VOLCANICITY AND EARTHQUAKES

·         Volcanoes are openings or cracks in the earth's surface that allow molten magma (or other material) to escape from the mantle beneath. Volcanoes are most well-known for releasing lava, but they may also release volcanic ash, rocks and gases.

Distribution of Volcanoes

·         As can be seen on the map below, the earth's active volcanoes are located in specific areas, including:

·         Around the edge of the Pacific Ocean

·         Down the centre of the Atlantic Ocean

·         In Southern Europe

·         Down the east coast of Africa

·         The reason for the distribution of volcanoes, is that they are located on or near tectonic plate boundaries, specifically destructive and constructive boundaries.

·         There are a few exceptions in the middle of the Pacific Ocean, noticeably on the islands of Hawaii. These volcanoes are caused by hotspots. The east coast of Africa is also not a plate boundary, but rather a plate (African Plate) ripping itself in half creating a rift valley lined by volcanoes.

·         Because volcanoes are normally found on plate boundaries their spatial concentration is limited. Their areal extent is also normally limited to areas immediately around the volcanoes, although volcanic ash clouds can potentially have global impacts by disrupting air travel and causing climate change.

·         Ring of fire: This is the name commonly given to the area around the Pacific Ocean. It gets its name because it has the biggest concentration of volcanoes.

·         Hot spots: These are volcanoes that are not found on plate boundaries. The most common explanation in mantle plume theory. This is when hot magma melts the crust above and escapes. Because tectonic plate are constantly moving, but mantle plumes stay stationary they normally create a chain of volcanoes e.g. the islands of Hawaii.

Key Volcanic Terminology

·         Lava: Molten rock above the surface of the earth.

·         Magma: Molten rock below the surface of the earth.

·         Magma Chamber: A store of magma found below the surface of the earth. When the pressure becomes to great in the magma chamber, volcanoes occur.

·         Vent: The main passage by which magma travels from the magma chamber to the crater. You can also get smaller secondary vents that often split off from the main vent.

·         Crater: A large hole or depression that has been created by a volcano. Lakes will often form in the bottom of lakes, they are known as crater lakes.

·         Eruption: A release of volcanic lava, ash or gas.

·         Active volcano: A volcano that regularly erupts and has erupted in recent history.

·         Dormant volcano: A volcano that has not erupted recently, but may erupt again in the future. It is unclear how long a volcano has to be dormant, before it is classified as dormant.

·         Extinct volcano: A volcano that will not erupt again in the future. It is unclear when an active or dormant volcano becomes extinct. Some people argue its is extinct if there is no reordered eruptions, others if it has not erupted for 10,000 years.

Volcanic Hazards

·         Volcanoes can cause multiple hazards (both primary and secondary hazards). Each hazard can have varying impacts. Below is a summary of volcanoes major hazards and their likely impact:

·         Primary Hazards: Hazards that are a direct result of the eruption and are caused by the released of substances during the eruption.

·         Lava Flow: The most commonly associated hazard with volcanoes. Lava flows are simply rivers of molten rock. Viscous (thick) lava flows are very slow, which means most lava flows can be avoided by humans. However, they can cause massive damage to land and property and trigger fires.

·         Tephra (Lava Bombs): Any material that is ejected from a volcano during an eruption. As long as you are standing a safe distance, humans should not be effected by tephra although they can damage buildings and start secondary fires.

·         Pyroclastic Flow: Probably the most dangerous of all volcanic hazards are pyroclastic flows which are superheated clouds of ash, gas and small tephra. They can travel at speeds up to 500km/hr and incinerate anything in their path.

·         Ash Cloud: Ash clouds are normally released into the atmosphere. Although they don't pose much immediate danger they can disrupt air travel and when the ash falls to ground it can crush buildings and bury farmland and also cause the secondary hazard of acid rain.

·         Poisonous Gases: Often released before a major eruption these gases can be deadly to animals and humans if inhaled in sufficient quantities.

·         Secondary Hazards: Hazards that happen as a result of primary hazards.

·         Lahar (mudslide): Volcanoes ash and/or lava can cause snow to melt or they can mix with river/rain water and create mudslides, commonly known as lahars.

·         Acid Rain: Gases released during an eruption e.g. sulphur dioxide can mix with water vapour in the atmosphere and create acid rain which can damage buildings and change the pH of soils and lakes killing plant and animal life.

