Vulkanen en aardbevingen

Volcanoes Magma - Deep within the earth it’s so hot rocks melt and form a thick flowing substance, magma. Volcanoes - openings in the earth’s surface through which magma and ash erupt onto the land. Lava - magma that has erupted (through volcanoes). Lahars - mudflows, formed when hot volcanic materials mix with water from streams. Some volcanic eruptions are explosive and others not, it depends on how runny or sticky the magma is. If magma is thin and runny, gases can escape from it so that it flows easily out of the volcano. Lava flows rarely kill anyone, because they move slowly enough for people to get out of their way, but they can destroy buildings in their path. If magma is viscous (thick), gases cannot escape easily. Pressure builds up until the gases escape violently and explode. In this type of eruption, the magma blasts into the air and breaks apart into pieces called volcanic bombs. Explosive volcanic eruptions can be dangerous and deadly. They cause fiery clouds which race down mountainsides, destroying everything in their path. When hot volcanic materials mix with water from streams, lahars (mudflows) form. These can bury entire settlements. There are many different ways to classify volcanoes. One is based on the shape of the volcanic landform. The most common types are; - strato volcanoes (sometimes calles composite). Strato volcanoes (60% of the earth’s volcanoes) are formed from layers of lava and ash. Their lave usually is thick and flows slowly, which causes them to form steep-sided volcanic domes. The thick lava also allows gas pressures to build up, causing explosive eruptions. - shield volcanoes. Shield volcanoes are the largest volcanoes on earth, and they erupt lava that is very fluid. Because of this these volcanoes are not steep – the lava runs downhill too easily to form a pile, an they also have very broad bases. Eruptions at shield volcanoes mainly consist of fountains of lava and lava flows. Volcanoes of a 3rd, less common type, caldera complexes, are huge and potentially very explosive. They often don’t even look like volcanoes because when they erupt they collapse in on themselves rather than building up into a cone. Earthquakes The earth’s crust is divided into ± 15 segments called tectonic plates, they ‘float’ on the mantle. Heat from within the earth creates movements in the mantle. These cause the tectonic plates to move slowly, sometimes only a few cm per year. Plates can move towards, away, from or past one another. The boundaries where the plates meet are called plate margins. These are active zones because this is where the most of our world’s earthquakes and volcanic eruptions occur. There are 2 types of crust; - continental crust (is lighter, thick (35-70 km) and very old (over 1500 million years). - oceanic crust (is heavier, thin (6-10 km) and young (mostly less than 200 million years). Crust - the solid, outermost and thinnest layer of the earth. Core - a mass of mainly iron at the centre of the earth. Mantle - a thick layer of semi-molten rocks which slowly move due to heat coming up from the core. Active zone - where the edges of the plates move towards, past and away from each other. Tectonic plates - huge segments of the earth’s crust. Plate margins - the boundaries where the plates meet

Constructive margin - two plates move away from each other and magma rises to the surface to form new crust. Destructive margin - oceanic crust moves towards continental crust, but, being heavier, it sinks beneath the continental rock and is destroyed. Collision zone - plates made of continental crust move towards each other. They can’t sink so they crumple upwards to form fold mountains. Conservative margin - two plates move past each other. Crust is neither formed nor destroyed. Active zones and destructive margins occur where a plate made of oceanic crust moves towards a plate made of continental crust. The oceanic crust is heavier, so it’s forced beneath the lighter continental crust and forms a deep sea trench. Because the plates don’t slide smoothly past one another, there is often a huge build up of tension. If the crust breaks, shock waves are sent out in all directions. This is called an earthquake. The point where the crust breaks is called the focus. The place where the shock waves reach the earth’s surface is called the epicentre. As the oceanic crust pushes beneath the continental crust, it melts. This is partly due to the heat caused by friction between the plates, and partly due to the increase in temperature as it reaches the mantle. This creates magma which escapes to the surface along lines of weakness in the earth’s crust called faults to from volcanoes. Volcanic eruptions at destructive plate margins can be very violent. An area where two plates made of continental crust move towards one another is called a collision zone. Because neither plate can sink beneath the other, their crust is crumpled upwards to form fold mountains, such as the Himalayas. Sometimes, as with destructive margins, tension may build up over time. Eventually the crust will break, sending out shock waves à an earthquake. At conservative margins two plates move past one another. Although the plates are travelling in the same direction, (for example) the Pacific Plate is moving faster, causing the plate margins to ‘grind’ past each other. As with destructive margins, when the two plates ‘stick’ it causes tension to build up. When this tension is suddenly released an earthquake occurs. Earthquakes happen daily along the San Andreas fault, but major events, causing loss of life, are much less frequent. At conservative margins, crust is neither created nor destroyed, so volcanic eruptions don’t occur. At constructive margins two plates move away from one another, this type of movement happens mostly under oceans. As the plates move apart magma rises from the mantle to form new oceanic crust. This creates a line of underwater volcanoes along the plate margins called a mid oceanic ridge. In places the volcanoes become large enough to appear above sea level as volcanic islands such as Iceland. The type of volcano most often formed at a constructive plate margin is a shield volcano. If scientists can predict when and where an earthquake will happen, people can be warned and lives saved, so scientists monitor active zones. - sensitive instruments measure earth movements and check the strain building up in rocks. - The nr. of earthquakes can be plotted to show if a major earthquake is likely. - Several earthquakes often strike in a hort time. Foreshocks occur before some large earthqueakes, so scientists can work out the chances of a larger mainshock following. - Hazard shaking maps show the risk of earthquakes. They can help governments to plan emergency services and earthquake education. Engineers can plan earthquake codes for buildings, bridges and roads. Many people are killed when earthquakes destroy buildings. Scientists can test how buildings respond to quakes, and improve their ability to survive, by using quake tables or platforms. People in Japan practise earthquake drills every year on ‘Disaster Day’ – 1 September. Similar drills are carried out in other places where earthquakes are a hazard. Governments must plan carefully to make sure the emergency services are prepared for possible earthquakes, and that relief supplies are ready. Good communication with earthquake monitoring stations can also save lives. This may be a problem for countries that can’t afford monitoring equipment and sophisticated communications.