4 Igneous Processes and Volcanoes
By the end of this chapter, students should be able to:
- Explain the origin of magma as related to plate tectonic settings.
- Describe how Bowen’s Reaction Series relates to the temperature of mineral crystallization and melting.
- Explain magma, and rock compositions and textures used to classify igneous rocks.
- Analyze the features of common igneous landforms and how they relate to their origin.
- Explain partial melting and fractionation, and how they change magma compositions.
- Describe how silica content affects magma viscosity and eruptive style of volcanoes.
- Describe volcano types, eruptive styles, composition, and tectonic setting.
- Describe volcanic hazards.
Igneous rocks form from liquid or molten material called magma (in the ground) or lava (on the surface). It is sometimes believed that the entire interior of the Earth is liquid; this is not the case. Except for the Earth’s outer core and a few minor pockets and zones near the surface where the materials are molten, the Earth is a solid body. Thus, the source for volcanoes and igneous rocks is controlled by geologic processes in the crust and upper mantle. This chapter will describe the classification of igneous rocks, the unique processes that form magmas, types of volcanoes and volcanic processes, volcanic hazards, and igneous landforms.
Lava generally cools quickly at the surface of the earth forming extrusive igneous rocks (also known as volcanic igneous rocks). In contrast, magma that cools slowly below the earth’s surface forms intrusive igneous rocks (also known as plutonic igneous rocks). This distinction between rapid and slow cooling is important in interpreting the geologic history of a mass of igneous rock.
4.1 Classification of Igneous Rocks
Igneous rocks are classified based on texture and composition. Texture of igneous rocks describes the character of the mineral components such as grain size and relates to the cooling history of molten rock. Composition is the specific mineralogy and chemical composition of a rock.
If the magma cools slowly at depth within the crust, the resulting rocks are called intrusive (or plutonic). Intrusive igneous rocks have a coarse-grained texture (phaneritic) in which most minerals are visible to the unaided eye because the minerals have had time to grow visibly large.
If a lava is extruded onto the surface (or intruded into shallow fissures near the surface), the resulting igneous rocks are called extrusive (or volcanic). Extrusive igneous rocks have a fine-grained texture (aphanitic) in which many mineral grains are too small to see with the unaided eye. In addition, quickly cooled extrusive material isn’t a mineral at all but rather is volcanic glass, which doesn’t have a crystalline structure and therefore isn’t a mineral. The fine-grained texture indicates that the lava cooled quickly and minerals didn’t have time to grow visible crystals. In fine-grained aphanitic rocks, mineral crystals can be studied under a petrographic microscope .
In addition to aphanitic and phaneritic texture, some igneous rocks have a few coarse-grained minerals surrounded by a fine-grained material in a texture called porphyritic. The large crystals are called phenocrysts and the fine-grained surrounded material is called the groundmass. This texture indicates a multi-stage cooling history in which the magma body was cooling slowing deeper under the surface and then later rose to shallower depth or extruded at the surface, thus indicating different stages of cooling.
Veins surrounding igneous intrusions may have very large crystals of felsic minerals like feldspar, quartz, and muscovite mica. The texture of such a rock is called pegmatitic and the rock is called pegmatite. Cleavage sheets from very large crystals of pegmatitic muscovite provided windows for wealthy houses in the Middle Ages.
Magma that rises to the surface contains dissolved gases in solution called volatiles. Pressure is released from rising magma causing dissolved volatiles to come out of solution, like bubbles coming out of solution when a soda bottle is opened. Bubbles of these gases that are trapped in the cooling lava create a texture called vesicular.
An extreme version of scoria occurs when volatile-rich lava is very quickly quenched and becomes a meringue-like froth of glass called pumice. Some pumice is so full of vesicles (pore space) that it can float.
Lava that oozes and cools so quickly that crystals cannot form (even microscopically) makes a volcanic glass. When most of a rock is volcanic glass it is called obsidian. Obsidian also shows an excellent illustration of conchoidal fracture.
As the solid parts of the eruption (called tephra) settle back to earth, they form a rock called pyroclastic, “pyro” referring to the igneous or fire nature of the material and “clastic” referring to rock fragments. Tephra materials are named based on size—ash (<2mm), lapilli (2-64 mm), blocks and bombs (>64mm). Pyroclastic texture is usually recognized by the angular shape of crystals and the presence of shards of glass and rock fragments. Rocks formed from these deposits are called tuff. If it accumulates while hot, crystals may be deformed and the mass may be welded together, forming a welded tuff.
Composition of igneous rocks refers to the chemical make-up and thus the mineral make-up of the rocks. For igneous rocks, composition is grouped into four groups, felsic, intermediate, mafic, and ultramafic. These terms refer to the amount of silica, iron, and magnesium in the rocks and are determined by the minerals present. Felsic refers to a predominance of feldspar (alkali feldspar), and silica in the form of quartz. Mafic refers to a abundance of ferromagnesian minerals (with magnesium and iron, chemical symbols Ma and Fe) plus plagioclase (Ca-rich) feldspar. Ultramafic refers to the extremely mafic rocks composed of mostly olivine and some pyroxene which have even more magnesium and iron. These minerals make up peridotite, the rock of the upper mantle. Intermediate rocks are those between mafic and felsic and are composed predominantly of plagioclase feldspar (Na-rich) and amphiboles.
