VOLCANOES - ENGINEERING GEOLOGY (StudyCivilEngg.com)

 VOLCANOES

SUBJECT: ENGINEERING GEOLOGY

Below are the topics covered in this post

• Volcanoes - Introduction and details
• Classification of Volcanoes
• Style or Modes of Eruption
• Types of Eruptions
• Periodicity
• Topography
• Products of Volcanoes
• Gaseous
• Liquid
• Solid

VOLCANO

• Volcanoes are the most spectacular and complex feature on the earth's surface. These are one of the oldest recorded volcanoes. The term 'volcano' is derived from the Latin word 'Vulcan' - the Roman God of Fire.
• Volcanoes along with earthquakes are the geological hazards of endogenic in origin. It can be said that earthquakes result from the movement of solid rocks; Volcanism is the process of movement of liquid rock (molten magma).
• A volcano behaves like a great chimney, conducting material from shallow depths in the earth to the surface. When a volcano erupts, hot liquid rock called lava, gases and rock fragments are flown onto the surface of the earth through its opening. The 'chimney' may take the form of a tall mountain cone. Volcanoes may also appear as gently sloping domes or flat lying high grounds.
• A volcanic eruption starts deep down in the mantle. Here, the rocks melts to form magma, a mixture of volcanic gases dissolved in liquid lava. Underground pressure from the weight of the surrounding rock (lithostatic pressure) forces the magma towards the surface.
• As the magma moves up, the pressure get reduced and the gases start to bubble out the liquid. The gases consist primarily of steam, most of which comes from water seeping through the rocks of the upper crust to meet the upwelling magma. The molten material also contains carbon dioxide, nitrogen, sulphur dioxide and small quantities of gases like hydrogen sulphide and hydrogen chloride.
• Finally, magma finds its way to the surface in a volcanic eruption; either through a vent or a fissure or by forcing its way up through new cracks in the crust. In some volcanic eruptions, the upper magma is only forced out and the magma immediately beneath degases explosively. As a result, there may not be one big explosion, but a series of continual blasts, each lasting a few seconds to minutes or over several days. Degassed magma may be poured out of the volcano as lava which, depending upon viscosity, output and slope of ground, can flow from a few metres to several hundred kilometres from the vent. All lava soon solidifies to form volcanic rocks (time for cooling varies from days to years).
• The energy of a volcanic event can be explained into four different parts, as follows:Energy released by volcanic tremors and earthquakes;
• Energy required to rupture the overburden;
• Energy expended in ejecting material; and
• Energy expended in creating atmospheric waves and rarely, Tsunamis.
• Although volcanoes can erupt with awesome fury and can destroy great properties and lives, their long term losses are lesser as compared with the losses due to other hazards. Furthermore, volcanic activity can be beneficial to human beings or to the society. Soils evolved from vulcanism are very fertile. Volcanic activity can be the supply of water and geothermal energy. They are also the sources of many minerals.

CLASSIFICATION OF VOLCANOES

Volcanoes can be classified on the basis of style and types of eruption, periodicity, topography etc.

Style or Mode of Eruption

Two different modes of eruption are recognised. They are namely central eruption and fissure eruption.

• Central Eruption: In this, the molten material is injected through an existing vent. As a result, rocks formed after cooling develop a gentle radial slope away from centre e.g. St. Helena, Hawaii Islands.
• Fissure Eruption: In this, the molten material is injected through fissures. Thus, thin tabular horizontal lava flows are developed. Deccan, Trap Basalts is the best example of fissure type of eruption. This type of eruption is often indicated by the presence of feeder dykes

Types of Eruptions

In this, volcanoes are classified on the basis of severity of their eruptions and degree of explosivity. The names are derived from the geographic location of the volcano that exemplifies the type of the activity.

• Pelean : Mt. Pelee, Martinique, an extreme case of eruption of viscous magma, form hot gas cloud.
• Vesuvian : Mt. Vesivi, Italy, an intermediate in explosiveness but michides high quantity of ash. As a result, a vast cloud develops.
• Strombolian : Stromboli, Italy. Minor explosions throw out lava which cools into pyroclasts.
• Hawaiian : Hawaii Island. Fluid lava outpouring essentially from craters and builds up huge shield volcanoes.
• Icelandic : Iceland, Quiet eruption of lava essentially through fissures and builds up horizontal lava flows.

Periodicity

Based on their period of activity, three major types of volcano are recognised.

