geol 100 chapter 5 volcanoes
Some of the largest active landslides on Earth are located on Hawaii
- 100 km (~60 mi.) wide - 10 km (~6 mi.) thick - 20 km (~12 mi.) long extending from volcanic rift zone to below sea-level
Recurrence interval of 350,000 years but capable of very destructive tsunami
- 17 major tsunamis identified - Several hundred meters in height (~1,000 ft.) tsunamis as measured on Hawaiian islands
Only the outer core is liquid, all other layers that comprise the earth are solid
Asthenosphere is mostly solid but is weak and able to flow very slowly because it is near its melting temperature- Prone to rapid melting and for melts to buoyantly rise and collect with changes in temperature, pressure and composition
Volcanic eruptions have shaped our modern atmosphere and provided many benefits along with their dangers
• Volcanic soils are nutrient rich and well drained although they take long to develop • Geothermal power can be harnessed from volcanically active areas • Volcanic rocks provide pumice, aggregate, materials for cement, plaster and cat litter as well as are associated with geologic processes contributing to metallic mineral resources • Recreational attractions • Formation of new land
• Debris flow is distinguished by having over half the particles larger than sand-sized
- 1985 Nevado del Ruiz eruption produced a flow that travelling 100 km/hr (~60 mph) at total of 45 km (~28 mi.) downslope to Armero where it killed 21,000 people as they slept
Gasses in sufficient concentrations can be a direct health hazard (deadly) to people and animal life
- 1986 Lake Nyos in Cameroon released CO2 cloud that travelled 23 km (~14 mi.) and killed 1742 and 3000 cattle
50-60 volcanoes erupt worldwide each year
- 500 million live in proximity to volcanoes worldwide - Migration to proximate urban areas elevating hazard most important factor of hazard
Flows killed more people than any other volcanic hazard
- 79 A.D. Mount Vesuvius eruption killed more than 16,000 in Pompeii and Herculaneum beneath 20 m (~65 ft.) of pyroclastic debris - 1902 Mount Pelée eruption on Martinique killed ~30,000 in St. Pierre - 1991 Mount Unzen eruption in Japan triggered a landslide causing a flow that killed 43 including three volcanologists (previous eruption was 220 years ago with 15,000 killed)Major eruptive gasses include water vapor (H2O) and carbon dioxide (CO2 ), comprising 90% of all emitted gasses
Gases within flows can reach temperatures of 1000°C (~1830°F) incinerating everything within its path
- AKA nuée ardentes (French for glowing clouds)
Volcanic eruptions of significant a size can have short-term affects on climate
- Aerosols of sulfuric acid and fine ash scatter sunlight leading to global cooling (typically 1-5 years)
Volatile content increases with increasing silica content
- Andesitic-rhyolitic magma has 2-5 wt. % dissolved gas • Explosive eruptions - Basaltic magma has <1 wt. % dissolved gas • Effusive eruptions
Very large caldera eruptions are known as super-volcanic
- Associated with rhyolitic magmas with high volatile content (magma mixing) - Associated with continental hot spots - Ejecting between 1000-2000 km3 (~240-480 mi.3 ) of volcanic ash and creating calderas hundreds of km wide
Pyroclastic Flows • Among the most lethal parts of an eruption • Triggered by collapses in lava domes and lateral blasts • Very fast moving and unpredictable
- Attaining speeds in excess of 400 km/hr (~250 mph) - Speed carries pumice, rock fragments and other debris
Lava - any magma that reaches the surface Pahoehoe - lava flow with smooth, ropey surface texture • A'a - blocky fractured surface to the flow
- Basaltic - lowest viscosity (low silica), highest eruptive temperature and fastest flow (15-35 km or ~10-20 mi. per hour) - Andesitic - moderate qualities of each - Rhyolitic - highest viscosity (high silica), lower temp and little flow
Magma is composed of melted silicate rocks and dissolved gases (silica is SiO2)
- Basaltic magmas (most common)• Lowest silica content (45-55%)• Mafic (magnesium and iron) in composition• Melting of asthenosphere- Rhyolitic magmas• Highest silica content (>65%)• Felsic (feldspar and silica) in composition• Mixing of continental crustal materials
People live near volcanic hazards for a number of reasons:
- Born there and may not have any other choice in the case of small volcanic hazards - Land is fertile and work farming - Optimistic perspective that an eruption is unlikely - Restricted in where they can live due to economics
Spreading apart of plates and their subduction interacts with earth materials, pressure and temperature to produce molten rock (magma)
- Called lava when it erupts onto the surface- Differences in magma control the variations in eruptions and volcano sizes and shapes
Continental Caldera: Formed from the collapse of the land surface or volcano following a partial emptying of chamber
