What lies beneath? Predicting the Earth with physics
Dr Christina Manning, Royal Holloway, University of London
Dr Manning is a Research Officer in Isotope Geochemistry in the Earth Science department at Royal Holloway, University of London. She studied Earth Sciences in the University of East Anglia, including an undergraduate dissertation on magma mixing as a trigger for the Minoan eruption of Santorini. Continuing on a volcanic theme she completed her PhD in 2009 investigating the compositional variation in the Icelandic mantle plume using the geochemistry of erupted volcanic rocks. Since then her research projects have ranged from, the use of mantle geochemistry to assess the role of lithospheric topography in controlling the distribution of plume material, the reconstruction of magma plumbing systems and then interactions between them to assess potential eruption triggers and improvement of the spatial resolution of laser ablation analyses to enable the measurement of diffusion profiles between zones and mineral grains.
We live on a special planet, one that has provided the components to enable the evolution and long-term survival of a diverse range of life forms. However, the consequence of this is that we now have to live alongside the dynamic processes that continue to build and shape our Earth.
From thermodynamics to fluid dynamics, gravity to geodesy, physics provides use with many tools with which to model, monitor and quantify the processes occurring where the eye cannot see. One of the most important applications of these techniques is in monitoring of active volcanoes. Commonly lured by the fertile volcanic soil, around 8% of the Earth’s population live in the shadow of an active volcano centre. The ability to provide early warnings of imminent volcanic activity and hazards has substantially reduced the death toll associated with volcanoes. Due to the danger of accessing volcanic areas many of these techniques utilize remote sensing and monitoring which can be transmitted and analysed remote from the area of interest.
The talk discussed the ways in which physics has helped further our understanding of volcanic activity; from how and where the melt that sources them is formed and stored, to the identification of eruption triggers and the prediction and mitigation of hazards.
What is Geology?
How most people imagine geologists, but it isn’t just about rocks.
In reality geology is the study of the Earth, its composition, its history, and it is constantly changing character.
Geologists study the origin and evolution of our planet; the chemical and physical properties of minerals, rocks, and fluids; the structure of our mobile crust (identifying plate boundaries and modelling the behaviour of the plates) – its newly forming ocean floors and its ancient drifting continents; the history of life; and the human adaptation to earthquakes, volcanoes, landslides and floods.
The subject matter of geology ranges from dinosaurs to the prediction of earthquakes. It is involved with the study of space, the atmosphere, the oceans and rocks, It is a multi-disciplined subject with connections to sciences such as physics.
Wikipedia describes geology and physics in the following terms:
Geology is an earth science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which they change. Geology can also refer generally to the study of the solid features of any celestial body (such as the geology of the Moon or Mars).
Geology gives insight into the history of the Earth by providing the primary evidence for plate tectonics, the evolutionary history of life, and past climates. Geology is important for mineral and hydrocarbon exploration and exploitation, evaluating water resources, understanding of natural hazards, the remediation of environmental problems, and for providing insights into past climate change. Geology also plays a role in geotechnical engineering and is a major academic discipline.
Physics is the natural science that involves the study of matter and its motion through space and time, along with related concepts such as energy and force.
Where does physics come into geology?
Without gravity we wouldn’t have our Sun, our planet or the rest of the Solar System.
Our Solar System came into being about 4.6 billion years ago when gravity caused a small part of a molecular cloud to collapse. Most of the mass collected in the centre resulting in the formation of the Sun. The rest flattened into a protoplanetary disc out of which the rest of the Solar System was formed.
All the stars in the universe along with their own planets would be formed in this way too.
Gravity is also responsible for crystals sinking down through the liquid they are formed from by virtue of their greater density. A type of igneous differentiation as the more dense crystals will settle below the less dense crystals.
Ideas about the evolution of igneous rocks go back to the early part of this century, and N.L. Bowen.
Norman Levi Bowen FRS was born in Kingston, Ontario, Canada June 21, 1887 and died on September 11, 1956. Bowen “revolutionized experimental petrology and our understanding of mineral crystallization”
Bowen’s core idea was that a silica rich parent rock gives rise to all other igneous rocks. He proposed that fractionating occurs during the crystallization process beginning with a magma slowly cooling. Crystallization begins with minerals highest in the reaction series. Because these minerals have the highest specific gravity they settle to the bottom of the magma chamber by gravity settling. Also, because these first formed minerals are high in Ca, Mg, and Fe, they take these elements to the bottom with them in greater quantities than their average composition in the original melt. The remaining melt is thus depleted in Ca, Mg, and Fe, and has a composition lower in the reaction series. Thus, the original magma of one composition is divided into two fractions. The first fraction is a cumulate (early formed crystals which ” accumulate” at the bottom of the magma chamber) collected at the bottom of the magma chamber composed of high density Ca, Mg, and Fe rich minerals from the top of Bowen’s Reaction Series. The second fraction is the lower density, more Na, K, and Si rich remaining melt with a composition lower down in BRS.