On the figure above, the top row has both plutonic and volcanic igneous rocks arranged from felsic on the left to intermediate, mafic, and ultramafic on the right. Rhyolite and granite thus refer to extrusive and intrusive felsic rocks respectively. Andesite and diorite refer to intermediate rocks (with dacite and granodiorite applying to rocks with composition between felsic and intermediate). Basalt and gabbro are the extrusive and intrusive names for mafic rocks, and peridotite is the ultramafic (with komatiite as the fine-grained equivalent). Note that komatiite is a rare rock; volcanic material direct from the mantle is not common although some examples can be found in ancient Archean rocks .
Igneous rocks are classified by both texture and composition described above. In this section, texture and composition will be used to name individual igneous rocks. The classification table below is arranged with composition represented with less silica from left to right (felsic, intermediate, mafic, and ultramafic). The approximate percentages of some minerals are included in each composition. Igneous texture is shown on the left side. Using this table, if an igneous rock is felsic and coarse-grained then it is a granite. If an igneous rock is felsic and vesicular then it is pumice.
|Granite is a course-crystalline felsic intrusive rock. The presence of quartz is a good indicator of granite. Granite commonly has salmon pink potassium feldspar or white plagioclase crystals that have visible cleavage planes. An example of granite is the Little Cottonwood Stock.||Rhyolite is a fine-crystalline felsic extrusive rock. Rhyolite is commonly pink and will often have glassy quartz phenocrysts. Examples of rhyolite is the altered rhyolite that makes up the Grand Canyon of the Yellowstone in Yellowstone National Park.|
|Diorite is a coarse-crystalline intermediate intrusive igneous rock. Diorite is identifiable by it’s Dalmatian-like appearance of black hornblende and biotite and white plagioclase feldspar. It is found in the Andes Mountains.||Andesite is a fine crystalline intermediate extrusive rock. It is commonly grey. It can be found in the Andes Mountains and in some island arcs. It is the compositional equivalent of diorite.|
4.1.4 Intrusion Types
Igneous rocks form intrusive landforms and extrusive landforms (i.e. volcanoes). The intrusive landforms are classified here. Volcanic landforms are created when lava reaches the surface and are discussed in a later section of this chapter.
Plutons—Any rock body that formed from a cooled and crystallized magma chamber is called a pluton. A magma chamber is a large reservoir under a volcano that holds a large supply of magma. The term stock can also be used for a pluton. The processes by which rising magma (sometimes known as a diapir) intrudes into overriding and surrounding rock are poorly understood. For example, what happens to the volume of pre-existing rock that is replaced by the pluton? That plutons exist and have intruded into pre-existing country rock is a matter of observation. Was the country rock shouldered aside? Was it consumed within the magma or stoped (i.e. pieces of it broke off and settled through the rising magma)? Is the idea of a magma chamber oversimplified, and really a series of dikes, combining, create a pluton? How plutons are emplaced is a subject of ongoing geological inquiry .
Batholiths—These are large masses of plutons (typically felsic) associated with the roots of mountain belts. The massive granite formations of Yosemite National Park and the Sierra Nevada Mountains are batholiths that were emplaced in the core of the Sierra Nevada when they formed millions of years ago and are now exposed by subsequent uplift and erosion. Batholiths and stocks are discordant intrusions, i.e. they cut across and through surrounding country rock. Fluid magmas may also intrude into near surface rocks by taking advantage of specific weaknesses in pre-existing rocks.
Dikes—When magma follows a cross cutting weakness like a crack or fissure, the resulting cross-cutting feature is called a dike. Because of this, dikes are often vertical or diagonal relative to the rock layers that they intersect. Like batholiths, dikes are discordant intrusions. Dikes are important to geologists, not only for the study of igneous rocks themselves but also for dating and interpreting the geologic history of an area. The dike is younger than the rocks it cuts across and, as discussed in the chapter on Geologic Time, may be used to assign actual dates to sedimentary sequences. Dikes often can be dated using radioactive isotopes and help determine the age of sedimentary rocks.
Sills—Magma may exploit a weakness between sedimentary layers by intruding between the layers. shouldering them apart and squeezing in between them. Such an intrusive structure is called a sill and is a concordant intrusion, i.e. it is parallel to the country rock. Sills are also important for the study of geologic history for they are younger than the sediment both below and above the intrusion. As seen above, radiometric dating of these igneous intrusions is an important method of relative dating sedimentary rocks.
Laccoliths—These are blister-like intrusions of magma between sedimentary layers (i.e.concordant). A famous example of a topographic feature formed by this process is the Henry Mountains of Utah. Laccoliths typically bulge upwards, while a downward-bulging and similar intrusion is called a lopolith.
