms from a liquid filled with gas bubbles into a gas filled with suspended liquid droplets

Magma fragmentation often precedes _______

explosive volcanic eruptions

______ of the escaping gas propels magma droplets high into the atmosphere

force

______ within the gas plume generates buoyancy, enabling the plume to rise further

heat

Types of Explosive Eruptions (Felsic)

• plinian

• vulcanian

• peléan

• strombolian

Types of Effusive Eruptions (Mafic, mostly); dominated by flowing lava

• Fissure

• Hawaiian

Types of Lava Flows

• pāhoehoe (mafic)

• ‘A’ā (mafic)

• intermediate to felsic “flows”

Scale of volcanoes

• shield volcanoes - mafic

• stratovolcanoes - intermediate or felsic

• cinder cones - variable

Shield volcanoes

• mafic

• form from layer upon layer of laterally-extensive, low-viscosity lava flows

• fissure eruptions and fountains can occur on the flanks of shield volcanoes

Composite or Stratovolcano

• intermediate to felsic (primarily)

• “strata” = layers (alternating layers of lava and ash)

• >60% of Earth’s volcanoes (above sea level) are stratovolcanoes

Cinder Cones

• composite varies; high volatile content

• steep, conical; made up of loose fragments of material that erupted explosively

• often found on flanks of other volcanoes

• gas-filled lava explodes into the air and breaks up into smaller pieces (cinders, ash)

Earth’s internal compositional structure

• continental crust (felsic)

• oceanic crust (mafic)

• mantle (ultramafic)

Felsic-dominated eruptions are associated with…

subduction zones, highly evolved magmas, and extensive partial melting of continental crust (felsic)

Examples of felsic-dominated eruptions

• Aleutian Islands (Subduction Zone)

• Yellowstone (hotspot)

• Andes (subduction zone)

Water is a type of volatile which…

makes an eruption more explosive (think bubbles)

The longer fractional crystallization goes on…

the more silica-rich (felsic) the remaining magma becomes.

Mafic magma can evolve into felsic magma via… (part 1)

cooling and fractional crystallization

mafic magma can evolve into felsic magma via… (part 2)

mafic magma can transfer heat to the felsic continental crust, causing it to partially melt to form felsic magma.

Rising magma can also become more felsic by…

assimilating minerals from continental crust

Mafic-dominated Eruptions are associated with…

MORs, continental rifts, oceanic hotspots, oceanic crust (mafic)

Examples of Mafic-Dominated Eruptions

• Mid-Atlantic Ridge (oceanic spreading center)

• East African Rift Valley (continental rift)

• Hawaii (hotspot)

Mafic eruptions are

fast and efficient

Large Calderas

• stratovolcano

• form when the roof of a magma chamber collapses

• diameter resembles that of underlying magma chamber

How is the Dense Rock Equivalent (DRE) calculated?

By comparing bulk density with the density of the tephra’s rock type

Volcanic Explosivity Index (VEI)

Based on volume of eruption products, height of eruption cloud, qualitative observations

• log scale

• *look at 3/5 slide 23

Products of eruptions that follow magma fragmentation

tephra/pyroclasts

• ash (<2mm)

• Lapilli (2mm-6.4cm)

• Bombs, blocks (>6.4cm)

Primary volcanic hazards

• pyroclastic flow (most deadly)

• air-fall tephra: mostly ash

• volcanic gases

Pyroclastic flows

• dense cloud of hot gases and smaller tephra

• as hot as 1000°C and travel <= 700km/h along the ground, down the slopes of a volcano

Mt. Pelée (1902)

• 3rd deadliest volcanic disaster in recorded history

• ~30,000 died in town of St. Pierre due to suffocation and/or burns

air-fall tephra

fragmented material that is ejected from the vent and falls to the ground; can cover 100’s to 1000’s of square kilometers

Ash Hazards for Aviation

• indistinguishable from normal clouds

• reduces visibility

• coats the engine due to the difference in temperature b/t the melting point of ash and the internal temperature of the plane’s engine (ash’s melting point is lower)

Hazards from Volcanic Gases

• strong acid rain

• vog = volcanic fog (gases & H20 vapor) —> respiratory and eye problems

• soil contaminations (from acid that adheres to falling ash particles) —> food and water supplies poisoned

Secondary Effects: Mass Wasting

• Lahars

  • Debris flows and mudflows made up of water-saturated volcanic ash and tephra (can occur long after an eruption)

VDAP

Volcano Disaster Assistance Program

Volcano Forecasting

• Goal: estimate probability of a volcanic eruption with a particular eruption style occurring at a particular time and place (similar to weather forecasting)

• accuracy improves with better knowledge of a particular volcanic system and its history

• involves monitoring various phenomena related to active volcanism (precursors)