It’s long been thought that the arches, columns and bridges of natural sandstone formations were created by years of erosion from wind and rain, but new research suggests that these fantastic shapes are actually the product of gravity instead.
Scientists from the Charles University of Prague explain that it’s not that erosion doesn’t play its part, but that it is the rock’s “internal stress fields” – the different areas of pressure caused by weight and gravity – that dictate the final form.
The gravitational pull from the Sun and the Moon gives rise to two high tides and two low tides every lunar day (24 hours and 50 minutes).
Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases. Measurements of gravitational acceleration and gravitational potential at the Earth’s surface and above it can be used to look for mineral deposits. The surface gravitational field provides information on the dynamics of tectonic plates. The geopotential surface called the geoid is one definition of the shape of the Earth. The geoid would be the global mean sea level if the oceans were in equilibrium and could be extended through the continents (such as with very narrow canals).
The image below shows a map of deviations in gravity from a perfectly smooth, idealized Earth.
2) Thermodynamics is a branch of physics concerned with heat and temperature and their relation to energy and work. It defines macroscopic variables, such as internal energy, entropy, and pressure that partly describe a body of matter or radiation. It states that the behaviour of those variables is subject to general constraints that are common to all materials, not the peculiar properties of particular materials. These general constraints are expressed in the four laws of thermodynamics. Thermodynamics describes the bulk behaviour of the body, not the microscopic behaviours of the very large numbers of its microscopic constituents, such as molecules. Its laws are explained by statistical mechanics, in terms of the microscopic constituents.
In geology, thermodynamics relates to the stability of rocks and minerals under varying heats and pressures. Geologists study these principles, which have applications to a number of geology specialities.
The temperature of the Earth increases with depth.
Some of the Earth’s internal heat is left over from its creation from the solar nebula, some is produced through radioactive decay, and some is produced from other sources. The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.
Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth’s heat production would have been much higher. Heat production was twice that of present-day at approximately 3 billion years ago, resulting in larger temperature gradients within the Earth, larger rates of mantle convection and plate tectonics, allowing the production of igneous rocks such as komatiites that are not formed anymore today.
Other sources of heat include heat of impact and compression released during the original formation of the Earth by accretion of in-falling meteorites. Heat released as abundant heavy metals (iron, nickel, copper) descended to the Earth’s core. Latent heat released as the liquid outer core crystallizes at the inner core boundary.
Heat may be generated by tidal force on the Earth as it rotates; since rock cannot flow as readily as water it compresses and distorts, generating heat.
Nature is always uncomfortable with any unequal distribution of heat. It constantly works to equalise any temperature difference in the most efficient way possible. The Earth has a basic obligation to the laws of physics to transfer its internal heat into the cold universe that surrounds us.
Because the interior of the earth is hot and under great pressure, it transfers much of its internal heat by a phenomenon called “convection.” Convection is the process by which hot materials rise, move laterally, cool, and then descend in a cycle.
Irregular convection cells transfer heat from the Earth’s core to the surface. Convection is the driving heat engine that powers the motion of the Earth’s great tectonic plates. Plate tectonics is the Earth’s way of expelling heat to space, thus fulfilling its obligation to the Second Law of Thermodynamics.
The image above is a model of thermal convection in the Earth’s mantle. The thin red columns are mantle plumes.
There are two thermal boundary layers – the core-mantle boundary and the lithosphere – in which heat is transported by conduction.
Heating is also responsible for the formation of igneous and metamorphic rocks.
Igneous rocks form from volcanic magma when a volcano erupts and are also referred to as volcanic rocks. Under the surface of the Earth heath causes rocks to melt and the resultant magma is kept liquid by high temperature and high pressure. As the volcano erupts hot magma reaches the surface. Afterward the lava rapidly cools down and solidifies. The crystals formed by cooling magma are usually small. Magma doesn’t always reach the surface. Sometimes it is trapped underground in pockets of other rocks. In this case the magma cools down more slowly forming larger crystals and coarse-grained rocks. How the rocks form will depend not only on the different cooling temperatures of the magma but also its chemical composition. Granite, basalt, and obsidian are examples of igneous rocks.
http://geology.com/rocks/granite.shtml (below left)
Granite is a light-coloured intrusive igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth’s surface.
http://geology.com/rocks/basalt.shtml (below centre)
Basalt most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies.
http://geology.com/rocks/obsidian.shtml (below right)
Obsidian is usually an extrusive rock – one that solidifies above Earth’s surface.
Metamorphic rocks are formed from sedimentary and igneous rocks which were subjected to more intense pressure or heat and as a result underwent a complete change. Metamorphic rocks form deep within the Earth’s crust. The process of metamorphism does not melt the rocks, but transforms them into other rocks which are denser and more compact. New minerals are created either by the rearrangement of a mineral’s components or by reactions with fluids that enter the rocks.
http://geology.com/rocks/gneiss.shtml (below left)
Gneiss usually forms by regional metamorphism at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallized under intense heat and pressure.
http://geology.com/rocks/marble.shtml (below right)
Most marble forms at convergent plate boundaries where large areas of Earth’s crust are exposed to regional metamorphism. Some marble also forms by contact metamorphism when a hot magma body heats adjacent limestone or dolostone.