4.2 Bowen’s Reaction Series
Norman L. Bowen (1887-1956) was an early 20th Century geologist who exemplifies application of the scientific method. Bowen studied igneous rocks and noticed that in igneous rocks, certain minerals always occur together and these mineral assemblages exclude other minerals. Curious as to why, and with the hypothesis in mind that it had to do with the temperature at which the rocks cooled, he set about conducting experiments on igneous rocks in the early 1900s. In those experiments, he ground rocks and combinations of rocks into powder, put the powder into metal capsules and sealed them. He heated them to various temperatures and then quenched the capsules. After opening the capsules, he cut thin slices (called thin sections) of the contents and studied the minerals present.
When he opened the quenched capsules, he found a glass surrounding mineral crystals that he could identify under his petrographic microscope. The results of many of these experiments, conducted at different temperatures over a period of several years, showed that the common igneous minerals crystallize from magma at different temperatures and that minerals occur together in rocks with others that crystallize within similar temperature ranges. Bowen’s work laid the foundation for understanding igneous petrology (the study of rocks) and resulted in his book, The Evolution of the Igneous Rocks in 1928 .
Bowen’s Reaction Series (shown above) describes the temperature at which minerals crystallize when cooling or melt when heated. The temperature scale ranges from around 700˚C (at the low end where all minerals have crystallized into solid rock) to around 1250˚C (at the upper end where all minerals have melted) . Since Bowen conducted his experiments at the pressure of the Earth’s surface, the temperature of crystallization would be different deeper in the Earth where pressures are much higher (discussed in the next section).
The compositions on the right side of the diagram, felsic, intermediate, mafic, and ultramafic, refer to the silica composition of each group. The arrows on the right show increasing abundance of key ions. Think in terms of the energy regimes at which these elements form bonds with other elements and the heat energy represented by atomic motion at the various temperatures. Energy is involved in the forming and breaking of bonds between ions and bonds form when the energy (temperature) is right for the ions involved.
4.3 Magma Generation
Magma and lava contains three components – melt, solids, and volatiles. The liquid part, called melt, is made of ions from minerals that have already melted. The solid part, called solids, are crystals of minerals that have not melted yet and are floating in the melt. Volatiles are gaseous components dissolved in the magma such as water vapor, carbon dioxide, sulfur, and chlorine . The presence and amount of these three components affect the physical behavior of the magma. This will be considered later in the chapter.
4.3.1 Geothermal Gradient
Although it is very hot under the Earth’s surface, the crust and mantle are mostly solid. This heat inside the Earth is caused by residual heat left over from the original formation of Earth and from radioactive decay. The rate at which temperature increases with depth is the geothermal gradient. The average geothermal gradient in the upper 100 kilometers of the crust is generally about 25°C per kilometer (km). So, for every kilometer of depth, the temperature increases by 25 °C.
Pressure-temperature diagrams illustrate the geothermal gradient and the behavior of rock by graphing depth (pressure) and temperature (see figure). The figure shows the geothermal gradient changing with depth through the the crust and mantle. The diagram shows geothermal gradient as a red line and at 100 km the temperature is about 1,200°C. In addition, the pressure at bottom of the crust (shown here as depth; 35 km deep) is about 10,000 bars . Bar is a measure of pressure, 1 bar being normal atmospheric pressure at sea-level. At these pressures and temperatures in the Earth, the crust and mantle rocks are solid. On the P-T diagram, the green solidus line shows the pressures and temperatures at which rocks start to melt. Since the geothermal gradient (red line) is always left of the solidus (green line) to a depth of 150 km, then the rocks are solid. The solidus line slopes to the right because the melting temperature of any substance depends on the pressure. A higher pressure at greater depth requires higher temperature to melt rock. In another example, water boils at 100°C at an atmospheric pressure close to 1 bar. But if the pressure is lowered, as shown on the video below, then water boils at a much lower temperature.
Three main ways that pressure and temperature conditions change to induce melting and create magma are: 1) lower the pressure (decompression melting), 2) add volatiles (flux melting),and 3) increase the temperature (heat).
The figure below uses P-T diagrams to show how melting can occur at three different plate tectonic settings. Setting A is a normal situation in the middle of a stable plate in which no magma is generated. Setting B is at a mid-ocean ridge (decompression melting). Setting C is a hotspot (decompression melting plus addition of heat), and setting D is a subduction zone (flux melting).
4.3.2 Decompression Melting
Magma is created at the mid-ocean ridge by decompression melting. The mantle is solid but is flowing under great pressure and temperatures due to convection. Rock is not a good conductor of heat so as mantle rock rises, pressure is reduced along with the melting point (the green line) but the rock temperature remains about the same and the rising rock begins to melt. Pressure changes instantaneously as the rock rises but temperature changes slowly because of the low heat conductivity of rock. On the figure above, setting B: mid-ocean ridge shows a mass of mantle rock at a pressure-temperature location X on the P-T diagram as well as its geographical location on the cross section under a mid-ocean ridge. At this location, the P-T diagram shows the red arrow increasing to the right. Thus, hotter rock is now shallower, at a lower pressure, and the new geothermal gradient (red line) shifts past the solidus (green line) and melting starts. As this magma continues to rise at divergent boundaries and encounters seawater, it cools and crystallizes to form new lithospheric crust.
