PLANETARY VOLCANISM
Next to meteoretic impact phenomena, volcanism modifies the surface of planets and offers us a glimpse into the thermal and compositional state inside planets and moons. A wealth of geological evidence indicates that the inner planets were once in a molten state. Their crusts formed with cooling and in the case of the Earth and the moons of Jupiter and perhaps Venus, is periodically penetrated by molten material called magma. This forms the science called volcanology. This branch of geosciences is multidisciplinary and requires the use of geophysics, geochemistry, mineralogy, petrology, structural geology and physical chemistry.
Volcanism involves gaseous, liquid and solid phases, as material is expelled from a dynamical planetary interior, where there are critical processes causing the melting.
Examples of Volcanoes on Earth
Recent eruptions
Mt. St. Helen, 1980.
Pinatubo in Philippines, 1991.
Unzen in Japan
Hawaii continuing eruptions
Mexico City
Recent movies: Dante's Peak
Inner planets: molten silicates , some liquid sulfur
Outer planets: water, water-ammonia mixture, liquid sulfur methane
The Nature and origin of Magma
Magma can consolidate on the surface as lava flows or it may be ejected explosively into the atmosphere to fall as fragments of glass that become the main components of PYROCLASTIC igneous rocks. At depth magma may cool completely without being erupted at the surface to form PLUTONIC igneous rocks such as GRANITE.
Only a portion of the magma is a liquid. Most magmas consist of three distinct phases. A phase is physically distinct and mechanically separable from the other parts of the system. (gas phase, one or two more liquid phases, and a number of solid phases such as included crystals of early formed minerals). PHASE DIAGRAMS describe the stability conditions of these phases.
The liquid phase of silicic magma is quasi-crystalline and is characterized by long-range order, comprised of POLYMERS of interconnected but distorted SiO2 tetrahedra. The solid phase of magma consists of crystals with high melting temperatures (over 1600 K) of OLIVINE , PYROXENE and PLAGIOCLASE. The abundance is controlled by a process called crystal growth, which is a complicated kinetic process that depends on temperature , pressure and composition. A magma of the same composition may show a different sequence of crystallization under different temperature and pressure conditions.
VESICULAR textures in some volcanic rocks are good examples that magma can contain a gas phase. The most spectacular manifestation of the gas phase of magma is the gaseous cloud during volcanic eruptions, which serves as a propellant for extensive ash showers and explosive activity. Gas phase can have a significant effect on the fluid dynamical behavior of the melt. composition of gas: water, carbon dioxide, carbon monoxide, sulfur dioxide, sulfur, chlorine, hydrogen, sulfur trioxide.
Certain aspects, of magmatic behavior - flow extrusion, crystal settling and fractionation, depends on the density and viscosity of the silicate melts, which are strongly dependent on temperature and composition.
Viscosity is a measure of the internal resistance of a liquid to flow. The unit of viscosity is POISE or Pa s (10 Poise)
Viscosity V = shear stress/ rate of shear strain
V depends on temperature , pressure and composition.
V(T, P, C)
Rule of thumb, lower SiO2 (polymeric contribution), the lower the viscosity.
Basaltic magma have lower viscosity by a factor of 10,000 than RHYOLITIC magma , which contains more silica. The temperature-dependence of magma depends exponentially on temperature, to a good approximation V ~ exp (-A*T) where T is the temperature from 800 to 1400 C viscosity varies 6 orders in magnitude in the absence of water, with increasing water content, the temperature dependence of magma viscosity decreases.
Table of properties of magmas, effects of water on the magmatic viscosity , temperature-dependence in the viscosity of common magmas.
Effects of pressure can decrease the viscosity of magma, which is different from mantle material, where pressure would increase the viscosity. The viscosity of magma can influence the eruption rate, the distance a magma will travel, the morphology of the volcanic edifices and the style of eruption. Highly viscous magmas, coupled with volatiles, will cause rapid vesiculation of a magma, explosive eruption and the development of pyroclastic materials. Rhyolite and andesite volcanoes tend to be more explosive than basaltic volcanoes (Hawaii).