- Causes an explosive release of the magma and its associated volatiles - Largest and most destructive eruptions in the world (rare) • None in the past few thousands of years in the U.S. • 10 such eruptions in the past 1 million years in North America
Cinder cones - relatively small accumulations of nut to fist size tephra near a volcanic vent
- Conical in shape due to the accumulation and piling of scoria - Scoria is the name for vesicular mafic volcanic rock typical of cinder cones formed from explosive ejection of basaltic magma with medium volatile content - Common on flanks of larger volcanoes, faults and fissures
Three melting processes
- Decompression melting- Addition of volatiles- Addition of heat
Volatile Content - concentration of dissolved volatiles within the magma
- Decompression of magma chamber forces high concentrations of volatiles to rapidly expand - Decompression of volatiles causes explosive eruption of the magma, forming the characteristic semicircular depression at the top of volcanoes called a crater
Stratovolcanoes - conical shape volcanoes
- Erupt a variety of lava compositions with intermediate to felsic dominating - High viscosity magma builds height of volcano through explosive eruptions (Plinian-type) • Composed of layered pyroclastic debris and effusive lava flows (composite cones) - High water content due to formation in conjunction to subduction zones - Responsible for 80% of volcanic eruptions and most of the destruction and death associated with volcanic eruptions
• Adjustments to Volcanic Hazards
- Evacuation is the primary human adjustment for dealing with volcanic hazards • Dependent on clear communication among scientists, leadership and the population of the imminent hazards and potential destructive force • People must have trust for the assessment of science and the threat that is assessed to exist - Where possible, avoidance of hazardous regions for dwellings and infrastructure • Mount Vesuvius is considered one of the most hazardous volcanoes on Earth • 3 million people live in the area surrounding the volcano • Italian government is offering 30,000 euros to anyone living in the 18 towns surrounding the volcano if they are willing to relocate
Eruptive behavior is classified and can generally be predicted by examining previous eruptions
- Every eruption will be unique and there will be variations even between eruptions of the same volcano
1200°C (~2200°F) melt temperature of rocks
- Exceeded by 50 km (~30 mi.) depth- Increasing pressure with depth keeps it solid
Lava Dome - small dome-shaped volcanic features that form independently or in the vent of a stratovolcano
- Formed from highly viscous felsic magma - Relatively small and low to moderate volatile content • Those forming in vents of stratovolcanoes following an eruption that degasses high-viscosity lava • May build, collapse and rebuild several times • Collapse can suddenly release trapped volatiles in volcanic avalanches of burning material - Common throughout the Ring of Fire
Lahar - general term for debris flows or mudflows from a volcanic eruption formed with water saturated tephra and volcanic ash
- Generally low temperature but may be relatively hot by environmental standards - Flows like water but is the consistency of wet concrete
Shield volcanoes - typically among the largest volcanoes, having broad and lowangled slopes
- Hawaiian Islands, Iceland, La Reunion - Low viscosity magmas allows erupted lavas to flow great distances from central vent (thousands of thin basaltic flows) • Much wider than they are tall - Common at hotspots in ocean crust and at divergent plate boundaries - Rather than an explosive crater, the vent is usually contained within a collapsed caldera
Typical eruptions:
- Hawaiian-type - effusive eruptions that are typically passive - Icelandic-type - generally effusive eruption from an elongated fissure that can be large-enough in scale to generate flood basalt features with devastating global consequences - Volcanian-type - (phreatomagmatic) explosive basaltic eruptions caused by the contact of magma with groundwater, snow or ice generating steam-based pryoclastic debris
• Large caldera-forming eruptions are known as ultra-Plinian
- Huge destructive explosive events that eject materials high into the sky - 100 km3 (~62 mi.3 ) pyroclastic debris - Ash deposits of 100 m (~300 ft.) thick near rim and even still 1 m (~3 ft.) thick 100 km (~60 mi.) away
Viscosity - resistance to flow of a fluid
- Influenced by both composition and temperature • Increased amounts of silica increases viscosity • Increased temperature decreases viscosity - Influences both subsurface mobility and velocity of flow and thickness at the surface • Basaltic lava (less viscous) flows rapidly, travels tens of kilometers and is thin at <3 m (~10 ft.) • Rhyolitic lava (more viscous) flows slowly, restricted to vent and steep sides of dome, and may be >30 m (~100 ft.) thick