4.3.3 Flux Melting
Another way that rocks melt is when volatiles gases (i.e. water vapor and carbon dioxide) are added to mantle rock from a descending subducting slab in a process called flux melting (or fluid-induced melting). The subducting slab contains oceanic lithosphere and hydrated minerals. As the slab descends and slowly increases in temperature, volatiles are expelled from these hydrated minerals, like squeezing water out of a sponge. The volatiles then rise into the overlying asthenospheric mantle lowering the melting point of the peridotite minerals (olivine and pyroxene). The pressure and temperature of the overlying mantle rock don’t change, but the addition of volatiles lowers the melting temperature. This is analogous to adding salt to an icy roadway. The salt lowers the melting/crystallization temperature of the solid water (ice) so that it melts. Another example is welders adding flux to lower the melting point of their welding materials.
Flux melting is illustrated in setting D: island arc (subduction zone) of the P-T diagram above. Volatiles added to mantle rock at location “Z“ act as a flux to lower the melting temperature. This is shown on the P-T diagram by the solidus (green line) shifting to the left. The solidus line moves past the geothermal gradient (red line) and melting begins. Magmas producing the volcanoes of the Ring of Fire, associated with the circum-Pacific subduction zones are a result of flux melting. As introduced in the minerals chapter, water ions can bond with other ions in the crystal structures of amphibole (and other silicates) and this is important in considering how magmas form in subduction zones by “flux melting.” Such hydrated minerals in subducting slabs contribute water to the flux melting process.
4.3.4 Heating-Induced Melting
The least common magma generation style is the simple process of adding heat to the mantle. On the figure above, setting C, a hotspot (mantle plume), the surrounding rock is exposed to the higher temperatures of the plume as shown by the red arrow and the geothermal gradient crosses the green solidus line and melting begins. There is rising material as well, meaning that decompression melting is a factor here too. A small amount of magma is also generated in intense metamorphism in collisions (see Chapter 6) resulting in a hybrid rock called migmatite in which veins of quartz and feldspar (the first minerals to melt on the Bowen’s Reaction Series diagram) occur in a gneiss.
4.4 Partial Melting and Crystallization
4.4.1 Partial Melting
According to Bowen’s Reaction Series (Section 4.2 above), each mineral has a unique melting and crystallization temperature. Since most rocks are made of many different minerals, when rocks start to melt, only some minerals melt. This process is known as partial melting and is responsible for magma having a different composition than the original rock from which it melted. For example, many magmas formed by decompression and flux melting come from partial melting of ultramafic mantle rock peridotite that is made of olivine and pyroxene. When the ultramafic rock starts to melt, the lowest temperature minerals melt first, meaning that the derived melt will be more silica-rich. Ultramafic rock partially melts to form mafic magmas; and mafic rocks partially melt to form intermediate magmas.
Early in Earth history when the continents were forming, less dense and more silica-rich magmas rose to the surface and solidified into silica-rich granitic continents. Today, the old granitic cores of the continents are shown below in orange as the shields. The next section describes how these silica-rich magmas evolve from ultramafic magmas.
4.4.2 Crystallization and Magmatic Differentiation
Since magma is liquid rock, it tends to be less dense than the surrounding rock and rises buoyantly through the mantle and crust. As the magma rises and begins to cool and crystallize, the chemistry of the magma changes, a process known as magmatic differentiation. As magma rises through the crust, two main changes can alter the chemistry: assimilation and fractionation .
During assimilation, pieces of country rock (with a different composition) are added to the magma. These pieces may melt and alter composition of the original magma, or these may simply be included in it as it cools. Such unmelted pieces of country rock within an igneous rock mass are called xenoliths. Xenoliths are also common in the processes of magma mixing and rejuvenation, two other processes of magmatic differentiation. These occurs when two different magmas come into contact and mix, though at times, the magmas can remain heterogeneous and create xenoliths, dikes, and other features.
Much of the thick continental lithosphere is felsic (granitic) in composition. When more mafic magma rises through the thick crust, it is a long journey and there is a lot of time to react with the surrounding country rock. Mafic magmas will tend to assimilate the more felsic rocks and become more silica-rich as they migrate through the lithosphere. Mafic magmas will thus become intermediate or felsic by the time they reach the surface.
Assimilation is not the only way in which a magma can evolve to a higher silica content and become more felsic. As the temperature drops on the magma diapir rising through the crust, some minerals will crystallize and settle to the bottom of the magma chamber leaving a remaining melt depleted in those elements (figure below). This process is called fractionation or fractional crystallization . For example, as an ultramafic magma cools olivine starts to crystallize and settle to the bottom of the magma chamber. Crystallizing of olivine depletes the magma of silica, iron, and magnesium and the remaining melt becomes more mafic in composition. The high temperature minerals at the top of Bowen’s Reaction Series crystallize first. As the magma further cools, the mafic magma then crystallizes calcium-rich plagioclase and pyroxene, depleting the residual magma of those elements leaving a magma that is more intermediate or felsic in composition. Cooling of a magma thus leaves mineral crystals and a resulting residual magma lower down on the Bowen Series diagram. This occurs in almost all magmas, but is more common in thicker crust, where more time is taken as the magma travels upward. Some differentiation takes place within oceanic lithosphere, but the formation of less dense, more differentiated (also known as highly evolved) felsic magmas is largely confined to continental regions on earth.