3) Plate tectonics is a scientific theory that describes the large-scale motion of Earth’s lithosphere.
There are seven or eight major plates (depending on how they are defined) and many minor plates. Where plates meet, their relative motion determines the type of boundary; convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative movement of the plates typically varies from zero to 100 mm annually.
The physics involved involves measuring the velocity of the plates and looking at how friction between plates can cause earthquakes. Convergent boundaries (Destructive) (or active margins) occur where two plates slide toward each other to form either a subduction zone (one plate moving underneath the other) or a continental collision. At zones of ocean-to-continent subduction (e.g., Western South America, and Cascade Mountains in Western United States), the dense oceanic lithosphere plunges beneath the less dense continent. Earthquakes then trace the path of the downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted plate partially melts, magma rises to form continental volcanoes.
When two plates move sideways against each other (at a transform plate boundary), there is a tremendous amount of friction which makes the movement jerky. The plates slip, then stick as the friction and pressure builds up to incredible levels. When the pressure is released suddenly, and the plates suddenly move apart, this is an earthquake.
4) Seismic waves are vibrations that travel through the Earth’s interior or along its surface. The entire Earth can also oscillate in forms that are called normal modes or free oscillations of the Earth. Ground motions from waves or normal modes are measured using seismographs. If the waves come from a localized source such as an earthquake or explosion, measurements at more than one location can be used to locate the source. The locations of earthquakes provide information on plate tectonics and mantle convection.
Measurements of seismic waves are a source of information on the region that the waves travel through. If the density or composition of the rock changes suddenly, some waves are reflected. Reflections can provide information on near-surface structure. Changes in the travel direction, called refraction, can be used to infer the deep structure of the Earth.
5) A variety of electric methods are used in geophysical survey. Some measure spontaneous potential, a potential that arises in the ground because of man-made or natural disturbances. Telluric currents flow in Earth and the oceans. They have two causes: electromagnetic induction by the time-varying, external-origin geomagnetic field and motion of conducting bodies (such as seawater) across the Earth’s permanent magnetic field. The distribution of telluric current density can be used to detect variations in electrical resistivity of underground structures. Geophysicists can also provide the electric current themselves (see induced polarization and electrical resistivity tomography).
6) Electromagnetic waves occur in the ionosphere and magnetosphere as well as the Earth’s outer core. In the Earth’s outer core, electric currents in the highly conductive liquid iron create magnetic fields by electromagnetic induction. Alfvén waves are magnetohydrodynamic waves in the magnetosphere or the Earth’s core. In the core, they probably have little observable effect on the geomagnetic field, but slower waves such as magnetic Rossby waves may be one source of geomagnetic secular variation.
Electromagnetic methods that are used for geophysical survey include transient electromagnetics and magnetotellurics.
7) The Earth’s magnetic field protects the Earth from the deadly solar wind and has long been used for navigation. It originates in the fluid motions of the Earth’s outer core. The magnetic field in the upper atmosphere gives rise to the auroras.
The Earth’s field is roughly like a tilted dipole, but it changes over time (a phenomenon called geomagnetic secular variation).
Geologists observed geomagnetic reversal recorded in volcanic rocks, through magnetostratigraphy correlation and their signature can be seen as parallel linear magnetic anomaly stripes on the seafloor. These stripes provide quantitative information on seafloor spreading, a part of plate tectonics. They are the basis of magnetostratigraphy, which correlates magnetic reversals with other stratigraphies to construct geologic time scales. In addition, the magnetization in rocks can be used to measure the motion of continents.
8) Radioactivity has been mentioned earlier. Radioactive decay accounts for about 80% of the Earth’s internal heat, powering the geodynamo and plate tectonics. The main heat-producing isotopes are potassium-40, uranium-238, uranium-235, and thorium-232. Radioactive elements are used for radiometric dating, the primary method for establishing an absolute time scale in geochronology.
9) Fluid motions occur in the magnetosphere, atmosphere, ocean, mantle and core. Even the mantle, though it has an enormous viscosity, flows like a fluid over long time intervals (see geodynamics). This flow is reflected in phenomena such as isostasy, post-glacial rebound and mantle plumes. The mantle flow drives plate tectonics and the flow in the Earth’s core drives the geodynamo.
Geophysical fluid dynamics is a primary tool in physical oceanography and meteorology. The rotation of the Earth has profound effects on the Earth’s fluid dynamics, often due to the Coriolis effect. In the atmosphere it gives rise to large-scale patterns like Rossby waves and determines the basic circulation patterns of storms. In the ocean they drive large-scale circulation patterns as well as Kelvin waves and Ekman spirals at the ocean surface. In the Earth’s core, the circulation of the molten iron is structured by Taylor columns.
