Earthquakes | Volcanoes | Fold Mountains
Volcanoes, earthquakes, and the formation of fold mountains are all evidence of the continental activity that takes place beneath the earth’s crust (surface).
This crust is made up mainly of igneous rocks, in particular granite and basalt. Beneath the crust is the mantle which is made up of hot molten rocks. This hot flowing material is known as magma .
The crust of the earth is not in one single piece. The crust is broken up into a number of sections called plates that fit together rather like a jigsaw. These plates move (float) on the molten rock beneath them. This movement is known as ‘Continental Drift’ and both earthquakes and the formation of fold mountains are closely associated with plate movements. Volcanoes tend to occur at plate boundaries or where the crust of the earth is particularly thin.
Earthquakes are sudden movements in the earth’s crust that cause a tremor or vibration. They usually occur along the edges of the plate boundaries; these edges are called fault lines . Where the plates collide or rub against one another is where the earthquake actually happens.
The term focus is used to describe the spot deep within the earth’s crust where the earthquake starts. The epicentre is the place on the earth’s surface directly above the focus. Shock waves are sent out from the epicentre and focus and they cause the severe damage often associated with earthquakes. The nearer the epicentre, the more severe the damage.
The instrument used to record the shock waves is called a seismograph and the scale used to measure the strength of an earthquake is called the Richter Scale . This scale ranges from 1-12 and each step up on the scale means that an earthquake is 10 times more powerful than the preceding step (e.g. a force 6 earthquake on the Richter Scale is 10 times more powerful than one that measures 5).
Predicting when earthquakes will occur has not yet proven possible. We can predict reasonably well where they will occur, that is on the lines where the plates meet (the fault lines) but not when they will occur as they can occur suddenly, without warning. They can cause great damage, especially in built-up areas. Earthquakes can also occur at sea and may then produce giant waves called Tsunamis. One wave on the coast of Alaska in 1958 measured over 500 metres high. If these waves reach land they can cause huge damage.
Students preparing for a question on earthquakes should have some information on one particular earthquake e.g. the great quake in Chile in 1960 which measured 9.5 on the Richter scale, or the 1998 quake in Afghanistan which measured 6.1 or the 2002 quake in southern Italy which measured 5.4.
Volcanoes occur when hot molten magma forces its way to the surface through a narrow channel in the earth’s crust called a vent. These vents are usually found at plate boundaries or where the earth’s crust is very thin. When the hot molten rock reaches the surface it is known as lava . The process of the lava reaching the surface is called an eruption. Repeated eruptions around a vent eventually build up a cone-shaped mountain. Volcanoes are classified as being active (erupting regularly), dormant (no eruption for a very long period) and extinct (when they have not erupted in recorded time and are considered unlikely to erupt again). The most significant area of active volcanoes in the world is around the Pacific Ocean. About two thirds of volcanoes are found in this region known as the ‘Pacific Ring of Fire.’ Italy contains some of Europe’s best-known volcanoes such as Vesuvius near Naples and Mount Etna on the island of Sicily. Etna’s last recorded eruption was in October 2002.
Volcanoes can have both constructive and destructive effects. Students should be aware of each. The constructive effects include:
- The soil produced by lava can be fertile and beneficial e.g. coffee growing regions of South America.
- New land can be created e.g. the island of Surtsey off the coast of Iceland.
- Volcanic activity my produce hot springs or geysers which gush out hot water from below the ground. These can be used to help generate electricity as in New Zealand.
- Volcanoes are often important tourist attractions.
The destructive effects include:
- Eruption often causes great loss of life and damage to property. This damage can be caused by the lava flows or by poisonous gasses that are often released during eruptions.
Fold mountains are found in places where the earth’s plates collide and the crust buckles up. The upfolds that occur become mountains (these upfolds are called anticlines) and the down folds are called synclines. Nearly all of the major mountain ranges of the world have been formed in this way. The three major mountain-building periods are known as the Caledonian mountain building period, the Armorican period and the Alpine period.
- The Caledonian mountains were formed about 400 million years ago and include such mountains as the Scandinavian Highlands and the mountains of Donegal.
- The Amorican mountains were formed about 250 million years ago. The mountains of north-west France and of the south of Ireland were formed at this time.
- The Alpine mountains include not only the Alps but also the Andes, the Rockies and the Himalayas. These are the youngest (formed about 35 million years ago) and consequently the highest mountains in the world.
The effects of mountains include:
- They can act as barriers between people.
- They can have an effect on the climate of a region by making a region colder due to their height or cutting off rain-bearing winds thus creating drought conditions.
- They can be suitable locations for coniferous forests.
- They can provide suitable sites for hydro-electric plants.
- They can prove attractive to tourists.