·         Climate Change: Gases released into the atmosphere e.g. Sulphur dioxide can enhance the greenhouse effect causing global warming. However, ash released into the atmosphere can also absorb or reflect incoming solar radiation and reduce global temperatures.

·         Fires: Tephra and lava flows can start fires which can cause widespread damage to buildings and land.

·         Predicting and Measuring Volcanoes

·         Volcanoes are easier to predict than earthquakes, volcanologist can try and predict volcanoes by looking for:

ü  Changes in the shape of a volcano

ü  Changes in the amount of gas being released

ü  Changes in the temperature

ü  Tectonic activity (earthquakes)

ü  Animal behaviour

ü  Changes in local hydrology

ü  Mass movements

Because there are many types of volcanoes and types of volcanic hazard, it is hard to compare one volcano with another. However, one method that has been used is the volcano explosivity index (VEI) shown to the right.. The scale is open-ended and measured by looking at:

ü  Volume of material released in eruption

ü  Cloud height and

ü  Qualitative observations

All material released (tephra, ash, etc) is all treated the same when calculating volume. The largest volcano over recorded in history was an 8.0 (Yellowstone and Toba). Both of these eruptions were classified as super volcanoes with a frequency of 10,000 years.

Key Earthquake Terminology

ü  Epicentre: The location on the earth's surface directly above the hypocentre.

ü  Hypocentre (focus): The actual site/location that an earthquake takes place.

ü  Aftershock: A smaller earthquake that takes place in the coming hours, days and weeks after the main earthquake.

ü  Seismic Waves: These are waves of energy that travel through the earth as a result of an earthquake. There are two types of waves; body waves that travel through the earth and can be divided into p-waves (more longitudinal) and s-waves (more transverse) and surface waves that travel across the surface.

ü  Tremor: A tremor is another name for an earthquake, but is all sometimes the name given to a lesser earthquake or the felt effects of a big earthquake by people living further from the epicentre.

Earthquake Hazards

Primary Hazards

Ground Shaking: The movements of the ground caused by the seismic waves can fell buildings, bridges, trees, etc. killing and trapping people.

Secondary Hazards

Tsunamis: The sudden shifting of tectonic plates under the sea can displace large amounts of water which can trigger massive tsunamis. The 2011 Japanese tsunami and the 2004 Indian Ocean tsunami both caused more deaths than the earthquakes that triggered them.

Fire: Earthquakes can break gas cables and knock over ovens and open fires which trigger secondary uncontrolled fires.

Liquefaction: This is where a saturated soil loses strength and rigidity because of applied stress, normally an earthquake. The changes in its state causes the ground to behave like water allowing things to sink into it. The recent earthquake in Christchurch New Zealand saw large scale liquefaction

Mass movements (landslides): The sudden movement of the earth and subsequent seismic waves can trigger landslides and avalanches which can bury and kill many people. The El Salvador earthquake of 2001 triggered a landslide in Santa Tecla which killed hundreds of people.

Floods: damage to flood defenses or even dams can cause widespread flooding.

Factors Affecting the Impact of Earthquakes

Depth: If the hypocenter of an earthquake is close to the surface then it is more likely to cause greater damage than a deep earthquake.

Duration: A longer earthquake is likely to cause greater damage than an earthquake that lasts only a few seconds.

Magnitude: Obviously a stronger earthquake is going to have a greater impact than a weaker one.

Time of Day: Time of day can be important. If people are sleeping and get trapped in their beds more people can be killed. In Japan an earthquake that struck while people were cooking their evening dinner caused widespread secondary hazards (fire) that caused more deaths.

Epicentre Location: If the epicentre of an earthquake is an uninhabited region it is going to have a lesser effect than one under a densely populated city.

Geology: If an earthquake occurs in solid bedrock it is likely to cause less damage than one centred below an alluvial floodplain which may lead to liquefaction.

Economic Development (buildings, planning, preparedness): Generally speak more developed countries have better zonal planning, building codes and preparedness mean the effects of the earthquake are less.

Can humans cause an earthquake?

