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Middle Years Science Program
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Return to Science Shows  Program: Christine Bellert (St Anne's); Judi Essing (St Jude's).
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Earthquakes: Earthquakes result from the movement of the Earth's crust because of currents of boiling hot magma under the Earth's crust. People experiencing earthquakes say the ground rumbles, hanging lamps begin to sway back and forth, shelves begin to rattle or spill their contents, the floor and walls shake. Even if you do not remember seeing or feeling an earthquake, you have probably lived through thousands of tiny earthquakes during your lifetime. The earth is constantly creating earthquakes. Earthquakes are among the most devastating natural events that occur on Earth and are a reminder that our planet is a dynamic, changing body. An
earthquake is the shaking of the earth caused by pieces
of the crust of the earth that suddenly shift. The crust,
the thin outer layer, is mostly cold and brittle rock compared
to the rock deeper inside. The most common cause of earthquakes
is faulting. A fault is a break in the earth's crust along
which movement occurs. The study of this movement is known
as plate tectonics. Most earthquakes occur in narrow belts along the boundaries of crustal plates, particularly where the plates push together or slide past each other. At times, the plates are locked together, unable to release the accumulating energy. When this energy grows strong enough, the plates break free. When two pieces that are next to each other get pushed in different directions, they will stick together for a long time (many years), but eventually the forces pushing on them will force them to break apart and move. This sudden shift in the rock shakes all of the ground around it. There are three types of plate boundaries: spreading zones, transform faults and subduction zones. At spreading zones, molten rock rises, pushing two plates apart and adding new material at their edges. Most spreading zones are found in oceans; for example, the North American and Eurasian plates are spreading apart along the mid-Atlantic ridge. Spreading zones usually have earthquakes at shallow depths (within 30 kilometers of the surface). Transform faults are found where plates slide past one another. An example of a transform-fault plate boundary is the San Andreas fault, along the coast of California and northwestern Mexico. Earthquakes at transform faults tend to occur at shallow depths and form fairly straight linear patterns. Subduction zones are found where one plate overrides, or subducts, another, pushing it downward into the mantle where it melts. An example of a subduction-zone plate boundary is found along the northwest coast of the United States, western Canada, and southern Alaska and the Aleutian Islands. Subduction zones are characterized by deep-ocean trenches, shallow to deep earthquakes, and mountain ranges containing active volcanoes.
Earthquakes can also occur within plates, although plate-boundary earthquakes are much more common. Less than 10 percent of all earthquakes occur within plate interiors. As plates continue to move and plate boundaries change over geologic time, weakened boundary regions become part of the interiors of the plates. These zones of weakness within the continents can cause earthquakes in response to stresses that originate at the edges of the plate or in the deeper crust. The New Madrid earthquakes of 1811-1812 and the 1886 Charleston earthquake occurred within the North American plate. The point beneath the Earth's surface where the rocks break and move is called the focus of the earthquake. The focus is the underground point of origin of an earthquake. Directly above the focus, on the Earth's surface, is the epicenter. Earthquake waves reach the epicenter first. During an earthquake, the most violent shaking is found at the epicenter. Earthquake waves are known as seismic waves. Scientists have learned much about earthquakes and the interior of the Earth by studying seismic waves. There are three main types of seismic waves. Each type of wave has a characteristic speed and manner of travel. Primary Waves Seismic waves that travel the fastest are called primary waves, or P waves. P waves arrive at a given point before any other type of seismic wave. P waves travel through solids, liquids and gases. They move through the Earth at different speeds, depending of the density of the material through which they are moving. As they move deeper into the Earth, where material is more dense, they speed up. P waves are push-pull waves. As P waves travel, they push rock particles into the particles ahead of them, thus compressing the particles. The rock particles then bounce back. They hit the particles behind them that are being pushed forward. The particles move back and forth in the direction the waves are moving. Secondary Waves Seismic waves that do not travel through the Earth as fast as P waves do are secondary waves, or S waves. S waves arrive at a given point after P waves do. S waves travel through solids but not through liquids and gases. Like P waves, S waves speed up when they pass through denser material. S waves cause rock particles to move from side to side. The rock particles move at right angles to the direction of the waves. The Richter Scale On the Richter scale the amount of damage is as follows ...