• success rate is high

Precursors for Volcanic Forecasting

• seismic activity

  • change/increase in EQ activity; audible rumblings

• Ground deformation

  • change(s) in the shape of the ground near the volcano; surface of volcanoe swells or is uplifted

• hydrothermal effects

  • greater output and/or higher temperature of hot springs and gas vents; melting of snow/ice on the volcano

• chemical changes

  • increase in SO2 and/or H2S content of gas vents, springs; withering of vegetation on the slopes of the volcano

Precursor activity ______________ leading up to a volcanic eruption

increases dramatically

Mass Wasting

the downslope movement of Earth materials under the influence of gravity

_____ leads to mass wasting

weathering

weathering

The breakdown of rock into smaller pieces and/or chemical components due to exposure to the atmosphere, water, and/or organisms

Physical weathering

breakdown of rock into smaller fragments by physical processes (involves NO CHANGE in the rock’s chemical composition)

Frost weathering

• results when water alternately freezes and thaws: cracks expand each time water freezes

• water expands by 9% when it freezes

• Talus (loose rock fragments)

Chemical weathering

breakdown of rocks via chemical destruction of the minerals’ internal structure

new minerals (different chemical composition) are produced in the process; these are often finer-grained and weaker as slope materials

Regolith

• a product of weathering

• blanket of loose debris (soil + colluvium) that forms a cover over the landscape (cm’s to 100s of m’s thick)

• primary material involved in mass wasting

Top layer of regolith (soil) is composed of:

• small rock fragments

• new minerals formed via weathering

• organic matter (e.g., decomposed plant material)

the remainder of regolith (colluvium) is composed of

• rock fragments of varying size

• new minerals formed via weathering

• does not include the active soil layer

The thickness of regolith is a function of

• bedrock type (which is susceptible to weathering)

  • more weathering = thicker regolith

  • Quartzite (100% silica) is much more resistant to weathering than shale (mostly clay materials)

• time exposed to surface processes

  • Older lava flow has been exposed to weathering for longer

• topography (shape of the landscape)

• climate

  • Weathering rates are much higher in warm and humid climates

  • faster weathering = thicker regolith

Why does regolith matter?

loose debris is much more susceptible to mass wasting than solid bedrock

mass wasting becomes more likely when the ________ increases

the angle of a slope (its steepness)

*road construction can also alter the angle of a slope, usually making it steeper

Relief (topographic)

difference in elevation b/t the highest and lowest points in a landscape

• rate of change influences stability

Tectonic mechanisms for increasing topographic relief

• volcanic arc at a subduction zone

• formation of mountains (via compression)

• up/downward bending of crust

• volcano growth

• formation of a rift basin (via extension)

plants add ___ to a slope

mass

*plants can negatively impact slope stability

_____ of plants can hold regolith in place

root systems

chemical changes occur at ____ temps

high

fire ____ vegetation and ____ soil properties

removes; alters

_____ can also cause slope to destabilize

seismicity

*force of EQs destabilizes regolith and causes mass wasting to occur

*shaking disrupts frictional contacts b/t grains of regolith that normally help hold slope material in place

How does liquefaction cause slope destabilization?

if water is present in the pore spaces of fine-grained regolith, the stabilizing effects of EQ shaking are enhanced due to liquefaction

Factors that influence driving (destabilizing)/resisting (stabilizing) forces:

• type of strength of slope material

  • loose sediment and finer-grained materials are weaker

• density/weight of slope material (includes water)

• permeability of slope material (can H2O flow through?)

• pore fluid pressure within slope material

• steepness and height of slope (topography and relief)

• climate, especially the frequency of rainfall, freeze/thaw

• vegetation presence/absence and type

Pore fluid pressure

• increases as more water infiltrates sediment

• higher pore fluid pressures tend to destabilize slope materials by removing the frictional contacts b/t grains

• can be enhanced by liquefaction

Slope stability equation

slope stability = resisting forces/driving forces = shear strength/shear stress

• slope stability > 1 = stable slope

• slope stability < 1 = unstable slope

gravity driving force

pull of gravity on the mass of the material on the slope; force is parallel to the slope surface

internal cohesion

sticking together or interlocking of granular particles (independent of overlying weight); can be caused by electrostatic forces (in clays) or cementation of particles by precipitated minerals

internal friction

resistance of particles to sliding across each other (depends on overlying weight)

normal force

pull of gravity on the mass of material on the slope; force is perpendicular to the slope surface

shear strength (resisting forces) =

• internal cohesion

• internal friction

  • normal force

shear stress (driving force) =

gravity driving force

Angle of repose

the maximum angle at which a slope made of loose sediment can remain stable

• governed by internal friction

• varies based on grain size, shape, and water content