Predicting and Measuring Earthquakes

Earthquakes are extremely hard to predict. Scientists can normally predict where earthquakes are likely to happen, but they can not predict when they will happen and how strong they will be. Scientists can attempt to predict by looking at:

ü  Microearthquakes

ü  Changes in rock stress

ü  Ground subsidence, uplift or tilt

ü  Changes in magnetic field and electrical resistivity of rocks

ü  Animal behaviour

ü  Seismic history

Richter Scale: The Richter Scale was developed by Charles Richter in 1935. It uses a base 10 logarithmic scale. The scale is normally seen from 0-10, but in theory could go above this. The scale measures the amplitude of waves on a seismograph. An earthquake of 5.0 is ten times stronger than one of 4.0. The largest earthquake ever recorded was a 9.5 of the coast of Chile in 1960.

Seismograph: The name commonly given to seismometers. It records movements in the earth caused by seismic waves. A picture of a seismograph is shown to the right.

Mercalli Scale: Instead of measuring an earthquakes energy like the Richter scale the Mercalli scale looks at the effects of an earthquake. The scale goes from 1 (hardly felt) up to 12 (total destruction). The scale is obtained by looking at the effects on humans, nature and structures.

MAJOR LANDFORMS

We all know that one – fourth of the earth’s surface is covered by land. The portion which forms land on earth’s surface is not the same everywhere. At some places the land may be too high, at some places very low, some areas would be lush green and certain areas are dry and barren. Our planet earth is a beautiful collaboration of various physical features.

These different physical features are called the various landforms on the surface of the earth. These are geographical features that control the ecosystem, climate, weather and the essence of life on earth. In simple terms, we say that any shape on the earth’s surface is known as a landform. The various landforms that we have, came into existence due to natural processes such as erosion, wind, rain, weather conditions such as ice, frost and chemical actions. Natural events and disasters such as earthquakes (the tectonic plates) and eruption of volcanoes created the various shapes of the land that we see.

The different major landforms are mountains, hills, valleys, plateaus, plains and deserts.

Facts about Mountains

·         A mountain is the highest landform on the surface of the earth. It is usually found to be conical in shape with steep sides and a pointed tip called a peak.

·         As compared to their surroundings, mountains are high points on the surface of the earth.

·         Mountain range is a series of mountains.

·         Mountains could be steep and snow covered or they could be gently sloping having rounded tops.

·         The highest mountain range in the world is the Himalayas. Some mountains are found under the sea and could be taller than the Mount Everest, which is the highest mountain peak in the world.

4 Types of Mountains

Ø  Volcanic Mountains

Ø  Fold Mountains

Ø  Block Mountains

Ø  Residual Mountains

·         Mountains could be formed when molten rocks from deep within the earth rise to the surface, pouring out in the form of lava from volcanoes.

·         Sometimes the tectonic plates on the earth’s crust move towards each other, the sediments deep below the earth’s surface are squeezed up to form mountain ranges.

·         There are many mountains that remain covered with snow throughout the year. These mountains are very cold and hence there is not much vegetation or life found in these hills. Trees like pine and conifer are found in the lower ranges or foothills.

·         Animals that have a thick fur coat can survive the extreme cold in the high mountain regions. The yak, the mountain puma, snow leopard or the male goat called the ibex are some of the animals found in the mountain areas.

·         Houses in the mountains have sloping roofs to enable the snow slide off easily. The houses are made of wood so that they remain warm. People usually work in small industrial units, farming and animal rearing form their main occupations.

·         Even besides harsh conditions, mountains are very useful to us as they act as shields for the country blocking the cold winds and also protect us from invading enemies.

·         Trees provide us with commercial and medicinal value.

·         Melting snow from the snowcapped mountains fills the rivers and they are a source of water.

·         They make beautiful tourist destinations.

Facts about Hills

·         Hills are lower than mountains but are higher than their surrounding areas.

·         Hills are lower in height than mountains, but they are higher than the surrounding areas. A number of hills together form a ‘range of hills’. Hills are usually covered with grass.

·         The climate in the hills is more pleasant than the climate in high snow covered mountains. It is usually neither too hot nor too cold. They make perfect tourist destinations.

·         The vegetation is thick, beautiful fruit orchards are found in the hills and it is good for crop cultivation like tea and coffee.

Facts about Valleys

·         Valleys are the low-lying areas between two mountains or hills.

·         When rivers flow down the mountainsides and hillsides, it wears off the rocks and soil. Over a period of time, the water carves out v- shaped grooves. These grooves get deeper and wider, finally forming low land areas called valleys.

·         Valleys formed by glaciers are U- shaped valleys.