Waves and other phenomena in the magnetosphere can be modelled using magnetohydrodynamics.
10) The physical properties of minerals must be understood to infer the composition of the Earth’s interior from seismology, the geothermal gradient and other sources of information. Mineral physicists study the elastic properties of minerals; their high-pressure phase diagrams, melting points and equations of state at high pressure; and the rheological properties of rocks, or their ability to flow. Deformation of rocks by creep make flow possible, although over short times the rocks are brittle. The viscosity of rocks is affected by temperature and pressure, and in turn determines the rates at which tectonic plates move.
Metamorphic rocks are produced by high pressures as well as heat from sedimentary and igneous rocks. All rocks are affected to some degree by friction forces due to weathering and erosion resulting in sedimentary rocks.
11) Satellites are used to investigate the Earth and produce accurate measurements of position, along with earth deformation and gravity. They collect data from not only the visible light region, but in other areas of the electromagnetic spectrum. The Earth can be characterized by its force fields: gravity and magnetic field, which are studied through geophysics and space physics.
Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine details of the gravity fields of the Earth (and other planets) to be mapped.
Primary geological hazards
Geologists utilise a range of techniques that rely on the laws of physics to measure, interpret and model data, to monitor areas which are susceptible to geological hazards.
Why study volcanoes
1) Volcanic activity poses a threat to human life. We need to be able to mitigate against the risks associated with volcanic hazards.
2) Volcanoes are windows into the deeper Earth – they allow geologists to learn about parts of the Earth we cannot physically get to.
One in ten people choose to live close to an active or potentially active volcano. Why?
Volcanic ash contains all the micronutrients required by plants
Above left shows Galeras volcano located in southern Colombia and above right shows Sakurajima located in Kyushu Japan.
How can geologists make living in the shadow of a volcano safer?
Firstly what exactly is a volcano?
A volcano is a rupture on the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
The Montserrat Volcano Observatory was established shortly after the first phreatic eruption of the Soufriere Hills Volcano on July 18th 1995. The Observatory is staffed by scientists from a variety of organisations working with local personnel. The scientific teams come mainly from the Seismic Research Unit (SRU) of the University of the West Indies in Trinidad and the British Geological Survey (BGS).
Guillaume Levieux published the following on 29th July 2011:
Vocanoes form at vents or openings in the Earth’s crust which allow hot, molten rock (magma) and gases from the interior of the Earth to reach the surface. Most of the time, the solid parts pile up around the vent to form a steep-sided mountain such as the Soufriere Hills Volcano in Montserrat but there are some which are simply holes or slits in the ground.
Above left is Guilaume Levieux and above right is the Monserrat Volcano Observatory.
What types of volcanic hazard are there?
1) Lava flows
Lava flows are the least hazardous of all processes in volcanic eruptions. They are streams of molten rock that pour or ooze from an erupting vent. Lava is erupted during either nonexplosive activity or explosive lava fountains. Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. The speed at which lava moves across the ground depends on several factors, including (1) type of lava erupted and its viscosity; (2) steepness of the ground over which it travels; (3) whether the lava flows as a broad sheet, through a confined channel, or down a lava tube; and (4) rate of lava production at the vent.
2) Ash fall
Volcanic ash consists of tiny jagged particles of rock and natural glass blasted into the air by a volcano. It can threaten the health of people and livestock, pose a hazard to flying jet aircraft, damage electronics and machinery, and interrupt power generation and telecommunications. Wind can carry ash thousands of miles, affecting far greater areas and many more people than other volcano hazards. Even after a series of ash-producing eruptions has ended, wind and human activity can stir up fallen ash for months or years, presenting a long-term health and economic hazard.
The ash can be 10x denser than snow. It is heavy enough to make roofs collapse, killing people inside the buildings. If inhaled from within a hot, dense pyroclastic flow or surge it almost always results in death from burns or asphyxiation. People exposed to ash fall and subsequent ash-filled air, commonly experience various eye, nose, and throat symptoms.
Because wet ash conducts electricity, it can cause short circuits and failure of electronic components, especially high-voltage circuits and transformers. Power outages are common in ash-fall areas, making backup power systems important for critical facilities, such as hospitals.
Agriculture can also be affected by volcanic ash fall. Crop damage can range from negligible to severe, depending on the thickness of ash, type and maturity of plants, and timing of subsequent rainfall. For farm animals, especially grazing livestock, ash fall can lead to health effects, including dehydration, starvation, and poisoning.
3) Pyroclastic flows
Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds. They may result from the explosive eruption of molten or solid rock fragments, or both. They may also result from the nonexplosive eruption of lava when parts of dome or a thick lava flow collapses down a steep slope. Most pyroclastic flows consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.