4.5.1. Distribution and Tectonics
Most volcanoes are located at active plate boundaries called interplate volcanism. The prefix “inter-“ means “between”. In contrast, some volcanoes are not associated with plate boundaries, but rather are located within the plate far from plate boundaries. These are called intraplate volcanoes and many are formed by hotspots and fissure eruptions. The prefix “intra-“ means “within.” The following discusses the location of volcanism in more detail with mid-ocean ridges, subduction zones, and continental rifts representing interplate volcanism, and hot spots representing intraplate volcanism.
Volcanoes at Mid-Ocean Ridges
Although most volcanism occurs on the ocean floor along the mid-ocean ridge (a type of divergent plate boundary), they are also the least observed since most are under 10,000 to 15,000 feet of ocean. As the oceanic plates diverge and thin, hot mantle rock is allowed to rise, pressure is released which causes the ultramafic mantle rock (peridotite) to partially melt. The resulting magma is basaltic in composition based on the concept of partial melting discussed earlier. Because most volcanoes on the ocean floor are basaltic, most of the oceanic lithosphere is also basaltic near the surface with gabbro and ultramafic peridotite forming underneath. Icelandic volcanism is an example of this, but above sea level .
What does an underwater volcanic eruption look like? Basaltic magma erupts underwater forming pillow basalts and/or in small explosive eruptions. In association with these seafloor eruptions, an entire underwater ecosystem thrives in parts of the mid-ocean ridge. This ecosystem exists around tall vents emitting black, hot mineral-rich water called deep-sea hydrothermal vents (also known as black smokers).
This hot water, up to 380 °C (716 °F), is heated by the magma and dissolves many elements that supports the ecosystem. Deep underwater where the sun cannot reach, this ecosystem of organisms depends on the heat of the vent for energy and vent chemicals as its foundation of life called chemosynthesis. The foundation of the ecosystem is hydrogen sulfide-oxidizing bacteria that live symbiotically with the larger organisms. Hydrogen sulfide (H2S – the gas that smells like rotten eggs) needed by these bacteria is contained in the volcanic gases emitted from the hydrothermal vents. The source of most of this sulfur and other elements is the Earth’s interior . Below are three short videos on a deep-sea submersible submarine and deep-sea hydrothermal vents.
Volcanoes at Subduction Zones
The second most common location of volcanoes is adjacent to subduction zones (a type of convergent plate boundary). As discussed previously, during subduction water is expelled from the hydrated minerals causing partial melting by flux melting in the overlying mantle rock. This creates a mafic magma that rises through the lithosphere and can change composition by interacting with surrounding continental crust as well as by magma differentiation. These changes then turn basaltic magma into more silica-rich rock in volcanoes and plutons. These silica-rich rocks are felsic to intermediate rocks such as andesite, rhyolite, pumice, and tuff. One area with an abundance of subduction, and thus volcanoes with silica-rich magma is in a ring around the Pacific Ocean known as the Pacific “Ring of Fire.” This volcanoes are discussed in more detail in the stratovolcano section.
Volcanoes at Continental Rifts
In addition to volcanoes at the mid-ocean ridge and subduction zones, some volcanoes are at continental rifts where the lithosphere is diverging and thinning such as in the Basin and Range Province in North America and the East African Rift Basin in Africa. The thinning allows for some of the lower crustal rocks or upper mantle rocks to rise releasing pressure and causing partial melting. The magma generated is less dense than the surrounding rock and rises through the crust to the surface erupting as basalt. These basaltic eruptions are usually in the form of flood basalts, cinder cones, and basaltic lava flows. For example, relatively young cinder cones are located in south-central Utah, the Black Rock Desert Volcanic Field, which is part of the Basin and Range crustal extension. The 1-minute video (below) illustrates volcanism in the Basin and Range Province. These Utah cinder cones and lava flows started erupting 6 million years ago with the last volcanic eruption 720 years ago .
The main source of intraplate volcanism are hotspots. Hotspots are when lithospheric plates glide over a hot mantle plume which is an ascending column of hot rock (not magma) originating from deep within the mantle. A chain of ancient volcanoes formerly active but now inactive for millions of years can be seen on the seafloor that leads to an active intraplate volcano. This indicates hotspot volcanism. The Pacific oceanic plate overrode a hotspot mantle plume producing a long volcanic island chain terminating at the Hawaiian Islands. When a continental plate overrides the mantle plume hhotspot, a chain of ancient volcanoes can form extending from Southwestern Idaho to the Yellowstone caldera.
Once the magma reaches the lithosphere it spreads out with a mushroom-shaped head that is tens to hundreds of kilometers across. Near the base of the lithosphere, the mantle plume and the lowest crust partially melt to form mafic magma that rises to feed volcanoes. Since most mantle plumes are beneath the oceanic lithosphere, the early stages of volcanism typically take place on the seafloor. Over time basaltic volcanoes may form islands like those in Hawaii or if the hotspot is under continental lithosphere then an explosive volcano with a more felsic to intermediate composition may form like the Yellowstone volcano. Two three-minute videos (below) illustrates hotspot volcanoes.
