NOTES ON LIVING WITH TECTONIC HAZARDS (N levels)

1.      What are tectonic hazards?

Tectonic hazards are caused by movements in the Earth’s crust e.g. Earthquakes, Volcanic eruptions, Tsunamis.

2.   What is the internal structure of the Earth?

The crust is the outer layer of the earth and it varies in thickness from 5 to 70 km.

Mantle is the second layer of the earth's structure which is 2900 km thick. It is divided into upper mantle and lower mantle. The upper mantle is a layer of solid rock and asthenosphere. Below the uppermost mantle, rocks are close to melting point and easily deformed as the temperature is between 800⁰C and 3,000⁰C.Convection currents carry heat from the hot inner mantle to the cooler outer mantle. The outer core is about 2100 km made up of molten iron-nickel (liquid) and the inner core is about 1200km consists of solid iron nickel and is very hot and under great pressure. 3. What is the difference between the tectonic plates? 

    The crust is cracked into pieces called tectonic plates which float on the mantle. The tectonic plates are part of the lithosphere, which includes the crust and the uppermost mantle. There are two types tectonic plates: 

     Continental crust – found beneath the earth’s continental land masses and under shallow seas close to continents, it is thicker between 30 and 60km but is made of lighter rocks including granite. 

     Oceanic crust – found beneath deep oceans, it is thinner between 5 and 8km but it is heavier and denser as it consist mainly of basalt

4. How do the tectonic plates move?

Movement of crustal plates is driven by convection currents and slab-pull force.

Convection currents occur when materials in the mantle is heated by the core, causing the mantle material to expand, rise and spread out beneath the plates. This causes plates to be dragged along and to move away from each other. Then the hot mantle material cools and sinks, pulling the plates along. The sinking mantle materials heats up again as it nears the core and the whole process repeats.

Slab-pull force occurs when the denser oceanic plate is force beneath the less dense continental plate resulting in subuction. As the plate subducts, it pulls the rest of the plates along. The subducting plate drives the downward moving portion of convection currents. The mantle material, which is found away from where the plate subduct, drives the rising portion of convection currents.

5. What are the types of plate boundaries?

Types of plate boundaries and examples: 

·         Divergent:  where plates move away from each other  - oceanic-oceanic  e.g. Mid-Atlantic Ridge, continental-continental e.g. Great Rift Valley of East Africa

·         Convergent: where plates move towards each other - oceanic-oceanic  e.g. Mariana Trench, continental-continental e.g. Himalayas, oceanic-continental e.g. Andes

·         Transform: where plates move past each other e.g. San Andreas Fault between the Pacific plate and the North American Plate.

6. Describe the characteristics of landforms associated with plate movements.

     Oceanic-oceanic plate divergence:  Mid-oceanic ridge and Volcanic islandsE.g. The Mid-Atlantic Ridge

Eg: The Azores (Chain of volcanic islands)

As the plates move apart due to convection currents inside the Earth. magma rises from the mantle to fill the gap between the plates as they diverge.

New sea floor is formed when the magma cools and solidifies. This process is called sea-floor spreading.

Magma rises at the zone of divergence/spreading zone to form a ridge of new ocean floor called mid-oceanic ridge.

The newly formed (youngest) rocks are closest to the middle of the ridge/plate boundaries.

At various points along the ridge, magma builds up above the ocean to form volcanic islands.

Continental–continental plate divergence: Rift valleys and block mountains

E.g. East African Rift Valley

When two continental plates diverge, they are stretched, causing fractures to form at the plate boundary.

The land in between the two continental plates sink, forming a linear depression known as a rift valley.

A number of active volcanoes and earthquake fractures can also be found along the East African Rift Valley.

Oceanic–oceanic plate convergence: Trench and islands, volcanoesE.g. Mariana Trench, Mariana Islands

When two oceanic plates converge, one subducts under the other. A subduction zone forms, creating a deep oceanic trench. The subduction of the oceanic plate causes the solid mantle material to melt and

magma is formed. The magma rises through the mantle and ocean floor to emerge as volcanoes.  Eventually a chain or arc of islands called island arc is formed. Earthquakes may also occur.Continental-continental plate convergence: Fold mountains

E.g. Himalayas

Plates made largely of continental crust may collide with other plates made largely of continental crust.

However, both plates have similar densities and hence, resist subduction. Instead, the plates break, slide along fractures in the crust and fold, forming fold

mountains.

Oceanic-continental plate convergence: Oceanic trench, Fold mountains and VolcanoesE.g. Sunda Trench, Barisan Mountains

When an oceanic plate meets a continental plate, the denser oceanic plate subducts under the less dense continental plate.

A subduction zone forms, creating a deep oceanic trench along the plate boundary.  The subduction of the continental plate causes the soild mantle material to melt and

magma is formed. The magma rises through the mantle and crust to emerge as volcanoes on land. The edge of thick continental plate buckles to form fold mountains. Earthquakes may also occur.

Transform plate boundaries

E.g. San Andreas Fault, United States of America - In 1906, an earthquake occurred in San Francisco, southern California between the Pacific Plate and the North American Plate.  This caused several hundred km of North American Plate to move an average of 2.5 m, and at one point almost 7 m all in less than 1 minute

Plates slide past each other. As they do so, tremendous stress builds up. This stress is eventually released, often as a violent earthquake.

7. How is a fold mountain formed?

E.g. The Himalayas, the Rocky Mountains and the Andes

Fold mountains are formed along convergent plate boundaries. The compressional force causes the layers of rocks to buckle and fold. This process is known as folding. When there is increasing compressional force on one limb of a fold, the rocks may

buckle until a fracture forms.

1. Describe the formation of Rift valleys and block mountains.

E.g. East African Rift Valley

Rift valleys and block mountains are formed at divergent plate boundaries where plates are pulled apart giving rise to faults.

The tensional forces from these movements result in parts of the crust being fractured.

A block mountain is a block of land with steep sides. It is formed when sections of the crust extend along fault lines and rock masses surrounding a central block sink due to tensional forces.

A rift valley is a valley with steep sides formed along fault lines.

2. Describe the formation of volcano.

A volcano is a landform formed by magma ejected from the mantle onto the earth’s surface. Magma is molten rock found below the earth’s surface. Magma that is ejected onto the surface is known as lava. The lava cools and solidifies in layers and form a cone-shaped mountain called a volcano.

Volcanoes vary in shapes and sizes due to the characteristics of lava. Low silica lava has low viscosity (stickiness) while high-silica has high viscosity.

3. Differentiate between the shield volcano and the stratovolcano.

Shield volcano have gently sloping sides and a broad summit. It is made up of fluid lava with low silica content. It is more mobile and spreads away from the vent before it solidifies forming gentle concave slopes.

E.g. Mauna Loa (Hawaii), Mount Washington, United States of America

Stratovolcano is steeper at the top and gentler at the base. There are periodic violent eruptions with alternate layers of lava and ash accumulated.

E.g. Mt Fujiyama in Japan, Mt Pinatubo in Philippines, Mount Mayon, Philippines

4. Explain what is an active, dormant or extinct volcano?

Active volcanoes – volcanoes which are currently erupting or are expected to erupt in the future

Dormant volcanoes – volcanoes which are currently inactive but may erupt in near future

Extinct volcanoes – volcanoes without current seismic activity with no geological evidence of eruptions for the past thousands of years.

12.   Describe the distribution of volcanoes.

Mainly along the Pacific Ring of Fire which is the boundaries of several converging plates – Pacific Plate, Nazca Plate, the Philippines Plate and the Eurasian Plate.

Also found where plates are diverging e.g. Atlantic Ocean and East Africa

13. Explain the cause of earthquake.

Earthquakes occur when there is plate movement along plate boundaries. The plate movements cause the slow build-up of stress on the rocks found on either side of the fault. When the rocks can no longer withstand the increasing stress, they can suddenly slip many metres, causing an earthquake.The release of tension in the form of seismic waves made the ground vibrate. After an earthquake, a series of smaller earthquakes called aftershocks occur along the fault line.

 A seismograph is used for recording earthquake. The intensity of earthquake is measured on the Richter scale graded from 1 to 9. The higher the number on the Richter scale the greater the intensity of the earthquake.

14. Explain why some earthquakes are more damaging than others.

Some earthquake causes more damage because the intensity of the earthquake differs. Places located near to the epicentre will experience higher intensity of the earthquake and thus suffer the greatest amount of damage.

The damage also depends on the depth of its origin – a deep-focus earthquake ( 70-700km below earth surface) has a smaller impact on the land compared to a shallow-focus earthquake (upper 70km of earth crust) as seismic waves take a longer time to reach the surface and would have lost most of their energy by then,

The extent of damage also depends on the amount of development in the areas where earthquake takes place. An earthquake which struck a desert is less damaging than in a city.

The foundations of the buildings and bridges are also important because if the foundation is good, it will withstand the vibration. Developed areas suffer more damages as water and gas pipes broke. Urban areas are heavily built up with dense population densities and heavy traffic movements. As residents in big urban areas usually live in high rise buildings because land is scarce in the cities, damage to properties and loss of lives can be phenomenal as the high rise building collapse when earthquakes occur. As building collapse, other related hazards usually occur such as fire from damaged power lines. Destruction to highways, streets, flyover and bridges leads to widespread traffic congestion and commuters may be killed and hurt. Telephone line and power supply will be disrupted, and this will affect communication with outside world which in turn will hamper rescue work.

The strength of the earthquake also depends on the the geology of the epicentre. e.g. Mexico, built on layers of mud and sand, vibrates like jelly in the 1985 earthquake which killed 7000.

In Christchurch, many houses and buildings had to be abandoned because of liquefaction where the ground becomes unstable and saturated soil flows like a liquid after the earthquake in 2011.

The damage also depends on the level of preparedness and time of occurrence. Preparations such as having evacuation plans, trained rescue workers and other action plans can make the damage of an earthquake more manageable if the people are more prepared.  If the earthquake occurs when most people are sleeping, there is a higher chance that more deaths will occur as they are trapped in their houses. E.g. more than 2400 people died when an earthquake occurred after midnight in the Sun Moon Lake Region in Taiwan in 1999.

15. Explain the impacts and hazards associated with earthquakes.

Disruption of services - An earthquake can disrupt services such as the supply of electricity, gas and water. The earthquake in Kobe, Japan, in 1995 disrupted electricity, gas and water supplies to about a million of Kobe city’s 1.4 million residents.

Fires are started due to rupture gas pipes which provide fuel to start fires as well as exposing electrical cables which ignite flammable items. The earthquake in Kobe, Japan in 1995 caused extensive fires that raged on an off for 2 days and it spread quickly due to strong winds. The firemen were unable to control the fires as there was no water supply due to ruptured water pipes.

Landslides caused by earthquakes which weaken the slopes of hills and mountains due to the shaking of the ground. In 1970, an earthquake off the coast

of Peru triggered a massive landslide on the slopes of Mount Huascaranand destroyed the town of Ranrahirca killing 18000 people within seconds.

Destruction of properties – the earthquake in Tohoku, Japan in 2011, caused a tsunami which travelled up to 10km inland, causing extensive structural damage resulting in hundreds of thousands of people forced from their homes. There was a severe shortage of housing and long-term consequence on the health of people.

Destruction of infrastructure –earthquakes cause cracks to form in infrastructure such as roads and bridges. Transportation can be disrupted as it is unsafe to use the damaged roads.

Loss of lives and threat of tsunami

Aftershocks -there could still be aftershocks of lower magnitudes as there are adjustments to the repositioning of the fault. As many buildings are already weakened by the main shock, the aftershocks will cause more collapse and there will be more casualties.  Dead animals and corpse will start to rot and if not disposed quickly, there might be an outbreak of epidemic such as Malaria.

16. Explain the cause of tsunamis.

Tsunamis may be formed by: Movement of the sea floor during a large earthquake at the subduction zones An explosive underwater volcanic eruption An underwater landslide; A landslide triggered by earthquake or volcanic eruption which causes materials to

plunge into the water.

17. Describe the benefits and risks of living in volcanic areas.

Benefits of living in volcanic areas

Volcanic regions are often rich in sulphur deposits which can be mined for industrial use. E.g. In East Java, Indonesia, the sulphur collected is used to make matches and fertilizers, and refine sugar.

Basic lava often produces fertile soil after weathering which is suitable for cultivation. e.g. fertile volcanic soils in Java and Deccan Plateau in India

Geothermal power may be utilised for making steam to drive turbines and generate electricity e.g. over 70% of homes in Iceland are heated by volcanic steam

Volcanic areas offer spectacularly beautiful attractions for tourists. e.g Mt Fuji inJapan. Volcanic areas can be rich in history e.g. ruins of Pompeii in Italy where Mt Vesuvius erupted in 79 CE and buried the town. Every year, almost 3 million people visit the unearthed archaeological site which revealed buildings, pottery and mosaics left intact.

Volcanic ash can be used to surface roads and manufacture bricks

In some parts of the world, valuable materials such as gold, iron and diamonds have been formed by volcanic activity, and large mining centres have developed. The old

volcanic rocks at Kimberly in South Africa are one of the world’s richest sources of diamond.

Risks of living in volcanic areas

Volcanic eruptions claim many lives and destroy buildings and property. The lava, with high temperatures of between 500ºC and 1400 ºC burns the area it flows through. Volcanic bombs of heated rocks destroy property around the volcano e.g. eruption of Kilauea in Hawaii destroyed many homes and highway.

Poisonous gases such as compounds of sulphur, carbon monoxide and carbon dioxide are produced. Inhaling the hot ash and gases can result in serious injury or death.

Landslide can occur due to collapse of a volcanic cone. Landslides can obstruct the flow of rivers causing floods, block roads, and bury villages and farmlands. The eruption of Nevado del Ruiz in the Andes mountain of South America in 1985 caused lahars which killed more than 20000 people in the town of Armero.

Ash and volcanic dust ejected by volcanoes may be blown away to pollute the air and disrupt human activities over a large area from the volcano. It can block sunlight, suffocate crops and cause severe respiratory problems for people and animals. The eruption of Eyjafjallajökull in Iceland in 2010 resulted in the closure of air space over much of Europe as the volcanic particles pose a serious danger to aircraft engines and structures. Connecting flights worldwide were cancelled and delays to 1.2 million passengers daily cost the airline industry a total of US$1.8 billion.

When snow-capped volcanoes erupt, a sudden flash flood will also result from the melting of snow and ice. Mudflow may also be produced.

Sulphur dioxide released from volcanic eruption may react with water vapour and other chemicals in the atmosphere to form sulphur-based participles which can reflect the sun’ energy back into the atmosphere and temporarily cool the earth. The 1815 eruption of Mount Tambora in Indonesia cause the global temperatures to drop by as much as 1.7ºC.