Density of magmas most important volcanic processes are controlled by the density of magmas, owing to the buoyancy forces. The rise of magmas from a source region by the density contrasts of the magma liquid and crystal densities. The mixing of magma is also controlled by the density differences. Mixing is facilitated by magma densities which are nearly matched. If the densities are very different, the magmas do not mix and remain separate. Density depends on the chemical composition of the magma and on the solid-, liquid and gas phases. Density values vary from 2.36 g/cm**3 for rhyolitic glasses to 2.77 for basaltic glasses. Iron-rich magma in general is denser. If higher pressure results in higher dissolved water content, the resulting change in density would lower magma density a lot.
Generation of Magmas
Solidus and Liquidus curves and phase diagrams. Schematic diagram of a two component system A and B Eutectic point, this point is the composition of the lowest melting temperature within this system. In the mixing of the A and B components, the melting temperature is decreased in proportion to the concentration of the added component. This is analogous to the depression of the freezing point of water by adding salt.
Melting temperature of anhydrous compounds generally are increased by an increase in lithostatic pressure. In contrast, an increase in lithostatic pressure causes a decrease in the melting temperature of many hydrous (water containing) compounds. Even small water pressures can significantly depress the solidus in silicate systems. Other volatile compounds (carbon dioxide, sulfur, chlorine and fluorine can lower melting temperatures under proper saturation conditions.
Decompression of hot solid rock can induce melting , if its temperature is already close to the liquidus. Upwelling under oceanic or continental rift zones. 30 % of melting can be produced by the mantle upwelling going up 60 km
KEY
(1.) Magmas do not form at one unique temperature and pressure, but rather over a significant range (Figure showing physical conditions under which magmas are produced)
(2.) During gradual melting, the liquid phase and the solid phase progressively change composition
(3.) the several mineral phases- olivines, pyroxenes, feldspars- that make up source rocks melt over a range of conditions with considerable overlap.
(4.) The composition of magma that come from partial melting depends entirely on the variables, T , P and composition X, and how these vary as the system changes from the solid to the liquid state.
Different types of magmas:
(Figure showing the generation of several types of magmas within the Earth's crust and mantle)
IMPORTANT TO KNOW THESE THREE BASIC KINDS OF VOLCANIC ROCKS
Basaltic: shallow magmas, peridotites can exist over a wide range of temperatures and pressures and upon melting can yield a liquid fraction of basaltic composition at depths of 60 km.
Andesitic Magma : volcanic rocks associated with island arcs (subduction zones)
Rhyolitic magmas: generated in the continental crust at 40 or so km. granitic composition at depths of 20 km in the presence of adequate volatile components. The most probable depth at which magma of granitic or rhyolitic composition can be generated is somewhere between 13 and 21 km.
Movement of Magma: rates of movement of magma from its source vary by several orders in magnitude and depend on (1) viscosity of the magma (2) volume of the magma at its source (3) depth to the source (4) difference in density between the magma and the surrounding country rock (5) lithospheric pressure (6) width and roughness of the conduit (7) rate of heat loss to cooler country rocks.
Bodies of magmas are formed by the partial melting of mantle or crustal rocks, followed by the segregation and separation of the liquid from its refractory residues. The degree of melting necessary for the liquid to segregate and move upward is between 5 and 22 %. The ascent of magma is very fast compared with the time during the segregation stage. Volcanic activity is episodic and flow rates highly variable.
In sum, magma is a naturally occurring mobile rock material, generated in planetary interiors. Magmas on the Earth's surface encompass a considerable range in chemical composition, temperature, density and viscosity. Viscosity of magmas is largely dependent on magma's temperature, volatile content and silica content.
Volcanic Products
Volcanic products are important not only for what they reveal about the interior but also because the chemical-mineralogical composition of lavas affects the eruptive style and the kinds of landforms built upon cooling.
Lava flows:
1. Pahoehoe is a lava with a smooth crust or surface wrinkled or twisted into ropy forms with folding. These forms result from the dragging of hot plastic crust by still-liquid magma underneath
2. Aa is lava characterized by a rough surface consisting of a layer of angular jagged fragments covered with small pines.