4.5.2 Volcano Features and Types
There are several different types of volcanoes based on their shape, eruption style, magmatic composition, and other features. The following are the main components of a volcano: A conduit is the narrow pipe that connects the magma chamber to the surface at the vent where lava erupts. A large depression at the top that is called a crater, and the largest craters are called caldera, such as the Crater Lake Caldera in Oregon. A parasitic cone is a small volcano located on the flank of a larger volcano such as Shastina on Mount Shasta. Kilauea sitting on the flank of Mauna Loa is not considered a parasitic cone because it has its own separate magma chamber . Many volcanic features are derived from one basic measure of a lava: viscosity. Viscosity is the resistance to flowing by a fluid. Counterintuitively, a high viscosity means a gooey, less able to flow magma, similar to toothpaste. A low viscosity magma will flow more easily, like the volcanism that occurs in Hawaii on shield volcanoes.
The largest volcano is a shield volcano and is characterized by broad, low-angle flanks, a small vent or groups of vents at the top, and basaltic magma. The name “shield” comes from the side view resembling a medieval warrior’s shield. They form slowly from many low-viscosity basaltic lava flows that can travel long distances, hence, making the low-angle flanks. Because the magma is basaltic and low viscosity, the eruption style is not explosive but rather effusive, meaning that volcanic eruptions are small, localized, and predictable. Therefore, this eruption styles isn’t much of a hazard. Mauna Loa (info) and the more active Kilauea (info) in Hawai’i are good examples. Shield volcanoes are also found at Iceland, the Galapagos Islands, Northern California, Oregon, and the East African Rift . The largest volcanic edifice in the Solar System is Olympus Mons on Mars. This is a shield cone as large as the state of Arizona indicating little if any plate tectonic activity on Mars as the volcano erupted over the same hotspot for millions of years .
Basaltic magma can form several rock types and special landforms. Based on magma temperature, composition, and content of dissolved gases and water vapor, there are two main types of basaltic volcanic rocks with Hawaiian names—pahoehoe and aa. Pahoehoe is a basaltic magma that flows easily into a “ropey” appearance. In contrast, aa (sometimes spelled a’a or ʻaʻā and pronounced “ah-ah”) has a crumbly blocky appearance . (Peterson and Tilling 1980).
In basaltic lava flows, the low viscosity lava can easily flow, and it tends to harden on the outside but continue to flow internally within a tube. Once the interior flowing lava subsides, the tube can be left as an empty lava tube. Lava tubes famously make caves (with or without collapsed roofs) in Hawai’i, Northern California, the Columbia River Basalt Plateau of Washington and Oregon, El Malpais National Monument in New Mexico, and Craters of the Moon National Monument in Idaho. Fissures, cracks that originate from shield-style eruptions, are also common. Magmas from fissures are typically very fluid and mafic. Some fissures are caused by the volcanic activity itself, and some can be influenced by tectonics, such as the common fissures parallel to the divergent boundary in Iceland.
Since basalt flows are thick accumulations of lava with a homogenous composition that flows quickly, when the lava begins to cool it can contract into columns with a hexagonal cross section called columnar jointing. This feature is common in basaltic lava flows but can be found in more felsic lavas and tuffs as well.
A stratovolcano, sometimes called composite cones, have steep flanks, symmetrical cone shapes, distinct craters, and rise prominently above the surrounding landscape. Examples include Mount Rainier in the Cascade Range in Washington and Mount Fuji in Japan. Stratovolcanoes can have magma with felsic to mafic composition. However, felsic to intermediate magmas are most common. The term “composite” refers to the alternating deposition of pyroclastic materials (like ash) and lava flows. The viscous nature of the more common intermediate and felsic magma results in steep flanks and explosive eruption styles. Stratovolcanoes are made of lava flows and ash .
Lava domes are relatively small accumulation of silica-rich volcanic rocks, such as rhyolite and obsidian, that are too viscous to flow, and therefore, pile high close to the vent. The domes often form within the collapsed crater of a stratovolcano near the vent and grow by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. A good example of a lava dome is inside of a collapsed stratovolcano crater is Mount Saint Helens. Examples of a stand alone lava dome are Chaiten in Chile and the Mammoth Mountain in California .
Calderas are usually large, steep-walled, basin-shaped depressions formed by the collapse of a volcanic edifice into an empty magma chamber. Calderas are generally very large with a diameter up to 15 miles. Although the word caldera only refers to the vent, many use caldera as a volcano type, typically formed by high-viscosity felsic volcanism with high volatile content. Crater Lake, Yellowstone, and Long Valley Caldera are good examples. At Crater Lake National Park in Oregon, about 6,800 years ago Mount Mazama was a composite volcano that erupted in a large explosive blast ejecting large amounts of volcanic ash. The eruption rapidly drained the underlying magma chamber causing the top to collapse forming a large depression that later filled with water. Today a resurgent dome is found rising up through the lake as cinder cone, called Wizard Island .