A pyroclastic flow will destroy nearly everything in its path. With rock fragments ranging in size from ash to boulders traveling across the ground at speeds typically greater than 80 km per hour, pyroclastic flows knock down, shatter, bury or carry away nearly all objects and structures in their way. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200°C and 700°C, can cause combustible material to burn, especially petroleum products, wood, vegetation, and houses. Even relatively small flows that move less than 5 km from a volcano can destroy buildings, forests, and farmland. And on the margins of pyroclastic flows, death and serious injury to people and animals may result from burns and inhalation of hot ash and gases.
A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley.
A hot lahar rushes down a river valley in Guatemala near the Santa Maria volcano, 1989
Lahars can be caused by:
(a) Damming or blocking tributary streams, which may cause water to form a lake behind the blockage, overtop and erode the blockage, and mix with the rock fragments as it rushes downstream (for example, see this case study at Pinatubo Volcano, Philippines)
(b) Increasing the rate of stream runoff and erosion during subsequent rainstorms. Hot pyroclastic flows and surges can also directly generate lahars by eroding and mixing with snow and ice on a volcano’s flanks, thereby sending a sudden torrent of water surging down adjacent valleys (see case study from Nevado del Ruiz volcano, Colombia).
Most pyroclastic flows consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.
4) Toxic gases
The image below right shows the active volcano, part of the ljen string of volcanoes in East Java, releasing sulphur fumes.
Volcanic gases were directly responsible for approximately 3% of all volcano-related deaths of humans between 1900 and 1986. Some volcanic gases kill by acidic corrosion; others kill by asphyxiation. The greenhouse gas, carbon dioxide, is emitted from volcanoes, accounting for nearly 1% of the annual global total. Some volcanic gases including sulphur dioxide, hydrogen chloride, hydrogen sulphide and hydrogen fluoride react with other atmospheric particles to form aerosols.
Acid rain can be produced when high concentrations of these gases are leached out of the atmosphere. When Katmai erupted in 1912, acid rain damaged clothes that were drying outside on a line 2000 km away from the erupting volcano in Vancouver, British Columbia (Bryant, 1991). High concentrations of CaF2 can burn vegetation and other material on contact. Fluoride and chloride can contaminate water. Livestock have died from drinking such contaminated water. Fluoride and chloride can also be irritating to the skin and eyes of animals, and can damage clothes and machinery. Carbon monoxide and carbon dioxide are usually produced in small amounts. However, large amounts of these gases will sometimes build up in low lying areas and can asphyxiate livestock and harm vegetation (Bryant, 1991 and Scott, 1989).
Recently, a new volcanic hazard involving tropical lakes in volcanic regions was discovered. Carbon dioxide built up at the bottom of tropical Lake Nyos was released from the bottom of the lake when the lake overturned. Fifteen hundred people were killed and 10,000 people burned in this disaster (Bryant, 1991).
Harmful concentrations of volcanic gases usually do not extend further than 10 km from the volcano (Scott, 1989). Remote sensing instruments have been used to track volcanic gases such as SO2. Other instruments have also been used to measure amounts and types of volcanic gases. Currently much research is being done on how volcanic gases may contribute to changes in climate.
5) Climate change – cooling.
Volcanic eruptions can affect climate in two main ways. First, they release the greenhouse gas carbon dioxide, contributing to warming of the atmosphere.
But the warming effect is very small. Volcanic carbon dioxide emissions since 1750 are at least 100 times smaller than those from fossil fuel burning, according to the latest report from the Intergovernmental Panel on Climate Change (IPCC).
As well as carbon dioxide, volcanic eruptions also blast a cloud of ash, dust and sulphur dioxide into the stratosphere, which is quickly blown around the globe.
Sulphur dioxide combines with oxygen and water to form sulphuric acid “aerosols”. These particles directly reflect sunlight and encourage clouds to form.
This cooling effect outweighs the warming contribution from carbon dioxide, causing an overall cooling that tends to lasts for about two years after a major eruption.
Deaths from Volcanic Hazards
What do all the volcanic hazards have in common?
They require magma to reach the surface
Illustration by Zina Deretsky, U.S. National Science Foundation
The most important aspect to monitor at volcanic areas is the amount of magma stored in the crust and if it is moving closer to the surface.
Tracking magma and magma movement
Seismic studies or Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake environmental effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic, atmospheric, and artificial processes (such as explosions). A related field that uses geology to infer information regarding past earthquakes is paleoseismology. A recording of earth motion as a function of time is called a seismogram. A seismologist is a scientist who does research in seismology.
Seismology studies investigate earthquake swarms. Earthquake swarms are events where a local area experiences sequences of many earthquakes striking in a relatively short period of time.
Seismic studies involve seismic tomography. Seismic tomography is a technique for imaging Earth’s sub-surface characteristics in an effort to understand deep geologic structure. Gathering ample compressional wave (P-wave) and shear wave (S-wave) travel time measurements allows us to compile 3D images of earth’s velocity structure
Ground deformation studies involve the use of the global positioning system.
The Global Positioning system consists of a constellation of 24 satellites. Each satellite orbits Earth twice a day at an altitude of about 20,000 km and continuously transmits information on specific radio frequencies to ground-based receivers. They can detect the build-up of stress and pressure caused by magma rising toward the ground surface.