Pyroclastic products
Pyroclastic products , commonly called tephra, are fragmentary volcanic rock materials that are ejected into the air. Blocks as heavy as 100 tons are known to have been thrown more than 10 kilometers in violent eruptions, while volcanic ash and dust can reach the upper levels of the atmosphere and be scattered throughout the world. During the 1815 explosion of Tambora ,Indonesia, an estimated 30 cubic kilometers of fragmental material was produced. Darken the planet for months.
Glowing avalanche or nuee ardente, internal temperature of about 800 C moving at a speed of 100 km/hour, killed 30,000 people in a few minutes in Mount Pelee in 1902. Lahars are landslides or mudflows of volcanic material. Fast moving lahars account for 50 % of the casualties from volcanic eruptions.
Gases
Many hot springs or geysers are thought to arise as a result of the heating of ground water in rocks over and around magmas or cooling igneous rocks.
Types and forms of volcanic activity
Magma rheology and ascent: Magma rheology is not Newtonian but is a Bingham fluid A Bingham fluid is one which requires a certain shear stress be applied before flow commences. One of the reasons for this non-Newtonian behavior is the presence of dispersed crystals and gas bubbles. As a consequence of the YIELD STRESS, there is a critical lava thickness for a given slope beneath which flow will not take place. Localization of magma flow on the sheet of rising magma in a long , narrow fissure into a number of discrete vertical zones.
Discharge rates. Discharge rate of magma is a major factor in determining lava flow dimensions. There is a close relationship between the flow length and the lava discharge rate. In large flood basalts, lava with a high discharge rates can flow hundreds of kilometers. The discharge rate is controlled by the rate magma flows through fractures and ascends through the crust. Volcanoes with high discharge rates will form no positive relief, as these lavas would be extensive sheets filling up all the holes.(Columbia River Plateau)
Other factors on the style of volcanic eruption are topography , effect of water (degree of magma fragmentation and intensity of explosions are enhanced), and tectonic control (fissure rift zones in Icelandic volcanics)
Styles of Eruption
Effusive eruption -quiet eruption with only the outpouring of fluid lava Examples : Iceland, Hawaiian (Kilauea), Stromoli, Italy. Nature of magma - very fluid
Explosive eruptions-erupt violently with the explosive release of great clouds of pyroclastics. On any planet with an appreciable atmosphere, (such as the Earth , Venus, or Mars) an explosively ejected cloud of gas and pyroclasts interacts strongly with atmospheric gas, which is entrained into the cloud and heated by the pyroclasts A high , convecting eruption cloud can form (Mt. Vesuvius) Krakatoa volcano erupted in 1883 and Crater Lake in Oregon about 6000 years ago. Two very large eruptions nature of the magma- viscous
Volcanic Forms (Structures)
Figure showing examples of volcanic land forms The morphology and geometry of volcanic deposits are useful indicators in determining deposit volumes, modes of emplacement and tectonic environments Terrestrial volcanism provides a key to the interpretation of deposits on other planets.
Shield Volcanoes
Shield volcanoes have broad flat cones and gentle slopes. The sides of shield volcanoes generally slope between 2 and 10 degrees. Icelandic shield volcanoes are rather small. Hawaiian shield volcanoes are quite different in scale and structure from Icelandic volcanoes. They have gigantic proportions and represent some of the largest structures on Earth. The shield volcanoes of the Hawaiian Islands rise 8 kilometers above the seafloor and 4 kilometers above sealevel.
Volcanic crater: is a more or less circular depression, lying directly above the conduit that feeds the volcano. Craters may result from the instability of the cone to build up directly over the vent-or they may form due to the collapse of the cone's summit after removal of support by magma withdrawal.
Calderas: A caldera is a large volcanic depression, often circular, measuring tens of kilometer in diameter. Calderas are large craters partly blown out by volcanic explosions, but mainly due to collapse when the underlying magma us drained out and the central part of the volcanic structure loses support. Valles Caldera in New Mexico. Long Valley Caldera in California. La Garita Caldera in the San Juan Mountains in Colorado measures 45 km across, as compared to 700 meter hole formed in recent nuclear bomb test in western India.