The Yellowstone caldera erupted three times in the recent past, at 2.1, 1.3, and 0.64 million years ago. Each eruption created large rhyolite flows and pyroclastic flows of ash that solidified into tuff. These extra large eruptions rapidly emptied the magma chamber causing the roof to collapse and form a caldera. Three calderas are still preserved from these eruptions and most of roads and hotels of Yellowstone National Park are located within the caldera. Two resurgent domes are located within the last caldera.
Yellowstone volcanism started as a hot spot under the North American lithosphere about 17-million years ago near the Oregon/Nevada border. As the North American plate slid over the stationary hotspot, surface volcanism moved through and helped form Idaho’s Snake River Plain, eventually moving to its current location in northwestern Wyoming. As the plate move over the stationary hotspot to the southwest, it left a track of past volcanic activities .
The Long Valley Caldera near Mammoth California is a large explosive volcano that erupted 760,000 years ago and dumped a large amount of ash throughout the United States, similar to the Yellowstone Eruptions. This ash formed the large Bishop Tuff deposit. Like the Yellowstone caldera, the Long Valley Caldera contains the town of Mammoth Lakes, a major ski resort, an airport, and a major highway. Further, there is a resurgent dome in the middle and active hot springs .
Cinder cones are small volcanoes with steep sides, made of cinders and volcanic bombs ejected from a pronounced central vent. Typically, they come from mafic lavas with high volatile content. Cinders form when hot lava is ejected into the air, cooling and solidifying before they reach the flank of the volcano. The largest cinders are called volcanic bombs. Cinder cones form in single eruption events that are short-lived and relatively common throughout the western United States .
A relatively recent and striking example of a short-lived cinder cone is the 1943 eruption near the village of Parícutin, Mexico . The cinder cone started with an explosive eruption shooting cinders out of a vent in the middle of a farmer’s field. Quickly, volcanism continued building the cone to a height of over 300 feet in a week and 1,200 feet in the first 8 months. After the initial explosive gases and cinders were released, growing the cone, basaltic lava poured out around the cone. This order of events is common for cinder cones: first violent eruption, then formation of cone and crater, followed by a low-viscosity lava flow. The Parícutin cinder cone was built over nine years and covered about 100-square miles with ashes, and destroyed the town of San Juan .
A rare and still unobserved volcanic eruption type are flood basalts. Flood basalts are some of the largest and lowest viscosity types of eruptions known. They are not known from any eruption in human history, so the exact mechanisms of eruption is still up for debate. Some famous examples include the Columbia River Flood Basalts in Washington, Oregon, and Idaho, the Deccan Traps, which cover about 1/3 of the country of India, and the Siberian Traps, which may have been responsible for Earth’s largest mass extinction at the end of the Permian (see chapter 8).
Arguably the most unusual volcanic activity produces carbonatites. These are a product of carbonate-based volcanism, instead of all other volcanism which is silicate based. Carbonatites only erupt from one volcano on Earth today: Ol Doinyo Lengai, in Tanzania. The lavas are incredibly low viscosity, relatively cold (for lava), black when erupted, but solidify to a brown/grey and eventually to white. These rocks are found occasionally in the geologic record, and are mostly associated with rifting .
4.5.3 Volcanic Hazards and Monitoring
Volcanic hazards have been famous for centuries, but recent eruptions are more well documented. The most obvious hazard is the lava itself found within a lava flow, but the hazards posed by volcanoes go far beyond a lava flow. For example, on May 18, 1980, Mount Saint Helens erupted explosively with a giant landslide that removed the upper 1,300 feet (400 m) of the mountain. This landslide was immediately followed by the lateral blast , and covered 230 square miles of forest with ash and debris. The effects of the blast are shown on the before and after images (Figure and Figure). The pyroclastic flow moved at speeds of 50 – 80 miles per hour (80-130 km/hr), flattened trees and ejected a large ash cloud into the air. Watch the 7-minute USGS video for an account of May 18, 1980 that killed 57 .
In 79 AD, Mount Vesuvius, located near Naples, Italy, violently erupted sending a pyroclastic flow over the Roman countryside, including the cities of Herculaneum and Pompeii. The buried towns were discovered as an archeological site in the 18th century . Pompeii famously contains the remains (casts) of people suffocated by ash and covered by 10 feet (3 m) of ash, pumice lapilli, and collapsed roofs .
The most dangerous volcanic hazard are pyroclastic flows (video). These flows are from collapses in the eruption column that run downhill. These are mainly explosive eruptions that rapidly eject a mix of lava blocks, pumice, ash, and hot gases between 400 to 1,300 ℉. The turbulent cloud of ash and gases races down the steep flanks at high speeds (>120 mph) in the valleys of composite volcanoes . Most explosive, silica-rich, high viscosity magma volcanoes such as composite cones usually have pyroclastic flows. The rock tuff and welded tuff is the solid form of ash from these pyroclastic flows.