Ground deformation studies involves the use of InSAR which is a remote-sensing technique used to study volcanoes and earthquakes. InSAR stands for Interferometric Synthetic Aperture Radar. Satellites record images of the Earth’s surface and these images can be combined to show subtle movements of the ground surface, called deformation. It produces a spatially complete map of ground deformation with centimetre-scale accuracy without subjecting field crews to hazardous conditions on the ground.
A pulse of radar energy is successively emitted from a satellite (below left), scattered by the Earth’s surface, and recorded back at the satellite (below right). The radar energy received by the satellite contains two important types of information.
The first type information is encoded in the strength or amplitude of the return signal, which is influenced by various physical properties of the surface including ground slope, particle size (i.e., sand versus boulders), and soil moisture. The second type of information contained in the return radar signal has to do with the round trip distance from the satellite to the ground and back again – a sort of invisible tape measure calibrated in units of the radar wavelength.
If two radar images are obtained at different times from exactly the same vantage point in space they can be compared and any movement of the ground surface toward or away from the satellite would show up as a phase difference between the images. For example, if a point on the ground moved toward the satellite (mostly upward) by one-half wavelength, the phase of the return signal from that point would increase by one full wavelength relative to the first image. This process is sometimes called “interfering” the images, because combining two waves causes them to either reinforce or cancel one another, depending on the relative phases.
InSAR results – A change of colours indicates a crustal movement
InSAR interferogram showing the effects of the earthquake in Hamadori, a coastal area in Fukushima prefecture. It shows the crust’s movement for 50 km in all directions. The three disconnected lines denote the location of the fault as it appeared on the surface. (courtesy: JAXA/GSI)
Gas monitoring – By measuring changes in the emission rate of certain key gases, especially sulphur dioxide and carbon dioxide, scientists can infer changes that may be occurring in a volcano’s magma reservoir and hydrothermal system. The emission rates of sulphur dioxide and carbon dioxide are measured using airborne or ground-based techniques. During large explosive eruptions, sulphur dioxide gas injected high into the atmosphere is measured by an instrument aboard a satellite.
The most common method for sampling volcanic gases is to collect them directly from fumaroles in solution-filled bottles, and then to analyse the mixtures in the laboratory.
The image above shows gases being drawn through a metal tube inserted into a fumarole at Mageik Volcano in Alaska; the sample was later analysed at a USGS laboratory in Menlo Park, California.
A fumarole is an opening in a planet’s crust, often in the neighbourhood of volcanoes, which emits steam and gases such as carbon dioxide, sulphur dioxide, hydrogen chloride, and hydrogen sulphide.
Seismic array set up
The seismometer needs to be deep enough and protected
Tracking magma – seismic signals
Earthquake activity beneath a volcano almost always increases before an eruption because magma and volcanic gas must first force their way up through shallow underground fractures and passageways. When magma and volcanic gases or fluids move, they will either cause rocks to break or cracks to vibrate. When rocks break high-frequency earthquakes are triggered. However, when cracks vibrate either low-frequency earthquakes or a continuous shaking called volcanic tremor is triggered.
Mantle features plumes etc.
What do different seismic signals mean?
Shadow waves can’t pass through liquids
Harmonic tremors indicate problems
Tracking magma – ascent to the surface
Prior to volcanic eruptions the number of seismic events increases exponentially and changes from mid-frequency to low-frequency as they move closer to the surface.
Daily count of the seismicity recorded at Merapi during the 2010 eruption. VT = Volcanotectonic; MP = Multiphase (= Hybrid earthquake); LF = low-frequency; Rockf = Rockfall earthquakes; Pyroclastic F = Pyroclastic flows; RSAM = Real-time Seismic Amplitude Measurement. (b) Location of earthquake prior and during the eruption.
Seismicity at Bárðarbunga and propagating along the dike that fed the Holuhraun eruption (Iceland Met Office, 2015)
Tracking magma – seismic surveys
Iceland meteorological office 2015
Imaging magma – seismic tomography
What do geologists use as a seismic wave source?
A seismic survey is one form of geophysical survey that aims at measuring the earth’s (geo-) properties by means of physical (-physics) principles such as magnetic, electric, gravitational, thermal, and elastic theories. It is based on the theory of elasticity and therefore tries to deduce elastic properties of materials by measuring their response to elastic disturbances called seismic (or elastic) waves.
A seismic source-such as sledgehammer-is used to generate seismic waves, sensed by receivers deployed along a preset geometry (called receiver array), and then recorded by a digital device called seismograph. Based on a typical propagation mechanism used in a seismic survey, seismic waves are grouped primarily into direct, reflected, refracted, and surface waves.