·         Valleys formed due to the effect of erosion are V – shaped valleys.

·         The valleys formed (that is V shaped or U shaped), depend upon the rate at which deepening and widening takes place.

·         Narrow valleys are called canyons.

·         The climate in the valleys is pleasant and favourable for living, hence many civilizations in ancient times were found in valleys where there were rivers flowing making water available for the people.

·         For example The Indus Valley Civilization that came up near the River Indus.

·         Due to ample water that is provided by the rivers and fertile soil, the vegetation is thick and valleys look green and beautiful.

·         They make great tourist destinations as well.

·         In Young Mountain areas the valleys found are steep sided.

Facts about Plateaus

·         A plateau is a flat topped highland with steep sides. Since it looks like a table, it is also called a tableland. They are basically areas of high flat land.

·         Plateaus are usually surrounded by steep rock faces called cliffs.

·         Some plateaus like the Plateau of Tibet lies between mountain ranges.

·         Plateaus are usually good for growing certain crops.

·         Plateaus are formed when magma pushes up towards the surface of the earth’s crust. This magma does not break through but it raises a portion of the crust up creating a plateau.

·         There are certain kinds of plateaus like the butte and the mesa. These are special kinds of plateaus.

Facts about Plains

·         Plains as you all are familiar with are areas of flat land.

·         The plains usually meet the oceans or seas, these are called coastal plains.

·         In India, we have the Eastern Coastal Plains and the Western Coastal Plains.

·         Some plains are formed by the action of rivers, these are called river plains.

·         In India the Northern Gangetic Plain is a river plain.

·         River plains are very fertile and good for growing crops.

·         You will find most big cities are located in plains. This is because it is easier to build houses, buildings, roads and other structures in the plains. Hence they are heavily populated.

Facts about Islands

·         An island is a piece of land surrounded by water on all sides.

·         The continent of Australia is an island.

·         Islands are formed due to volcanic activity or due to hot spots in the lithosphere.

·         Coral islands are formed when the skeletal material of the corals piles up over a long period of time. These look beautiful.

·         A large group of islands close to each other together form an archipelago. The Lakshadweep islands are an example of an archipelago. The largest archipelago in the world is Indonesia.

Deserts : Facts and Types

·         Deserts are large, dry and hot areas of land which receive little or no rainfall throughout the year. The vegetation is scanty due to the shortage of water. Deserts are covered with sand.

·         Sand dunes are formed in deserts. Sand dunes are huge hills of sand formed by the winds.

·         Deserts have extreme weather conditions, days could be very hot and nights very cold. This is because the sand absorbs heat fast during the day and gives off heat quickly at night.

·         The main vegetation found in the deserts are the cacti and the baobab trees.

·         The baobab tree can store nearly up to 1000 litres of water in its trunk which enables it to survive the harsh conditions.

·         There are two types of deserts – Hot Deserts and Cold Deserts.

Hot Deserts

·         As the name suggests, hot deserts are vast areas of land that are covered with sand and dust. These areas receive little or no rainfall and are very dry.

·         The animals found in the hot deserts are camels, snakes, lizards and rats.

·         Thar Desert in India is a hot desert.

Cold Deserts

·         The cold deserts are large areas of land covered with snow. These deserts receive little or no rainfall. They receive snowfall during the winters. Animals such as penguins, whales and fur seals survive in the cold deserts.

·         The Antarctica is the world’s biggest cold desert.

·         Life in these cold deserts is impossible.

·         There are some rocky deserts like the Gobi desert in Asia.

·         Some other Landforms

·         Peninsula

·         A peninsula is a piece of land that is surrounded by water from three sides. For example the southern part of India is a peninsula as it is surrounded by the Arabian sea, Bay of Bengal and the Indian ocean and is joined to land on the fourth side.

STRUCTURE OF THE EARTH

structure of the earth

The Earth consists of four concentric layers: inner core, outer core, mantle and crust. The crust is made up of tectonic plates, which are in constant motion. Earthquakes and volcanoes are most likely to occur at plate boundaries.