Sinuous Rilles
Sinuous rilles are meandering channels that range in width from tens of meters to 3 km and from a few to several hundred km in length. They are particularly common in lunar maria. Lava channels
Coronae
Coronae are circular or ovoidal structures usually between 200 and 250 km across, with a typical relief of less than 2 km. These features are unique to Venus and have not been observed in other planetary bodies. Basaltic volcanism is often associated with coronae.
Flood basalts
Regions of flood basalt: western United States, Yellowstone, Deccan traps, Iceland, Hawaii, Siberia, their rates of magma generation were enormous , 10 times more than the rate generated today at Hawaii. must involve some large plume blast from the mantle the source of these flood basalts is unknown ,more likely from the upper mantle than from the deep lower mantle. Total area of flood basalts on Earth is not large compared to the total surface, in contrast to Venus where the basalt flood plains cover a large portion of the Venusian surface, can be due to massive gushing of hot' material from the Venusian lower mantle across the phase transition zone.
Moon
Mare volcanism the maria (sea in Latin) because of their distinctive albedo and morphology, are the most striking volcanic features on the moon. The occurrence of the vast majority of mare deposits is related to the presence of the major lunar basins cratering modification helps the mare deposits to flood the low-lying areas. Mare volcanism might have started early at 4.3 Byrs ago and lasted until around 3.2 Byrs ago. The mare lava flows are generally confined to basin interiors. Some time may elapse between the formation of large basins and its subsequent filling with basaltic flows. Mare magma are basaltic in origin.
Venus
About 85 percent of Venus surface is made up of plains of volcanic origins at a nearly uniform altitude close to the average radius of the planet (6050 km).
Cratering
Analysis from Magellan mission (1991 to 1993) reveals a very low density of craters on Venus, considerably lower than on Mercury. Surface of Venus does not preserve a record of heavy bombardment from the early history of the Solar System. crater retention age of the surface of Venus is estimated to be between 300 to 500 Myr. the density of impact craters on large volcanic structures is less than the global average, This argues for a significant volcanic activity after a major RESURFACING event some 300 to 500 Myrs ago. Big debate about mega-plumes coming up from the Venusian mantle. active research topic.
Volcanic Plains and Flow fields
Vast sheets of lava fields and associated sinuous channels. By terrestrial standards, many of these individual flow fields are large, indicating very fluid lava and very high rates of eruption (volumes as great as 8000 km**3, a total length of around 1000 km. area of 180,000 km.)
Small shield volcanoes (about 200 meters high) on Venus.
Volcanic centers are not arranged in linear patterns but are scattered and look similar to the intraplte volcanism (hot-spots) on Earth. The abundance of volcanic centers in the regions of extension
Coronae are defined as predominately circular or ovoidal structures between 60 and 1,100 km across with concentric and radial fracture systems. Altitude is less than 2 km over the surrounding plains. about 360 coronae have been mapped on Venus. one of the best working models for the origin of coronae is the ascent by mantle diapirs, (detached plume).
Basaltic volcanism is dominant on Venus. Upper-mantle composition is similar to terrestrial mantle material.
Mars
The total relief of Mars is 31 km, 1.5 times that on Earth. Mars is markedly asymmetric in the distribution of its physiographic features. Two-thirds of the southern hemisphere is covered with an ancient , densely cratered terrain. The southern hemisphere is mostly higher than the northern and a broad bulge is centered on the equator in the volcanic province of Tharsis.(Megaplume under Tharsis region and its relationship to the depth of the phase transitions in the Martian mantle).
Volcanic plains
The young plains in the Northern Hemisphere such as those close to the large Tharsis shields may be only a few hundred million years old. In the equatorial latitudes, most of the plains appear to be volcanic and have features such as flow fronts, wrinkle ridges, graben and channels. The plains account for a lot of volcanic activity on Mars.
Shield volcanoes
The most spectacular volcanic features on Mars are the extremely enormous shield volcanoes. They are much larger than the Hawaiian shield volcanoes by a long shot. They appear to have been active over a very long time. This is due to the absence of plate tectonics on Mars.
Martian shield volcanoes are concentrated in the regions of Tharsis (centered on the equator) and Elysium (centered at 25 N).