There are three major types of seismic surveys: refraction, reflection, and surface-wave, depending on the specific type of waves being utilized. Each type of seismic survey utilizes a specific type of wave (for example, reflected waves for reflection survey) and its specific arrival pattern on a multichannel record. Seismic waves for the survey can be generated in two ways: actively or passively. They can be generated actively by using an impact source like a sledgehammer or passively by natural (for example, tidal motion and thunder) and cultural (for example, traffic) activities. Most of the seismic surveys historically implemented have been the active type. Seismic waves propagating within the vertical plane holding both source and receivers are also called inline waves, whereas those coming off the plane are called offline waves.
Major types of seismic waves based on propagation characteristics.
The reflected and refracted waves are body waves and are propagated through the body whereas surface waves travel along the surface of the medium
Seismic inversion, in Geophysics is the process of transforming seismic reflection data into a quantitative rock-property description of a reservoir. Seismic inversion may be pre- or post-stack, deterministic, random or geostatistical, and typically includes other reservoir measurements such as well logs and cores.
Inversion is the set of methods used to infer properties through physical measurements. Surface wave inversion is the method by which elastic properties, density, and thickness of layers in the subsurface are attained through analysis of surface wave dispersion. The entire inversion process requires the gathering of seismic data, the creation of dispersion curves, and finally the inference of subsurface properties.
P-wave reflection imaging is also utilized often by seismologists to image geologic structures sometimes kilometres below the earth’s surface, and is useful for locating faults or geologic features that contain oil.
Magma near the surface – ground deformation
Inflation begins as magma rises into the summit reservoir.
As the magma reservoir becomes inflated, the ground around it cracks to accommodate its increasing volume.
The summit magma reservoir begins to deflate when magma moves laterally into a rift zone and either erupts or is stored there.
Simulating ground deformation due to magma near the surface
This method of measuring ground deformation, caused by magma near the surface, is low tech but high risk. It is good for movements smaller than are resolvable by GPS
The method illustrated below is lower risk but requires high tech. It is good for long term monitoring over a small area where large movements are expected. It is expensive to maintain in active areas.
InSAR is a low risk, high tech method. It is excellent for long term monitoring. No equipment necessary on the ground but requires that the volcano is not in a military black zone to get the best satellite coverage.
Dr Kiyoo Mogi, born 1929 in Yamagata Prefecture, Japan) is a prominent seismologist. He is regarded as Japan’s foremost authority on earthquake prediction.
In 1958 Mogi was responsible for a major advance in understanding the dynamics of volcanos. After studying data from several sources, he concluded that a mathematical solution developed by Yamakawa in 1955 could be used in the modelling of the deformation of a volcano caused by pressure changes in its magma chamber. The ‘Mogi model’ (also known as the ‘Mogi-Yamakawa model’) subsequently became the first commonly used quantitative method in volcanology, and is still widely used today.
Mogi Modeling is used to estimate the location and size of anomalies in the earth when there is a change in size or density. This approach is taken when trying to investigate the nature of injection of magma in the crust.
1) Ash plume 2) Lapilli 3) Lava fountain 4) Volcanic ash rain
5) Volcanic bomb 6) Lava flow 7) Layers of lava and ash 8) Stratum
9) Sill 10) Magma conduit 11) Magma chamber 12) Dike
Magma near the surface – measuring ground deformation. The problems
The density can change but goes undetected. It is assumed that the crust is homogenous and that the magma is coming from a fixed point source (image 1 below).
Modelling ground deformation in the classroom
Inflation of a water balloon in a “crust” of gelatine can allow observation and measurement of ground deformation which can then be fed into a Mogi model to calculate the change in volume of the magma chamber.
Monitoring gas emissions
Ground Based Ultraviolet Remote Sensing of Volcanic Gas Plumes
Principle of spectroscopic remote sensing
Spectra are collected both with and without the volcanic gas plume in the optical path. Identification of the wavelengths at which absorption occurs, and the depths of these features, provides information on which plume gases are present, and in what abundances, respectively.
In June 1991, the second largest volcanic eruption of the twentieth century took place on the island of Luzon in the Philippines.
Sulphur dioxide is the most common emission
Tracking atmospheric effects of volcanic plumes
Sulphur dioxide, SO2, can be recorded from space, both in the TIR (thermal infra-red) and the ultraviolet. The diagram below illustrates two examples, one from the 2009 eruption of Redoubt volcano using NASA Ozone Monitoring Instrument (OMI) data in the ultraviolet, Figure A, and another example using a MODIS decorrelation stretch on the 2009 eruption of Sarychev Volcano, Russia, Figure B. These two sensors have very different spatial resolutions and so the OMI data looks ‘blocky’ in comparison to the MODIS data.
A – OMI SO2 data from March 23, 2009, showing a large SO2 cloud dispersing to the east from Redoubt volcano, adapted from Webley et al. (2013) and B – High altitude SO2-rich volcanic clouds on June 16, 2009 at 00:50 UTC from Sarychev volcano, Russia, as depicted in the MODIS decorrelation stretch. Here, the SO2 clouds are displayed in yellow and the ash clouds are displayed in red/magenta, with ice crystals are displayed in blue, adapted from Rybin et al. (2011)
Climatic effects of volcanic gases
Amount of sulphur gases put into the air by recent volcanic eruptions. Note that the 1989 eruption of Redoubt put only 1/100 the amount of sulphur dioxide (SO2) into the air that the 1991 eruption of Mt. Pinatubo did. Image credit: TOMS Volcanic Emissions Group.