Surface Waves The slowest-moving seismic waves are called surface waves, or L waves. L waves arrive at a given point after primary and secondary waves do. L waves originate at the epicenter. Surface waves travel along the surface of the earth, rather than down into the earth. Although they are the slowest of all the earthquake waves, L waves usually cause more damage than P or S waves. Measuring Earthquakes The severity of an earthquake can be expressed in several ways. The magnitude of an earthquake, usually expressed by the Richter Scale, is a measure of the amplitude of the seismic waves. The moment magnitude of an earthquake is a measure of the amount of energy released -- an amount that can be estimated from seismograph readings. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location. The Richter Scale, named after Dr. Charles F. Richter, is the best known scale for measuring the magnitude of earthquakes. This scale is logarithmic so that a recording of 7, for example, indicates a disturbance with ground motion 10 times as large as a recording of 6. A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a Richter value of 6 or more or commonly considered major; great earthquakes have a magnitude of 8 or more on the Richter Scale. The Modified Mercalli Scale expresses the intensity of an earthquake's effects in a given locality in values ranging from I to XII. The most commonly used adaptation covers the range of intensity from the condition of "I -- not felt except by a very few under especially favorable conditions, " to "XII -- damage total. Lines of sight and level are distorted. Objects thrown upward into the air." Evaluation of earthquake intensity can be made only after eyewitness reports and results of field investigations are studied and interpreted. The maximum intensity experienced in the Alaska earthquake of 1964 was X; damage from the San Francisco and New Madrid earthquakes reached a maximum intensity of XI. An earthquake's destructiveness depends on many factors. In addition to magnitude and the local geologic conditions, these factors include the focal depth, the distance from the epicenter, and the design of buildings and other structures. The extent of damage also depends on the density of population and construction in the area shaken by the quake.
Earthquakes in History The scientific study of earthquakes is comparatively new. Until the 18th century, few factual descriptions of earthquakes were recorded, and the natural cause of earthquakes was not very well understood. Those who did look for natural causes often reached conclusions that seem ridiculous today; one popular theory was that earthquakes were caused by air rushing out of caverns deep in the Earth's interior. The earliest earthquake for which we have descriptive information occurred in China in 1177 B.C. The Chinese earthquake catalog describes several dozen large earthquakes in China during the next few thousand years. Earthquakes in Europe are mentioned as early as 580 B.C., but the earliest for which we have some descriptive information occured in the mid-16th century. The San Francisco earthquake of 1906 was one of the most destructive in the recorded history of North America. The earthquake and the fire that followed killed nearly 700 people and left the city in ruins. The Alaska earthquake of March 27, 1964, was of greater magnitude than the San Francisco earthquake; it released perhaps twice as much energy and was felt over an area of almost 500,000 square miles. The ground motion near the epicenter was so violent that the tops of some trees were snapped off. One hundred and fourteen people (some as far away as California) died as a result of this earthquake, but loss of life and property would have been far greater had Alaska been more densely populated. Volcanoes The temperature of different layers of lava can be deduced from its colour ...
The largest explosion ever recorded resulted from a volcano erupting in 1883 on an island called Krakatoa. The eruption destroyed the island and produced 39m high tidal waves that swept 16km to Java killing 36000 people. There are about 1300 volcanoes around the world of which 600 are still active. Some of them even erupt every day.There are no active volcanoes in Australia. Mt Gambier in South Australia erupted 1400 years ago and can now be said to be extinct. Volcanoes occur at weak spots on the Earth's surface usually at the edges of tectonic plates that float on molten rock of the mantle. Pressure from gases squeeze the molten rock upwards and explode through like a pimple. They explode with lava, ash and steam creating an eruption (if in one spot only) or fissures if in numerous locations. Most release clouds of steam called fume which carries with it large amounts of fine rock dust. Magma forms in chambers underneath the volcano and pushes upwards and emerges as lava. Lava is made up of magma and gases such as hydrogen sulphide and steam. It flows down at speeds of 10km/h and will cool to form solid rock. The cloud above Krakatoa was estimated to have reached 80km into the sky. It eventually settles back down on Earth as a thick blanket. In 79 AD Pompeii in Italy suffocated many people living there when Mt Vesuvius buried the town. Rain can turn the dust into rivers of mud flow called lahar. Volcanic dust can travel around the planet in jet-stream winds that can block the sun. Gas explosions can destroy parts of the volcano itself. Storms Storms are quite frightening. A familiar occurrence in coastal Australia, storms form a very important component of our weather systems. Their violence is often destructive, their rain welcome and power awe inspiring. Severe storms are more common than