Category The Earth, Earth Science, Planet Earth

Steep-sided, cone-shaped andesitic volcanoes are formed by melted plates exploding to the surface. These types of volcano are extremely violent, and their eruptions are very destructive. Basaltic volcanoes form where molten rock rises slowly to the surface from the mantle. They are broad and low and when they break the surface they can spray their lava into the air, producing blobs of lava known as volcanic bombs.

There are three main types of volcano – composite or strato, shield and dome.

Composite Volcanoes

Composite volcanoes, sometimes known as strato volcanoes, are steep sided cones formed from layers of ash and [lava] flows. The eruptions from these volcanoes may be a pyroclastic flow rather than a flow of lava. A pyroclastic flow is a superheated mixture of hot steam, ash, rock and dust. A pyroclastic flow can travel down the side of a volcano at very high speeds with temperatures over 400 degrees celsius. Composite volcanoes can rise over 8000 feet.

When composite volcanoes erupt they are explosive and pose a threat to nearby life and property. Eruptions are explosive due to the thick, highly viscous lava that is produced by composite cone volcanoes. This viscous lava has a lot to do with why they are shaped the way they are. The thick lava cannot travel far down the slope of the volcano before it cools.

Composite volcanoes are usually found at destructive plate margins. Examples of composite volcanoes include Mount Fuji (Japan), Mount St Helens (USA) and Mount Pinatubo (Philippines).

Shield Volcanoes

Shield volcanoes are low with gently sloping sides and are formed from layers of lava. Eruptions are typically non-explosive. Shield volcanoes produce fast flowing fluid [lava] that can flow for many miles. Eruptions tend to be frequent but relatively gentle. Although these eruptions destroy property, death or injury to humans rarely occurs.

Shield volcanoes are usually found at constructive boundaries and sometimes at volcanic hotspots. Examples of shield volcanoes include Mount Kilauea and Maunaloa on Hawaii.

Dome (Acid Lava Cones)

Acid [lava] is much thicker than [lava] which flows from shield volcanoes. Dome volcanoes have much steeper sides than shield volcanoes. This is because the lava is thick and sticky. It cannot flow very far before ot cools and hardens. An example is Puy de Dome in the Auvergne region of France which last erupted over 1 million years ago.

Picture Credit : Google

Volcanoes erupt when molten rock, known as magma, is forced to the Earth’s surface by the movement of the Earth’s tectonic plates. Sometimes a volcano explodes, sending thick clouds of ash high into the atmosphere. Other volcanic eruptions produce rivers of red-hot lava that flow over the landscape covering everything in their path. Whichever way a volcano erupts, it is one of the natural world’s most powerful and destructive forces.

Many of us only notice volcanoes when they are about to explode or disrupt our travel plans, but these spectacular forces of nature can have a significant impact on people living in the local area. While volcanoes can be destructive, they are also responsible for creating rich agricultural soil, minerals like gold and silver, diamonds, hot springs and geothermal energy.

A volcano is like a chimney that allows hot liquid rock, called magma, to flow from a layer within the Earth and erupt onto the surface. The magma can come from as far down as 200 kilometres in the mantle and once it erupts — at a piping hot 700 to 1,200 degrees Celsius — it is called lava.

As magma rises through many kilometres to the Earth’s surface, dissolved gases contained within it form expanding bubbles. These bubbles increase the pressure of the magma and, if this pressure is great enough, the volcano will erupt. The amount, temperature and composition of magma, including the amount of trapped gas contained in it, determines the type of volcano formed. The three most common large types of volcanoes are strato, shield and caldera.

Strato volcanoes are cone-shaped mountains that have been built up from layers of ash and lava. They are generally the tallest type of volcano and are known for their violent explosions. Bubbles of gas build up in the magma — which has a high silica content — and explode creating volcanic ash, consisting of tiny gritty sharp fragments of glassy snap-frozen magma and rock from the sides of the volcano vent.

Examples of strato volcanoes include Agung in Bali, Yasur in Vanuatu, Etna in Italy and Fuji in Japan.

Picture Credit : Google

Modern building technologies mean that homes, offices and other buildings can be designed to withstand the effect of an earthquake. Tall buildings are built with a strong central column from which the structure “hangs”. Conical or triangular designs are able to absorb shocks more easily, while the use of new materials allows buildings to be constructed in earthquake zones at a relatively low cost.

After the massive earthquake near Japan one wonders if it’s possible to build an earthquake-proof building. The answer is yes and no. There are of course, engineering techniques that can be used to create a very sound structure that will endure a modest or even strong quake. However, during a very strong earthquake, even the best engineered building may suffer severe damage. Engineers design buildings to withstand as much sideways motion as possible in order to minimize damage to the structure and give the occupants time to get out safely.

Buildings are basically designed to support a vertical load in order to support the walls, roof and all the stuff inside to keep them standing. Earthquakes present a lateral, or sideways, load to the building structure that is a bit more complicated to account for. One way to make a simple structure more resistant to these lateral forces is to tie the walls, floor, roof, and foundations into a rigid box that holds together when shaken by a quake.

