geol105 - exam 3

volcano

a hill or mountain that forms around a vent where lava, pyroclastic materials, and gases erupt, so it is a landform

volcanism

refers to all processes related to the rise and discharge of magma and gases at the surface or into the atmosphere

• accounts for the origin of all volcanic (extrusive igneous) rocks, such as basalt, tuff, and obsidian; as well as features like volcanoes, fissures, and basalt plateaus

volcanic gases

water vapor, carbon dioxide, sulfur dioxide, and others

• Most volcanic gases quickly dissipate into the atmosphere and pose little danger to humans, but on several occasions they have had far-reaching climatic effects or have cause fatalities

physical characteristics of lava

• amount of volatiles

• viscosity

• temperature

amount of volatiles

more dissolved gases in the magma the more volatile

• Ease or difficulty of gases escaping to atmosphere

viscosity

a fluid’s resistance to flow

• Higher silica contents produce higher viscosity

• Lower silica content produces lower viscosity

temperature

temp of the lava

• Cooler lavas have higher viscosity.

• Hotter lavas have lower viscosity

lava tube

forms within a flow when its margins and upper surface solidify, thereby forming a conduit through which lawva can move rapidly and for great distances

Pahoehoe (pronounced pay-hoy-hoy)

has a smooth ropy surface much like taffy

Aa (pronounced ah-ah)

flow consists of rough, jagged, angular blocks and fragments

pillow lava

bulbous masses of basalt that resemble pillows

columnar jointing

a pattern of columns bounded by fractures

• Mafic lava flows (sometimes intermediate flows) show

  • to form, the lava flow must cease moving, and cool and contract, setting up for the fractures known as joint to open

  • commonly polygonal (six-sided) that extend downward, outlining the columns with their long axes perpendicular to the cooling surface

volcanic ash

pyroclastic materials

• a designation for pyroclastic particles that measure less than 2.0mm

ashfall

fall from the sky due to gravity

ashflow

when ash flows along/close to the land surface

volcanoes also erupt

• lapilli

• blocks/bombs

lapilli

pyroclastic material measuring 2-64mm

blocks

are angular pieces of rock ripped from a volcanic conduit or pieces of a solidified crust of a lava flow

bombs

twisted, streamlined shape, indicating that they were erupted as globs of magma that cooled and solidified during their flight through the air.

crater

circular depression at their summit

caldera

huge structure that forms following voluminous eruptions during which part of the magma chamber drains and the mountains summit collapses into the vacated space below

shield volcanoes

• look much like the outer surface of a shield laying on the ground with the convex side up

  • Composed almost entirely of low-viscosity mafic lava flows so the flow spreads out and thin

  • rather quiet type of eruptions in comparison to other volcanoes

  • Most common in ocean basins, but some present on continents

  • Common in CA, OR, HI islands=5 huge shield volcanoes and East Africa

cinder cones volcanoes

• Small, steep-sided, made up of particles resembling cinders from when pyroclastic materials accumulate around a vent from which they erupted.

  • Rarely exceeding 400 m high, with slope angles up to 33 degrees

  • Most are perfectly symmetrical unless prevailing winds cause the pyroclastic material to build up higher on the downwind side of the vent

  • Common in Rocky Mountain states, particularly New Mexico and Arizona, as well as CA, OR, WA, H

composite volcanoes (stratovolcanoes)

• have pyroclastic, as well as intermediate lava flow layers. As the lava flows cool, they typically form andesite

  • steep-sided, conical mountains and the angle of their slope decreases towards the base

  • Most common at convergent plate boundaries

  • Ex: Mount Shasta in CA

lava domes

• less common than all of the volcanoes previously discussed.

  • 
    

  • steep-sided, bulbous mountains that form when viscous felsic magma, and occasionally intermediate magma, is forced toward the surface

  • eruptions are some of the most violent and destructive

supervolcano eruptions

explosive eruption of hundreds of cubic kilometers of pyroclastic materials and the origin of a huge caldera.

• Geologists think that these huge eruptions were caused by a rising mantle plume

  • mantle plume- a cylindrical mass of magma probably rhyolitic in composition

basalt plateau

Lava flows build up a plateau shaped landform made of basalt. Result of a very large volume eruption of basalt lava.

fissure eruption

Rather than erupting from central vents, lava flows from long cracks in Earth’s crust as a linear volcanic vent.

pyroclastic sheet deposits

xtensive, relatively flat-lying accumulations of pyroclastic material.. These result from numerous eruption of very large volumes of extremely fluid, basaltic lavas, which accumulate as sheet-like flows covering large areas.

circum-Pacific belt (ring of fire)

has more than 60% of all active volcanoes.

mediterranean belt

about 20% of all active volcanism takes place in this belt

plate tectonics, volcanoes, and plutons

reminders

1. Mafic magma is generated beneath spreading ridges at a divergent boundary, creating oceanic crust

2. Intermediate or felsic magma forms where an oceanic plate is subducted beneath another oceanic/continental plate at a convergent boundary

igneous activity at divergent boundary

Mafic lava is very fluid, allowing gases to escape easily, and at great depth in the oceans, the water pressure is so great that explosive volcanism is prevented

• no pyroclastic material

• Effusive eruptions of basaltic magmas and pillow lavas

• Formation of most of the sea floor

igneous activity at convergent boundary

Most large well–known volcanoes, located in the circum-Pacific Belt

• Explosive composite volcanoes, pyroclastic debris

• The convergence leading to subduction causes a chain of continental volcanoes called a volcanic arc

• Viscous andesitic or felsic lavas

intraplate volcanism

interior of a plate (not convergent or divergent)

• Mantle Plumes (Hot Spots)

• Hawaiian islands begin as a submarine volcano until they protrude out of the water, creating a volcanic island.