The above images are simplified diagrams showing the effect on the atmosphere by (a) small scale eruption and (b) strong volcanic eruption. Volcanic eruptions caused cooling.
The graph below is from the NOAA ESRL Solar Radiation program. Lidar measures backscatter from volcanic particles and other aerosols high in the atmosphere (15.8-33km). The volcanic particles are composed mainly of water and sulphuric acid and persist for up to a decade following major eruptions such as occurred in 1982 and 1991. These particles influence the ozone layer, and as illustrated in the graph below of apparent solar transmission, reduce the amount of sunlight reaching the earth.
Lidar (also written LIDAR, LiDAR or LADAR) is a remote sensing technology that measures distance by illuminating a target with a laser and analysing the reflected light.
Is living beneath a volcano safer?
Deaths from volcanic hazards are reducing due to improved monitoring and hazard forecasting, but people do still die and in a large number of cases these deaths occur within exclusion zones set up by scientists.
A hazard mitigation failure: Nevada Del Ruiz (1985)
The Nevado del Ruiz also known as La Mesa de Herveo or Kumanday in the language of the local pre-Columbian indigenous people, is a volcano located on the border of the departments of Caldas and Tolima in Colombia, about 129 kilometers west of the capital city Bogotá. It is a stratovolcano, composed of many layers of lava alternating with hardened volcanic ash and other pyroclastic rocks. Nevado del Ruiz has been active for about two million years, since the early Pleistocene or late Pliocene epoch, with three major eruptive periods. The current volcanic cone formed during the present eruptive period, which began 150 thousand years ago.
The Armero tragedy was one of the major consequences of the eruption of the Nevado del Ruiz stratovolcano in Tolima, Colombia, on November 13, 1985. After 69 years of dormancy, the volcano’s eruption caught nearby towns unaware, even though the government had received warnings from multiple volcanological organizations to evacuate the area when volcanic activity had been detected in September 1985.
In November 1984 there was significant seismic activity and unusual fumaroles in the summit crater. Experts and equipment was called in from WOVO.
WOVO is an organization of and for volcano observatories of the world. Members are institutions that are engaged in volcano surveillance and, in most cases, are responsible for warning authorities and the public about hazardous volcanic unrest.
The first lahar flowed for 20km down the river. The Swiss seismologists’ note that danger from further lahars is evident but the newspaper La Patria reports that Ruiz activity not dangerous.
Three provinces Tolima, Caldas and Risaralda agree to manage their own hazards plans but there is a disparity between the attentions of local governments to the crisis.
The above image shows a map generated by INGEOMINAS in October 1985 which was published by newspapers but changes to the colours and lack of a key led to confusion.
November 13 1985:
At 3.06pm phreatic eruption occurred. A phreatic eruption, also called a phreatic explosion or ultravulcanian eruption, occurs when magma heats ground or surface water. The extreme temperature of the magma (anywhere from 500 to 1,170 °C) causes near-instantaneous evaporation to steam, resulting in an explosion of steam, water, ash, rock, and volcanic bombs.
1) Water vapour cloud 2) Magma conduit 3) Layers of lava and ash 4) Stratum 5) Water table 6) Explosion 7) Magma chamber
Heavy rain and ash fell in N. Armero and headquarters were alerted to prepare for lahars.
At 9.08pm strong eruptions occurred and produced several pyroclastic flows. Much of the snow pack melted and the mixing of hot pyroclastic material and melt water mobilised lahars in the river channels.
At 9.30pm an evacuation notice was given at Chinchina. The Lahar hit at 10.40pm and the evacuation order enabled hundreds of lives to be saved.
Between 9.45pm and 10pm officials attempted to contact Armero but storms had cut off power and communication to the village. Local attempts to warn the population were ignored.
At 11.35pm the lahar exited the canyon as a 40m high torrent. It split into 3 channels each with a flow depth of 2-5m and a speed of 8ms-1
How can the implementation of hazard plans be improved?
1) Improve education of locals about prolonged nature of volcanic activity
2) Improve the understanding of scientists to the beliefs and needs of the local population that make them more susceptible to risk from volcanic activity.
3) Introduce a new discipline – social volcanology
4) The understanding of volcanic systems now includes the physical system using geochemistry and geophysics, and the social system to improve the implementation of evacuation and exclusion programs.
1) Vast improvements have been made in how we can image, track and quantify the volume of magma underlying volcanoes.
2) This has helped to better predict volcanic eruptions.
3) Despite this there are still deaths related to volcanic eruptions.
4) This is in part due to poor education and communication with local communities.
5) With efforts made to improve this we should see further reduction in volcanic deaths.