Landslides: May be triggered by eruption itself or other events such as changes in moisture, slope loading or even bulging of the surface of the volcano
- Landslides may be triggered far from the volcano eruption - Landslides often release surface pressures and allow sudden explosive eruptions - Major landslides near coastlines or on volcanic islands may result in massive tsunamis
Primary Effects of Eruptions
- Lava flows, ash fall, pyroclastic flows, volcanic blasts (lateral), and volcanic gasses
Attempts to control eruptions is limited to efforts to redirect lava flows through hydraulic chilling and diversion walls
- Limited effectiveness - Best example was Eldfell volcano on Icelandic island of Heimaey in 1973 - Other areas will be threatened instead
Addition of volatiles - refers to adding chemical compounds that evaporate easily and exist in a gaseous state at the surface (H2O, CO2, SO2)
- Lowers melting temp by breaking chemical bonds- Most important when added as a fluid to rocks
• March 1980 seismic activity and small explosions mark the awakening of Mount St. Helens
- May 1 a prominent bulge on northern flank appears and grows 1.5 m (~5 ft.) per day - May 18, 1980 8:32 am a 5.1 M earthquake below the volcano triggered a landslide of 2.3 km3 (~0.6 mi.3 ) - Seconds following the landslide, a massive lateral blast burst the northern slope of the volcano, leveling the landscape nearly 30 km (~19 mi.) and devastating 600 km2 (~230 mi.2 )
Forecasting is based on
- Monitoring seismic activity - Monitoring thermal, magnetic, and hydrologic conditions - Monitoring the land surface to detect tilting or swelling - Monitoring volcanic gas emissions - Studying the geologic history of a volcano through mapping of flows and pyroclastic deposits • Used to form hazard maps and coordinate disaster preparedness
Forecasting is based on
- Monitoring seismic activity - Monitoring thermal, magnetic, and hydrologic conditions - Monitoring the land surface to detect tilting or swelling • Satellite-based radar and GPS networks used
Major eruptive gasses include water vapor (H2O) and carbon dioxide (CO2 ), comprising 90% of all emitted gasses
- Other gasses include carbon monoxide (CO), sulfur dioxide (SO2 ), hydrogen sulfide (H2S)
-Volcanic risks associated with plate tectonic-influenced regions -Hot spot locations -Larger regional threats from very infrequent but significant eruptions
- Ring of fire - Divergent plate boundaries and rift zones
Volcanic Explosivity Index (VEI) - relative scale for comparisons of different eruptions based on height of explosivity and the volume of ejected material
- Scale is logarithmic (except VEI 0-2)
Ash Fall • Covers hundreds or even thousands of square kilometers downwind from eruption • Thickness controlled by distance and particulars of wind direction, strength and consistence
- Smothers crops and vegetation starving environment and livestock - Contamination of surface waters by sediments and toxic compounds (increased acidity) - Structural damage to roofs associated with ash buildup and rainfall - Health affects with irritation of eyes and lungs - Aviation hazards of ash in turbine engines
Volume
- The last of the "V's" this characteristic is an important determination of the overall extent of the volcanic feature and its ability to affect the surrounding environment
Variations in volcanoes (size, shape, composition and behavior during eruption) is controlled by:
- Viscosity, volatiles and volume of magma
100 km (~62 mi.)
- Volatile-free melting temp of 1500°C- Water present melting temp of 800°C
Addition of heat - induces melting of surrounding rocks as hot magma rises within the mantle and then through the crust
- Widespread process wherever rising magma occurs- Extent depends on temperature of magma, temp of surrounding rock and amount of magma
Magma evolution - crustal assimilation along the way or within the chamber will cause magma to become more felsic
-Greater volatile content the higher the evolution - Slower rise through crust also enhances evolution
Volcanic activity is closely linked to plate tectonics
2/3 of activity volcanoes above sea level are located within the Pacific "Ring of Fire"
Tephra - pyroclastic debris ranging in size from sand-sized ash (< 2 mm) to small gravel lapilla (2- 64 mm) to large angular block and smoothed bombs (> 64 mm)
Pyroclastic deposits when hot can fuse themselves together, forming pyroclastic rocks when cool
Volcanic vent - opening at the surface from which lava and other materials are extruded or ejected
May be circular or fissures (linear shaped cracks)
• Forecasting is based on
Monitoring seismic activity - Monitoring thermal, magnetic, and hydrologic conditions • Rising hot magma alters the magnetic properties of existing igneous rocks, alters groundwater flows and may even be thermally imaged off the surface
Secondary Effects of Eruptions
Secondary Effects of Eruptions