• Active: If eruption has occurred during historic times yielding catastrophic results, then the volcanoes are known as active. For example Mount Etna. This type of activity creates a cinder cone, as fragments of solidified lava pile up around the vent. However, some magma is forced into the surrounding rock, thus forms sill, laccolith, lopolith, dykes etc.
• Dormant: Also called as Inactive. These can remain dormant or quiet for hundreds or thousands of years. The vents of a dormant or extinct volcano are plugged by lava. However, pressure may build under a dormant volcano causing it to erupt after a long period of inactivity. For example : Mount Fusiyama/Fujiyama.
• Dead: In this, volcanic activity is completely ceased and no further eruptions can be expected from this.

Topography

Four major categories may be recognised -

• Cinder Cones : These are normal steep sided piles of pyroclastic materials with a central vent. Cinder cones may be rapidly formed and may attain an elevation of 400 metres.
• Shield Volcanoes : These are domal in shape with very gentle slopes and are characteristic by basaltic lava flows. For example, Mauna Loa, Hawaii Island. The molten material is mobile and commonly produces quiet eruptions.
• Composite Volcanoes : They are symmetrical in outline with very steep walls. Composite volcanoes are also called as strato volcanoes. The molten material is highly viscous and possess capacity to retain gases. As a result, the eruptions are highly explosive. For example, St. Helenas. Commonly, the slopes are made of alternate layers of andesites and pyroclasts.
• Volcanic Domes : In this, acidic magma is highly viscous and can not flow for longer distance. It tries to solidify near the vent volcanic domes, may be intrusive or extrusive in origin. However, it must be mentioned that the classification of observed volcanoes on the basis of topography is difficult to apply

PRODUCTS OF VOLCANO

Volcanic eruptions vary from a relatively quiet outpouring of lava to violent explosions accompanied by showers of volcanic debris. Thus, based on the states of matter, the volcanic products can be classified into three types, viz. gaseous, liquid and solid.

Gaseous

The gaseous products are the main cause for the violent eruption of the volcanoes and at all stages; the volcanicity is associated with emission of steam and gases from the vents, fissures or cracks. The most abundant volcanic gas is steam which may vary from 60 to 90 % of the total gaseous content. The next in abundance are carbon dioxide, nitrogen and sulphur, with minor amounts of carbon monoxide, hydrogen, chlorine and fluorine. The steam is mainly derived from the magma, which is molten rock matter containing about 0.8 % of water (H20) and partly from the ground water or the crater lakes, whereas carbon dioxide (COz) is derived from limestone. The source for the other gases is the magma itself.

Liquid

The liquid products extruded during a volcanic eruption mainly consist of a molten and hot magma that forms the volcanic rocks on cooling. The hot magma erupted on the surface is known as lava after escape of its gases. The initial temperature of the lava may vary from 600°C to 1200°C. Based on the chemical composition of the lava they are classified as acidic, intermediate or basic. The acidic lava has a silica content of 60% or more and is very viscous. Due to their low mobility, they solidify near the volcanic vents. These lavas do not allow the gases to escape easily through them and therefore, when it cools they form block-like structures called as 'blocky lavas' or in Hawaiian terminology, they are called as Aa Aa lava. The lavas of intermediate composition are rare and have silica content of 55% to 60%. The basic lavas have a silica content less than 55% and their viscosity being very low, they are more mobile and therefore, are able to flow for a long distance forming thick sheets of lava e.g., plateau basalts. Many a times, they show a ropy structure, which in Hawaiian terminology is called as pahoehoe lava.

Solid

The solid products of volcano consist of rock fragments of already cooled lava, along with the country-rock fragments. The terms pyroclastic material or tephra are used to describe solid volcanic products of various sizes. The term pyroclastic literally means 'fire broken'. Depending on the size of the solid fragments these are described differently as follows.

• The angular particles greater than 32 mm in diameter are known as blocks, whereas the elliptical or spindle shaped fragments are called as volcanic bombs.
• The particles in the range of 4-32 mm are called lapilli if they are not vesicular, but are termed as cinders when highly vesicular.When the grain size is about 4 mm then the fragments are called as volcanic sand. The fine fragments which are less than 4 mm constitute the ash.
• Sometimes the volcanic ash is blown into the atmosphere and it travels to great height and distances before settling down. Such solidified deposits are known as a tuff.
• The solid and large angular fragments, when compacted in the atmosphere, give rise to volcanic breccia.

Glassy solidified lava is called obsidian while the magma that has trapped bubbles of steam or gases solidifies as a porous pumice deposit.

OTHER SUBJECTS

• SURVEYING - STUDY ONLINE
• BUILDING MATERIALS AND CONSTRUCTION - STUDY ONLINE
• FLUID MECHANICS - STUDY ONLINE
• SOIL MECHANICS - STUDY ONLINE