The principal products of volcanic eruptions may be grouped into several broad categories according to the type of material ejected and its mode of transport from the vents to its place of deposition: ash falls, pyroclastics flows, lava flow and gas emission.

Must read: Volcano – Causes , Types , Effects and Distribution

Several other hazardous phenomena are directly associated with eruption. There are ground fracture, ground subsidence, debris avalanche, lahars, glacier bursts, volcanic earthquakes and tsunamis.

However the material that is expelled in a volcanic eruption can be classified into three types:

1 . Lava, the name of magma when it emerges and flows over the surface

2 . Pyroclastic Materials particles of solid material of all shapes and sizes ejected and thrown through the air

3 . Volcanic gases, a mixture made mostly of steam, carbon dioxide, and a sulfur compound (either sulfur dioxide, SO₂, or hydrogen sulfide, H₂S, depending on the temperature)

Lava

The ease with which lava flows and the structures it forms depend on how much silica and gas the lava contains. The more silica, the more polymerization (formation of long molecules) occurs, stiffening the lava.

Lava can be broadly classified into four different compositions:

1 . If the erupted magma contains a high percentage (>63%) of silica, the lava is described as felsic.

2 . If the erupted magma contains 52–63% silica, the lava is of intermediate composition or andesitic.

3 . If the erupted magma contains <52% and >45% silica, the lava is called mafic (because it contains higher percentages of magnesium (Mg) and iron (Fe)) or basaltic.

4 . Some erupted magmas contain ≤45% silica and produce ultramafic lava. Ultramafic flows, also known as komatiites, are very rare.

The stiffness of lava is described in terms of viscosity– lava that flows easily has low viscosity, and lava that is sticky and stiff has high viscosity.

Highly viscous lava generally does not advance far, but commonly piles, up above an active vent as a lava dome. Such domes can collapse repeatedly and generate dangerous hot block and ash flows and hot surges and blasts.

Pyroclastic Materials

Collectively, loose material thrown from a volcano is referred to as tephra. Individual fragments are referred to in general terms as pyroclasts, so sometimes tephra is also referred to as pyroclastic debris. Pyroclasts are classified according to size:

Volcanic Ash

Particles less than 2 mm in diameter are called volcanic ash. Volcanic ash consists of small mineral grains and glass.

Lapilli

Fragments with dimensions between 2 mm and 64 mm are classified as lapilli.

Blocks and Bombs

Fragments larger than 64 mm are classified as blocks or bombs, depending on their origin.

Blocks are solid fragments of the volcano that form when an explosive eruption shatters the pre-existing rocks

Bombs form when lava is thrown from the volcano and cools as it travels through the air.

Effects of Gas on Lapilli and Bombs

The presence of gas in erupting lava can cause lapilli and bombs to take on distinctive forms as the lava freezes around the gas bubbles, giving the rocks a vesicular (hole-filled) texture.

Pumice forms from gas-filled felsic lava. Pumice floats on water because some of the holes are completely enclosed, and air-filled.

Volcanic gases

Volcanic gases are gases given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating from lava, from volcanic craters or vents. Volcanic gases can also be emitted through groundwater heated by volcanic action.

The sources of volcanic gases on Earth include:

• primordial and recycled constituents from the Earth’s mantle,

• assimilated constituents from the Earth’s crust,

• groundwater and the Earth’s atmosphere.

Substances that may become gaseous or give off gases when heated are termed volatile substances.

Components of volcanic gases

The principal components of volcanic gases are water vapor (H₂O), carbon dioxide (CO₂), sulfur either as sulfur dioxide (SO₂) (high-temperature volcanic gases) or hydrogen sulfide (H₂S) (low-temperature volcanic gases), nitrogen, argon, helium, neon, methane, carbon monoxide and hydrogen.

Other compounds detected in volcanic gases are oxygen (meteoric), hydrogen chloride, hydrogen fluoride, hydrogen bromide, sulfur hexafluoride, carbonyl sulfide, and organic compounds. Exotic trace compounds include mercury, halocarbons (including CFCs), and halogen oxide radicals.

Percentage of different gases

The abundance of gases varies considerably from volcano to volcano, with volcanic activity and with tectonic setting.

Water vapour is consistently the most abundant volcanic gas, normally comprising more than 60% of total emissions. Carbon dioxide typically accounts for 10 to 40% of emissions.

Volcanoes located at convergent plate boundaries emit more water vapor and chlorine than volcanoes at hot spots or divergent plate boundaries. This is caused by the addition of seawater into magmas formed at subduction zones.

External link: https://openpress.usask.ca/physicalgeology/chapter/11-2-materials-produced-by-volcanic-eruptions/

PRACTICE QUESTIONS

QUES . Consider the following: UPSC 2024

1 . Pyroclastic debris

2 . Ash and dust

3 . Nitrogen compounds

4 . Sulphur compounds

How many of the above are products of volcanic eruptions?

(a) Only one

(b) Only two

(c) Only three

(d) All four

Show Answer