The structure of the Earth

 The Earth is made up of four distinct layers:

•          The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C. With its immense heat energy, the inner core is like the engine room of the Earth.
•       The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel. It is still extremely hot, with temperatures similar to the inner core.
•          The mantle is the widest section of the Earth. It has a thickness of approximately 2,900 km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down the rock is soft and beginning to melt.
•          The crust is the outer layer of the earth. It is a thin layer between 0-60 km thick. The crust is the solid rock layer upon which we live.
•    There are two different types of crust: continental crust, which carries land, and oceanic crust, which carries water.
•    The diagram below shows the structure of the earth. In geography, taking a slice through a structure to see inside is called a cross section

Distribution

•       The Earth's crust is broken up into pieces called plates. Heat rising and falling inside the mantle creates convection currents generated by radioactive decay in the core. The convection currents move the plates. Where convection currents diverge near the Earth's crust, plates move apart. Where convection currents converge, plates move towards each other. The movement of the plates, and the activity inside the Earth, is called plate tectonics.
•     Plate tectonics cause earthquakes and volcanoes. The point where two plates meet is called a plate boundary. Earthquakes and volcanoes are most likely to occur either on or near plate boundaries. 

Different plate boundaries

•          At a tensional, constructive or divergent boundary the plates move apart.
•          At a compressional, destructive or convergent boundary the plates move towards each other.
•          At a conservative or transform boundary the plates slide past each other.
• Different exam boards and textbooks may use different names for each of the boundary types. For example, a destructive boundary may also be called a collision boundary. Use any term so long as you use it correctly, but it is best to stick to the terms you have been taught.

The diagram above shows the zone of subduction, oceanic ridge and an oceanic trench.

rocks

Ø  There are three main classes of Rocks. They are classified according to how they originated.

Ø   Igneous rocks form from cooling bodies of magma.

Ø  Over time, various weathering processes erode these rocks and the resulting particles or chemicals settle into beds and are compressed and cemented into sedimentary rocks.

Ø  If these rocks are buried, heated and highly compressed they will be made into metamorphic rocks.

Ø  If these rocks continue to be heated and compressed to the point that they melt, then the molten rock might eventually form another igneous rock.  This is called the rock cycle.  It forms a complete circle as one rock can be turned into another.  They can even form different rocks of their own class.  A sedimentary rock such as a sandstone can be weathered and eroded and those fragments might eventually end up as part of a shale, a different sedimentary rock.

Ø  Understanding rocks, their origins and classifications, is important to understanding the minerals that are found in them, found on them or are a part of them. Below are the major types of rocks: Rocks are made of minerals. They can be made of a single mineral or a combination of several minerals.

There are three main types of rocks:

·         Igneous (IG-nee-us) rocks are formed from hot, molten rock, called magma. Granite and basalt are examples of igneous rocks. These are first stage rocks or primary rocks because they were the first to be formed. These rocks are divided into plutonic and volcanic also known as intrusive and extrusive.

Ø  Plutonic/ Intrusive rocks are formed through the solidification of magma inside the earth’s crust. These rocks have large crystals and take time to solidify.

Ø  Volcanic/ Extrusive rocks are formed due to the solidification of magma outside of the earth’s crust. These have small crystals and quick to solidify.

·         Sedimentary rocks are formed from bits and pieces of rock which have broken off. These bits and pieces are called sediments. The sediments were carried by water and wind and dropped to a new location. The sediments are piled layer upon layer and pressed or cemented together to form a new kind of rock. The most common sedimentary rocks are sandstone, limestone, shale, and conglomerates. These are secondary rocks because they were formed from the sediments rom weathered material from other rocks.

Ø  These rocks are divided into mechanically, organically and chemically formed types.

Ø  Mechanically formed

Ø  These have been formed through the sedimentation of fine silt (sand) to formed layered rocks which are hard examples include sand stone.

Ø  Organically formed these were formed from the sedimentation of plant and animal remains that have been cemented by weather elements.

Ø  Rocks that originate from plants include coal and those that originate from animal include chalk.

Ø  Chemically formed rocks are as a result of chemical reaction of minerals that have been deposited by water which were dissolved from other rocks to form other rocks these include potash and gypsum.     

·         Metamorphic rocks are igneous or sedimentary rocks which have been changed by heat, pressure or chemical action. Examples of metamorphic rocks are slate which is formed from shale, and marble which is formed from limestone.

FOLDING AND FAULTING

• v  Folding is the bending of the earth’s crust due to compressional forces to form up-folds and down-folds called anticlines and synclines respectively
• v  Faulting is the cracking or breaking of the earth’s crust or sedimentary rocks due to compressional and tensional forces.

A geological fold occurs when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Syn-sedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales