PLATE TECTONICS

EARTHQUAKES:

• They occur in long narrow belts. This belts include those which:

• 
  Encircle the whole of the pacific ocean

• Extend down the entire length of the mid-Atlantic Ocean

• Stretch across the continents of Europe and Asia, linking the Atlantic and the Pacific Oceans.

• Extend into the Pacific Ocean from the west coast of South America.

• Earthquakes occur where two areas of the earth's crust try to move in different directions. If friction prevents movement between these two areas then there will be a lot of pressure. When friction is released by a sudden movement, the result is an earthquake. Earthquakes are measured on the Richter scale. Each level is ten times greater than the level below it.
  THE KOBE EARTHQUAKE

WHY DID IT HAPPEN?

Kobe is on a minor fault which lies above a destructive plate margin. It is there that the Philippine Plate (oceanic crust) is forced downwards on contact with the Eurasian Plate (continental crust).

WHEN DID IT HAPPEN?

The earthquake occurred at 5.46 on the morning of 17 January 1995. It recorded 7.2 on the Richter scale and lasted for 20 seconds. During this time the ground moved 18 cm. horizontally and 12 cm. vertically. The widespread devastation was due to the earthquakes focus being so near to the surface and its epicenter being so close to Kobe.

PRIMARY EFFECTS:

• 200000 buildings collapsed.

• 120 of 150 quays were destroyed

SECONDARY EFFECTS:

• Electricity, gas and water were cut.

• Many fires were produced.

• Main roads and others were completely destroyed.

• 230000 people became homeless.

• People didn't want to return home because they were afraid of another possible earthquake.

• Many industries were forced to close.

VOLCANOES:

• MOUNT ST. HELENS

The oceanic crust (Juan de Fuca) moves eastwards towards the continental crust (North American Plate) and is forced downwards. This movement creates friction which produces earthquakes and due to an increase in temperature, destroys the oceanic crust. Magma rises to the earth's surface causing volcanic eruptions. The cascades mountain range was formed by volcanic eruptions.

TIMETABLE OF EVENTS:

• Spring 1980: 20 March minor earthquake (4.1 on the Richter scale). 27 March there was a small eruption of ash and steam. Bulge develops on north side of the mountain.

• 18 May (08:30): ash and steam erupted.

• 18 May (08:32): earthquake causes bulge to move outwards. Landslides, mudflows and floodwaters affect surrounding areas.

• 18 May (08:33): blast wave from main explosion kills everything within 25 km to north.

• Rest of morning: gas, ash and volcanic bombs continually ejected.

• Three days later, 21st of May: Ash plume reaches east coast of USA.

• Several days later: ash completely encircles the world.

CONSEQUENCES:

• Human life: 61 people died, mainly due to poisonous gases which accompanied the blast waves.

• Settlements: logging camps were destroyed, but as the disaster happened on a Sunday, nobody was working there.

• Rivers and lakes: the river's and lake's temperature rose because ash fell into them, and also sediments and mud obstructed the channel. These killed the fish. Spirit Lake was filled in.

• Communications: floodwaters wash away some roads and railway bridges. The ash also made car engines to work wrongly.

• Forestry: all the trees in the blast zone were completely destroyed. As the rivers carried them, they caused a log jam.

• Services: electricity and telephone wires were cut.

• Wildlife: any animal survived n the blast zone.

• Farming: 12 per cent of the crop was ruined by settling dust. Crops and livestock on valley floors were lost due to flooding.

CONSTRUCTIVE PLATE MARGIN

Two plates move away from each other. Molten rock or magma immediately rises to fill any possible “gap”, forming new oceanic crust.

DESTRUCTIVE PLATE MARGIN

COLLISION PLATE MARGIN

As both plates consist of continental crust, neither can sink. The land between them has been buckled and pushed upwards to form the Himalayas. This movement accounts for massive earthquakes which periodically occur where the plates meet.

CONSERVATIVE PLATE MARGIN

Two plates are forced to slide past one another. Although crust is neither formed nor destroyed at this point, earthquakes can occur if the plates “stick”.

MONTSERRAT-A VOLCANIC ERUPTION:

• Montserrat was formed by volcanic action. It is located on a destructive plate margin where the Atlantic and Caribbean Plates move towards each other, have formed an island arc.

• Soufrière Hills volcano has been dormant for 400 years. The soils surrounding it, consisting of weathered volcanic material, were extremely fertile and, together with the hot, wet climate had attracted many farmers to the area.

• 1995: During July, people living in the capital of Plymouth, became aware of an increase in sulphur fumes, but it was not until the steam was seen coming from the summit. When the first explosions began, nearby farmers collected the harvest that they could. Further eruptions caused ash to fall. Inhabitants were evacuated to a safe zone in the north. Although the south of the island became increasingly covered in ash, there was no big eruption.

• 1996: Magma pushed upwards to create a dome. Pressure built up under the dome until it was forcibly removed by an eruption. The eruptions sent rivers of hot gas, ash and mud (pyroclastic flows).

• 1997: The biggest explosion to date took place in June. It caused large pyroclastic flows which, for the first time, over-ran several small settlements and caused the death of 19 people. Ash also fell in the safe zone. An eruption on 8 august created a pyroclastic flow, which virtually destroyed what was left of Plymouth. This resulted in the danger zone being extended. Bye the end of the year, 7000 people had left the island.

• 1998: Will the predicted big explosion come, causing possible destruction of an island that owes its existence to volcanic activity? Will volcanic activity decrease, allowing those remaining on the island to return to their homes and re-establish their lives? Will the periodic eruptions of 1996-1997 continue, leaving the remaining islanders with an uncertain future?

ASSISI-AN EARTHQUAKE

Assisi is one of the many hilltop towns in central Italy. Assisi is also situated near to a complex plate boundary where the small Adriatic Plate is squeezed in between the two larger Eurasian and African Plates. To the south, is a destructive plate margin with volcanic activity (Etna and Vesuvius) and earthquakes. To the north is a collision margin with Fold Mountains and earthquakes.

• 19 September 1997: An earthquake struck Assisi and the surrounding region in the early hours of 19 September. In mid-morning, a second earthquake 5.7 caused considerable damage and loss of life. Strong aftershocks were still being experienced two weeks after the initial event. These later shockwaves caused further damage to the Basilica and other buildings.

• The day after the event: Six people were killed and 80% of housing was damaged. Part of the roof of the thirteenth-century Upper Basilica of St Francis of Assisi collapsed. Two Franciscan monks and two surveyors were killed by falling masonry. Two frescoes had been seriously damaged. The twelfth-century cathedral was also made unstable.

• A week after the event: The tomb of St Francis is said to be safe. Some 30 years ago, wooden beams had been replaced by reinforced concrete. The mayor's attempt to close the town to visitors failed, when a lot of people complained that 80% of Assisi's revenue comes from tourism. Thousands of people, many elderly, were still spending the chilly nights in the temporary shelters.

Where the plates meet, the oceanic plate is forced downwards to form a sub duction zone. The increase in pressure can bring severe earthquakes. As the crust continues to descend, it melts, partly due to heat resulting from friction caused by contact with the continental plate, and partly due to an increase in temperature as it re-enters the mantle. The new magma is lighter than the mantle. Some of it, rises to the surface to form volcanoes and long chains of Fold Mountains.