There are numerous examples of deadly pyroclastic flows. In 2014, the Mount Ontake pyroclastic flow in Japan killed 47 people. The flow was caused by magma heating groundwater into steam, which then rapidly ejected with ash and volcanic bombs. Some were killed by inhalation of toxic gases and hot ash, while others were struck by volcanic bombs . Two short videos below document eye-witness video of pyroclastic flows. In early 1990’s, Mount Unzen erupted several times with pyroclastic flows. The pyroclastic flow shown in this famous short video killed 41 people. In 1902, on the Caribbean Island Martinique, Mount Pelee erupted with a violent pyroclastic flow that destroyed the entire town of St. Pierre and killing 28,000 people .
Landslides and Landslide-Generated Tsunamis
The flanks of a volcano are steep and unstable which can lead to slope failure and generate dangerous landslides. For example, the landslide at Mount St. Helens 1980 released a huge amount of materials as the entire north flank collapsed. The landslide moved at speeds of 100-180 mph. These landslide can be triggered by movement of magma, explosive eruptions, large earthquakes, and heavy rainfall. In unique situations, the landslide material can reach water and cause a tsunami. In 1792 in Japan, Mount Unzen erupted causing a large landslide that reached the Ariaka Sea and made a tsunami that killed 15,000 people on opposite shore (info) .
A lahar is an Indonesian word for a mudflow that is a mixture of water, ash, rock fragments, and other debris moving down the flanks of a volcano (or other nearby mountains covered with freshly-erupted ash) and entering adjacent river valleys. They form from the rapid melting of snow or glaciers on volcanoes. They are similar to a slurry of concrete but can flow up to 50 mph while still on the steep flanks. Since lahars are slurry-like they can travel long distances in river valleys almost like a flash flood.
During the 1980 Mount St. Helens eruption, lahars reached 17-miles (27 km) down the North Fork of the Toutle River. Prehistoric lahar flows have been mapped at major volcanoes such as Mount Rainier near Tacoma, Washington (Rosi et. al. 1999). Prehistoric lahars are occupy river floodplain where large cities are located today as shown on the map. Similarly Mount Baker poses a hazard as shown by this hazards map for Mount Baker north of Seattle, Washington . A recent scenario played out when a lahar from the volcano Nevado del Ruiz in Colombia buried a town in 1985 and killed an estimated 25,000 people.
Tephra and Ash
Volcanoes, especially composite volcanoes, eject large amounts of tephra (ejected rock materials) and ash (fragments less than 0.08 inches [2 mm]). Tephra is heavier and falls closer to the vent. Larger blocks and bombs pose hazards to those close to the eruption such as at the 2014 Mount Ontake disaster in Japan discussed earlier. Ash is fine and can be carried long distances away from the vent, and can cause building collapses and respiratory issues like silicosis. Hot ash can be dangerous to those close to the eruption and disrupt services such as airline transportation farther away . For example, in 2010 the Eyjafjallajökull volcano in Iceland created a large ash cloud in the upper atmosphere that caused the largest air travel disruption in northern Europe since a seven-day airline shut down during World War II. No one was hurt but the cost to the world economy was estimated to be billions of dollars .
Magma contains dissolved gases. As rising magma reaches the surface the confining pressure decreases allowing gases to escape. This is similar to gases coming out of solution after opening a soda bottle. Therefore, volcanoes when not erupting release hazardous gases such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S) and hydrogen halides (HF, HCl, or HBr). Carbon dioxide can sink and accumulate in low lying depressions on the earth’s surface. For example, Mammoth Mountain Ski Resort in Mammoth Lakes, California is located within the Long Valley Caldera. Therefore the whole ski resort and town are within the caldera. In 2006, three ski patrol members were killed after skiing into snow depressions near fumaroles that had filled with carbon dioxide (info). Therefore, in volcanic areas where carbon dioxide emissions occur, avoid low-lying areas that may trap carbon dioxide . In rare cases, a volcano can suddenly release gases without warning. Called a limnic eruption, this commonly occurs in crater lakes as gases pour from the water. It infamously occurred in 1986 in Lake Nyos, Cameroon, killing almost 2,000 people due to carbon dioxide asphyxiation.
Volcano monitoring requires geologist to use many instruments to detect changes that may indicate an eruption is imminent . Some of the main observations include regular monitoring for: earthquakes (including special vibrational earthquakes called harmonic tremor, caused by magma movement), changes in the orientation and elevation of the land surface, and increases gas emission. Very short videos (below) summarize how an increased frequency of earthquakes can show that magma is moving and that an eruption may occur soon. Another video (below) shows how gas monitoring is used to monitor volcanoes and predict eruption. As the magma gets closer to the surface, the gases come out of the magma. A rapid increase of gas emission can indicate an eruption is imminent. This last video (below) shows how a GPS unit and tiltmeter can detect movement of the land indicating that the magma is moving underneath.
Igneous rocks are broken into two major groups: intrusive, which cool underground, and extrusive, which erupt of the surface. Magma is generated at several tectonics settings, and is usually generated by volatiles lowering the melting temperature or decompression. Magma is changed by several processes dealing with the differences in the melting temperatures of mineral components (Bowen’s Reaction Series), including partial melting, magmatic differentiation, and assimilation. Volcanism is hazardous to human civilization, but can be monitored and mitigated with careful study.