any other natural hazard and they occur everywhere in Australia. On average, unclassified storms are responsible for more damage each year than tropical cyclones, earthquakes, floods and bushfires. Each year between 5 and 10 people are killed by lightning. Tornadoes have caused at least 41 deaths in Australia's short recorded history. A storm is produced when hot air and cold air mix, releasing huge quantities of energy and creating strong winds, rain, hail, lightning and thunder. Sometimes very large storms are driven by massive quantities of hot moist tropical air, these types of storms are known as cyclones in Australia or as hurricanes and typhoons overseas. Tropical cyclones occur in northern Australia during summer when the overhead Sun evaporates immense quantities of water creating huge reserves of stored thermal energy. The warm moist rising causes wind to blow in towards the centre of the storm while the rising air soon cools forming storm clouds and releasing some of the stored energy from the moist air. As more and more energy is released and more hot moist air is fed into the storm, it becomes larger and stronger eventually reaching proportions large enough to sustain itself and become a cyclone. Tropical cyclones soon die out when they move over land because the source of energy, moisture from the sea, is no longer available to drive the cyclone. Tornadoes Tornadoes occur when hot moist tropical air meets cold dry air. Tornadoes are especially well known over the southern states of the U.S.A. but also occur in Australia, mainly in south-eastern Australia. A tornado is a very violent windstorm in which the air whirls rapidly upwards in a vortex, forming a funnel shaped cloud. Tornadoes are associated with larger thunderstorm activity and form in the edges of the storm clouds and descend until they reach the ground. They can range in width from a few metres to hundreds of metres, their winds have been measured at more than 450 km/h. There is no generally agreed theory for the formation and maintenance of tornadoes, waterspouts, and other vortices. Eyewitness accounts associate particularly bright blue lightning in the eye of tornadoes and theories involve both electrical activity and the formation of circulating winds formed by the enormous energy released when large quantities of warm moist air are suddenly cooled when two air masses meet. The largest and most dangerous tornadoes definitely occur in the southern U.S.A. with over 1000 tornadoes recorded in some years. In Australia we quite often see much smaller vortices such as 'Willy-Willys', 'Dust Devils' or 'Whirl Winds'. Waterspouts are tornadoes that occur over water. The whirling air sucks up water creating a very well defined column of rising water. Fish caught in waterspouts have been known to fall to the ground many kilometres from the sea having been carried by the waterspout and associated storm. Lightning The lightning flashes that accompany thunderstorms are enormous electrical sparks caused when electrical charges build up in the storm clouds. Most lightning occurs within or between adjacent storm clouds. Friction within clouds created by updraughts and the movement of air creates the build up of negative and positive charge within a storm cloud. Lightning within clouds occurs when enough charge difference is built up and an electrical discharge jumps from a negative region to a positive region within the cloud. Lightning strikes from cloud to Earth occur slightly differently. Here negative charge build up within a cloud repels negatively charged electrons within the ground and induces (or creates) a positive charge on the surface of the Earth. Lightning will tend to strike the nearest place of accumulated charge on the ground, hence church spires, trees and even golf clubs can act as lightning conductors as they allow the accumulation of charge to occur. Lightning itself involves a 'leader' stream of electrons descending from the cloud towards the ground in a series of jagged steps and branches as it tries to find the path of least resistance. Once the lightning strikes the ground, it tries to dissipate its charge again along the route of least electrical resistance. This may be back up to the cloud along the original lightning strike path, so what at first appears to be a single movement of the lightning can actually be a series of rebounds back and forth between the cloud and the ground. The light from lightning is created as the electrons steaming along the lightning path, smash into and ionise molecules in the air making them emit light that we see as the flash. Thunder Thunder occurs when lightning dramatically heats nearby air, expanding the air very quickly. This creates an area of low pressure around the lightning flash. Upon cooling, surrounding air rushes back into the low-pressure zone. The sound we call thunder is created as part of the initial explosion of air and as the inrushing air meets again. Wind Strong winds are classified as 40-50km/h; 51-62km/h whole trees move; at 63 - 75 km/h gale winds break twigs from trees and you cannot walk; 76 - 87 km/h dislodged roofs and large branches break; storm occur at 88 - 102 km/h with trees uprooting and buildings damaged. Beyond 103 km/h, extensive and widespread damage to buildings and infrastructure occurs. Weather There is a number of different features that can be looked at regarding weather. Each of these can be monitored daily and trends can be discovered. A chart similar to the one below can be drawn up ...
Weather stations would conduct measurements of this kind on a daily basis. The weather patterns around Australia include ...
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