The most dangerous building construction, from an earthquake point of view, is unreinforced brick or concrete block.  Generally, this type of construction has walls that are made of bricks stacked on top of each other and held together with mortar.  The roof is laid across the top.  The weight of the roof is carried straight down through the wall to the foundation.  When this type of construction is subject to a lateral force from an earthquake the walls tip over or crumble and the roof falls in like a house of cards.

Construction techniques can have a huge impact on the death toll from earthquakes. An 8.8-magnitude earthquake in Chile in 2010 killed more than 700 people. On January 12, 2010, a less powerful earthquake, measuring 7.0, killed more than 200,000 in Haiti.

The difference in those death tolls comes from building construction and technology. In Haiti, the buildings were constructed quickly and cheaply. Chile, a richer and more industrialized nation, adheres to more stringent building codes.

Picture Credit : Google

Perhaps the world’s best known fault line is the San Andreas Fault. Situated in California, USA, it is an area of the world where earthquakes and tremors occur frequently. The citizens of San Francisco know that a very powerful quake (often referred to as “The Big One”) could occur at any time.

Viewed from space, the San Andreas Fault looks like a long, narrow valley that marks where the North America plate meets the Pacific plate. This narrow break between the two plates is called a fault. But viewed up close, there are actually many fractures and faults that mark the zone where the two plates slide past one each other. Sometimes the boundary is a zone of several smaller faults, one or more of which may break during an earthquake. Sometimes it is a single fault. 

On the ground, one can find the San Andreas Fault by looking for landforms it created. For example, sharp cliffs called scarps form when the two sides of the fault slide past each other during earthquakes. “The dominant motion along the fault is primarily horizontal, but some areas also have vertical motion,” noted Shimon Wdowinski, a geophysicist at the University of Miami’s Rosentiel School of Marine & Atmospheric Sciences who has studied the San Andreas Fault. And stream channels with sharp jogs — the channels are offset across the fault line — can be visited in the central California’s Carrizo Plain National Monument.

On the west side of the fault sits most of California’s population, riding the Pacific Plate northwest while the rest of North America inches south. The Pacific Plate is moving to the northwest at 3 inches (8 centimeters) each year, and the North American Plate is heading south at about 1 inch (2.3 cm) per year.

The San Andreas Fault was born about 30 million years ago in California, when the Pacific Plate and the North America plate first met. Before then, another oceanic plate, the Farallon plate, was disappearing beneath North America at a subduction zone, another type of plate boundary. The new configuration meant the two plates slid past one another instead of crashing into each other, a boundary called a strike-slip fault.

Researchers have measured identical rocks offset by 150 miles (241 kilometers) across either side of the fault. For example, the volcanic rocks in Pinnacles National Park south of Monterey match volcanic rocks in Los Angeles County (called the Neenach volcanics). Geologists think the total amount of displacement along the fault is at least 350 miles (563 km) since it formed.

Picture Credit : Google

The shock wave of a powerful earthquake can easily destroy buildings and other structures, but there are some side-effects of the quake itself. Underground gas pipes may rupture, leading to serious fires and explosions. The health of survivors is but at risk by damaged sewerage systems allowing disease to spread. In mountainous areas, landslides or avalanches can be triggered, and an undersea earthquake can generate a huge wave called a tsunami.

An earthquake is a sudden shaking movement of the surface of the earth. It is known as a quake, tremblor or tremor. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.

So far, there have been sixty-two earthquakes in India. The first recorded earthquake in India was on 6th June 1505 it occurred in Saldang, Karnali zone. And the most recent one happened in India as on 31st January 2018 and occurred in Kashmir, Pakistan, Afghanistan, and Tajikistan.

An earthquake is measured on Richter’s scale. A seismometer detects the vibrations caused by an earthquake. It plots these vibrations on a seismograph. The strength, or magnitude, of an earthquake, is measured using the Richter scale. Quakes measuring around 7 or 8 on the Richter scale can be devastating.

Most earthquake-related deaths are caused by the collapse of structures and the construction practices play a tremendous role in the death toll of an earthquake. In southern Italy in 1909 more than 100,000 people perished in an earthquake that struck the region. Almost half of the people living in the region of Messina were killed due to the easily collapsible structures that dominated the villages of the region. A larger earthquake that struck San Francisco three years earlier had killed fewer people (about 700) because building construction practices were different type (predominantly wood). Survival rates in the San Francisco earthquake was about 98%, that in the Messina earthquake was between 33% and 45%) (Zebrowski, 1997). Building practices can make all the difference in earthquakes, even a moderate rupture beneath a city with structures unprepared for shaking can produce tens of thousands of casualties.

Although probably the most important, direct shaking effects are not the only hazard associated with earthquakes, other effects such as landslides, liquefaction, and tsunamis have also played important part in destruction produced by earthquakes.

Picture Credit : Google