• Basaltic magma/lava.

volcanic hazards

risk of living near volcano

• Lava flows with few exceptions pose little threat to humans but can destroy property

• Nuee ardentes and volcanic gases are threats during an eruption

• Lahars and landslides may take place even with no eruption

Volcanic Explosivity Index (VEI)

most widely used indication of the size of a volcanic eruption

• ranges from 0 (gentle) to 8 (cataclysmic) and is based on several aspects of an eruption, such as

  • volume of material explosively ejected

  • height of the eruption plume

  • does NOT consider the volume of lava, fatalities, and property damage

monitoring eruptions

Geologists monitor things like:

• Gas emissions, Groundwater level change, Temperature, Hot spring activity, Local magnetic and electrical fields, Volcano slope changes, from inflating as magma rises, Paleo Volcanoes (past eruptions)

  • The most important: the detection of a volcanic tremor- continuous ground motion that lasts for minutes to hours.

    • This is different than an earthquake that usually are sudden, sharp jolts.

    • Volcanic tremors,also known as harmonic tremors, indicate that magma is moving beneath the surface

forecasting eruptions

Monitoring volcanoes can help predict future eruptions, geologist have successfully gave timely warnings to many impending eruptions. While some eruptions can be easier to predict than others; geologist have even issued low-level warnings where nothing happened but we can prepare ourselves as best as we can for when volcanoes do erupt.

volcano status

• active

• dormant

• extinct

active

if currently or recently eruptive (Approximately 500 in the world today).

dormant

if it hasn’t erupted in many thousands of years but is expected to erupt in the future

extinct

haven’t erupted in many years and show no signs of any future eruptions

importance of volcanoes

• Atmosphere was originally created from gases released from volcanic eruption.

• Hydrosphere was produced by condensation of volcanic water vapor.

• Lava flows and ash weather to produce fertile soils.

• Violent eruptions can destroy nearly all life in their paths so it is important to monitor them.

• Large amounts of ash and volcanic gases in atmosphere can trigger rapid climate changes and contribute to mass extinctions (which they already have).

earthquake

a trembling or shaking of the ground caused by the sudden release of energy, usually as a result of faulting, which involves the displacement of rocks along fractures.

aftershocks

After an earthquake, continuing adjustments along a fault may generate a series of earthquakes

seismic waves

form energy is released

elastic rebound theory

earthquakes are a sudden release of stress progressively stored in rocks that bend until they finally break and move along a fault

• analogy: the bending of a long, straight stick over your knee. As the stick bends, it deforms and eventually reaches the point at which it breaks. When this happens, the two pieces of the original stick snap back to their original straight position

seismology

the study of earthquake, emerged as a true science during the 1880s with the development of seismographs

seismographs

instruments that detect, record, and measure vibrations produced by an earthquake

seismograms

permanent paper (or digital) records of the earthquake vibrations

seismic waves

• focus

• epicenter

focus (hypocenter)

the point of initial breakage and movement along a fault, where seismic waves originate

epicenter

point on Earth’s surface directly above the focus

three categories of earthquakes

1. shallow-focus: focal depth less than 70 km

2. intermediate-focus: between 70-300 km

3. deep-focus: more than 300 km deep

• these are not evenly distributed between categories

seismic waves (boundary relationship)

a definite relationship exists between earthquake foci and plate boundaries

• Divergent and transform boundaries are majority shallow-focus

• Convergent plate boundary have many shallow-focus, intermediate-focus, and deep-focus earthquakes

body waves

travel outward from the focus in all directions through Earth’s interior

• two types

  • p waves

  • s waves

p waves

• “primary”, interior (focus)

• fastest seismic waves

• can travel through solids, liquids, gases

• compressional (push-pull) waves

  • cmopress and expand when traveling

• parallel force

s wave

• “secondary”, exterior

• somewhat slower than p waves

• only travel through solids

• shear waves

• perpendicular force

surface waves

travel outward from the epicenter along the surface of the Earth, these waves are the slowest type of seismic waves produced by earthquakes

• two types

  • love waves

  • rayleigh waves

love waves

side to side motion of the ground surface, in a horizontal plane perpendicular to wave propagation

rayleigh waves

slower of the two, the ground moves in an elliptical path opposite the direction of wave propagation; extremely destructive to buildings

determinng the lcation of an earthquake

1. p-s wave time interval

• ex) p wave arrival: 5 sec

  • s wave arrival: 12 sec

    • 7 sec gap between the two

2. plot on p-s wave time travel curve graph

• ex) see where the gap falls between the two lines, plot interval, follow interval down to bottom, finds seismograph distance away from origination (do 3 times for 3 stations)

• obtain distance

3. plot on map using Triangulation

• intersection of three circles is location of earthquake’s epicenter

measuring earthquake intensity

intensity: a subjective or qualitative measure of the kind of damage done by an earthquake as well as people’s reaction too

• Modified Mercalli Scale: Factors affecting intensity: Distance from epicenter, focal depth, population density, geology of the area, type of building construction employed, and duration of shaking.

Modified Mercalli Scale

measuring earthquake magnitude

magnitude: a measure of the amount of energy released by an earthquake by measuring the amplitude of seismic waves on a seismogram

• Ricter Scale

  • Logarithmic scale-A logarithmic scale is a way to show and study a wide range of values in a simple, compact way. Log scales grow very quickly with large numbers.

  • Each whole number increase in magnitude represents a 10-fold increase in wave amplitude.

Ricter Scale

destructive effects of earthquakes