The Tharsis province of Mars is approximately 8000 km in diameter and occupies an area, that equal to 25% of the Martian surface. It had a long history.
Olympus Mons , located at 1600 km northwest of Tharsis shields, is the largest volcano on Mars and probably in the Solar System. It has a volume 50 to 100 times that of the largest volcano on Earth-Mauna Loa in Hawaii. An escarpment in places around 6.5 km.
The large size of the Martian shield volcanoes probably results from a deep source of magma due to a megaplume and the lack of plate tectonics, allowing the build-up of magmas. The history of volcanic activity on Mars became more restricted with time.
Outer Satellites
Cryovolcanism
With the photographs from the Voyager I and II missions, we now see that the traditional meaning of volcanism involving molten magmas of silicate composition must now take on a new meaning to include compounds of water, methane, ammonia and nitrogen. The term CRYOVOLCANISM (ice volcanism) has been applied to the extrusion of these "exotic " magmas .
In the outer parts of the solar system, ice is not usually H2O (pure water) but is contaminated by ammonia, methane, which causes it to melt at lower temperatures in ways analogous to partial melting of terrestrial rocks. The presence of ammonia in water lowers the melting point of eutectic mixture to about 175 K !! Such a melting temperature is low enough to be attained. This provides a process whereby resurfacing could be achieved on larger Jovian moons. A melt of NH3 .2H2O is less dense than the remaining solid ice. It will therefore move upward. This peculiar melt would have physical properties (especially viscosity and yield strength) that under the low surface gravity of an ice satellite would cause it to behave in a manner similar to a basaltic lava flow on Earth. This ammonia -water melt system can provide a variety of icy volcanic landforms which will remind one of terrestrial volcanic analogues (area of active research).
Tidal forces are ubiquitous in the solar system. It is important for the Earth and also for heating up the interiors of the Jovian moons. The heating-rate inside Jovian moons is several hundred times that of natural radioactivity in the Earth's mantle. The surface of Io produces more heat than it receives. Very hot magma , 1500 K are found. Volcanism is reminiscent to the early Earth, very vigorous.
The most remarkable volcanism in outer satellites are found on Io (Jupiter) and Triton (Neptune) which show not only concrete evidence of resurfacing but also active volcanism with large eruption plumes. Europa (Jupiter) and Enceladus (Saturn) are two other places which may harbor active volcanism today. SEE WEB SITES FROM GALILEO mission.
Io - lots of resurfacing and recycling of the lithosphere and upper-mantle system. During the Voyager mission, volcanic plumes (reaching heights of 300 km) were observed. These plumes have sulfur as their dominant volatile phase. Plumes have temperatures of 325 to 625 C which is very hot compared to the long-term surface temperature of -170 C.
Europa
Continued heating of Europa's crust by tidal dissipation could have resulted in further differentiation of the crust, thereby forming magma chambers or a saline magma ocean. Crystallization of brines in the lower crust would have caused volume expansion and fracturing of the upper crust, giving rise to extrusion of brines. There is no direct evidence of volcanic activity on Europa but its surface appears to be very young. The average age , based on crater counting, may be as young as 100 Myr. Evidence is mounting for an ocean under the surface of Europa from the Galileo flyby.
Ganymede
Ganymede is the largest satellite of the solar system and is larger than both Pluto and Mercury. Its low density of 1.93 g/cm**3 indicates that it contains a significant fraction of ice. Planetary surface of Ganymede from Voyager shows that it has been severely deformed. evidence for cryovolcanic flooding from the surface morphology. Ganymede seems to have undergone periods of intense tectonic and cryovolcanic surface modification.
Triton
Triton is the largest satellite of Neptune It has a density of 2.1 g/cm**3 , indicating that the rocky material accounts for 70 to 75 percent of Triton's mass The remaining portion appears to consist of layers of ice. A layer of liquid water may exist between the rocky core and the thick ice layers. The surface of Triton looks complex and the terrain may have formed by repeated episodes of localized melting and the collapse of the Triton's icy crust. From crater counting a relatively young surface is suggested. Indications of nitrogen-driven volcanism on Triton today.