Background
More than six months ago, on June 27, a volcanic eruption occurred in Hawaii, its lava continuing to bubble till September. It crawled its way from the erupted volcano till the edges of the nearest village, causing people to evacuate from their homes. The leading edge of the lava flow was about 110 meters wide with an average speed of 5 meters per hour. The volcanic eruption occurred at the mountain Kilauea. Volcano experts quote Kilauea as “the most continuously active volcano on Earth; lava streaming into the ocean at a steady rate of around 5 [metres per second] for years on end.” Though Hawaii is a volcanic island, a typical Hawaiian eruption involves thin lava discharge, not viscous or explosive like other greater volcanoes like the Vesuvius or Krakatua. The eruptions of Hawaiian volcanoes occur along thin linear cracks or fissures along the surface of the earth’s crust, spilling out vast amounts of lava, forming an extensive lava field. Volcanic eruptions generally generate huge amount of gases in varied volumes, but are generally dangerous to human contact. Since Hawaiian volcanoes are gentle and not really hazardous, the sulphur dioxide gas that resulted from the Kilauea eruption was expelled from the eruptive cleft and not from the surface of the lava.
Hawaiian Volcanoes
Hawaii was formed by volcanoes and they still continue to boil below – thus most of the settled areas here are vulnerable to destruction by lava flows. Hawaiian volcanoes are hotspot volcanoes. This means that the volcanoes are formed by hotspots. Hot spots are extremely hot places on the earth’s mantle, so hot that they melt through a hole on the earth’s crust. When the earth’s techtonic plates move over these hot spots, new volcanoes are formed. When a volcanic eruption occurs, magma is released from the mantle. It is hot molten rock that assumes the name of lava when it escapes to the surface of the earth. Volcanoes are dormant most of the time, boiling inside quietly. However, sometimes the heat below the earth creates enough pressure for it to push up outside through weak spots over the crust – for example, plate edges or hot spots – causing volcanic eruptions. When a volcano erupts, it can destroy life and release a lot of ash and molten lava. A type of volcano, called composite volcano is the most explosive and dangerous volcano forming from the tallest and largest mountains. Many poisonous gases escape from the volcanic vent, such as sulphur dioxide and carbon dioxide. It also produces large waves of ash and small rocks that sweep down the surface.
Chemical effects of Volcano
The volcanic eruptions affect the earth in many disturbing ways. The gases from the eruptions are chemically and microphysically active, consisting of solid aerosol particles. A typical volcanic eruption can infuse tens of teragrams of such gases into the earth’s atmosphere, more specifically, to the stratosphere. This results in disturbing the chemical and radioactive equilibrium of the atmosphere. The gases form an aerosol cloud – this cloud is formed by the conversion of sulphur dioxide to sulphur aerosol. Another poisonous gas emitted along with sulphur dioxide is hydrogen sulfide (H2S). Other gases emitted by volcanoes include water vapor (H2O) (most prevalent), nitrogen (N2) and carbon dioxide (CO2). Another major component of volcanic eruptions is magnetic material, referred to as ash or tephra. Tephras are pyroclastic fragments that can be classified as ash, lapilli and volcanic bombs. Hawaiian eruptions are effusive eruptions consisting of liquid basaltic lava flows. Its plume is less than 1000 meters and typically consists of ash and lava. However, very less pyroclastic elements are released. Hydrogen Chloride (HCl) is the main halogen component of the emitted gases. Hydrogen Fluoride (HF), with a fraction of 1 ppm among all the volcanic gases.
Magma and Lava
The lava is initially the magma resting under the earth. Magma can be mafic or basaltic depending on the melting of the mantle. Magma also includes melted crust, which mainly consists of silicon. The main types of Magma and their details are given in the table below.
The most primitive composition of magma consists of high Magnesium Oxide (MgO) and low Silicon Dioxide (SiO2). They also contain volatile compounds like water and carbon dioxide. Other elements are – Silicon, Aluminum, Iron, Calcium, Magnesium, Sodium, Potassium, Hydrogen, and Oxygen. A major feature of the magma is crystal fractionation. Here, the magma solidifies into rock over time, and each of the mineral in it crystallizes at different temperatures. This results in crystals being removed from the rock over time. It is said to be influenced by gravity to some extent. Removal of these crystals from the liquid magma changes its composition. Crystal fractionation provides an explanation to the mechanism by which basaltic magma transforms to andesitic magma to rhyolitic magma.
Bowen’s reaction series
The order in which minerals crystallizes depends upon the temperature. In basaltic magma, the minerals that crystallize first are Olivine and Ca-rich plagioclase. Olivine reacts with the magma liquid to form pyroxene. Once the two minerals are crystallized out, the remaining component in the liquid is SiO2. As the process continues, basaltic magma can convert to andesitic magma as temperature continues falling.
Figure 1. Bowen’s Reaction Series
The lava consists of igneous rocks that are expelled from the earth’s crust due to increase in the compression stress on the crust. It is of three types:
1. Felsic – it is rich in SiO2
2. Andesitic – it is rich in Magnesium and Iron (Mg and Fe)
3. Mafic – low in SiO2
The viscosity of lava fluid flow can be determined by the relationship between the temperature and composition, with an important parameter called the yield strength. Lava flows have an ideal Newtonian behavior at high temperatures, which means that they are yield strength-free. This kind of behavior is also known as pseudo-plastic fluid behavior. Volatile content of the magma is released during eruption, but a small proportion (0.5%) is retained by the magma. However, during high eruptions when the magma emissions have a low volume, large amount of volatile content is trapped within the lava flow, thereby increasing the viscosity of the flow.
References
"Emissions from volcanoes." Emissions of Chemical Compounds and Aerosols in the Atmosphere. Ed. Claire Granier, Claire Reeves and Paulo Artaxo. Springer, 2004.
Manga, Michael and Guido Ventura. Kinematics and Dynamics of Lava Flows . Geological Society of America, 2005.
Parfitt, Liz and Lionel Wilson. Fundamentals of Physical Volcanology. John Wiley & Sons, 2009.
Robock, Alan. "Volcanic Eruptions." The Earth system: physical and chemical dimensions of global environmental change. Vol. 1. Chichester: John Wiley & Sons, Ltd, 2002. 738-744.
Young, Jennifer. Kilauea Volcanic Eruption: Hawaii Just Keeps on Growing. 2 November 2014. <http://www.decodedscience.com/kilauea-volcanic-eruption-hawaii-just-keeps-growing/50501>.
