Friday, 22 April 2011
MAKSUD GUNUNG BERAPI
Answer Question A.
Definition of Valcano
A volcano is a vent in the Earth surface through which magma (molten rock) and associated gas and ash ejected from the earth’s interior. When these erupted products accumulated on the earth’s surface , a volcanic mountain is created.
A volcano constitutes a vent, a pipe, a crater, and a cone. The vent is an opening at the Earth's surface. The pipe is a passageway in the volcano in which the magma rises through to the surface during an eruption.
The crater is a bowl-shaped depression at the top of the volcano where volcanic materials like, ash, lava, and other pyroclastic materials are released. Solidified lava, ashes, and cinder form the cone. Layers of lava, alternate with layers of ash to build the steep sided cone higher and higher.
Structure of a Volcano
Types of volcano
Volcanologists have classified volcanoes into groups based on the shape of the volcano, the materials they are built of, and the way the volcano erupts.
The groups are:
- Composite Volcanoes (also called strato volcanoes),
- Shield Volcanoes (also called shields),
- Cinder Cones,
- Spatter Cones, and
- Complex Volcanoes (also called compound volcanoes).
| Types of valconic | Features and explanation | Picture |
| · also called strato volcanoes, are formed by alternating layers of lava and rock fragments. This is the reason they are called composite. · usually erupt in an explosive way. This is usually caused by viscous magma. When very viscous magma rises to the surface, it usually clogs the crater- pipe, and gas in the crater-pipe gets locked up. Therefore, the pressure will increase resulting in an explosive eruption. · usually large and conical, we can distinguish different shapes of them: concave (like Agua), pyramidal (like Stromboli),convex-concave (like Vesuvius), helmet-shaped (like Mount Rainier), collapse caldera (like Graciosa), nested (like El Piton in Teide), multiple summits (like Shasta), elongated along a fissure (like Hekla). · are constructed along subduction zones. Examples of composite volcanoes include Mount Hood, Mount Rainier, Mount Shasta, Mount Fugi, Mount Mayon, and Vesuvius
| Composite Volcano. ( Different Shapes of Composite Volcanoes. | |
| Shield Volcanoes | · a volcano with shallow-sloping sides. Shield volcanoes normally form from fluid lava flows that can travel long distances across slight inclines, resulting in their relatively flat, broad profile. In contrast, steeply sloped stratovolcanoes better match the popular stereotype of a volcano. · are huge in size. They are built by many layers of runny lava flows. Lava spills out of a central vent or group of vents. A broad shaped, gently sloping cone is formed. This is caused by the very fluid, basaltic lava which can't be piled up into steep mounds. · produced by hot spots which lay far away from the edges of tectonic plates. Shields also occur along the mid-oceanic ridge, where sea-floor spreading is in progress and along subduction related volcanic arcs. · The eruptions of shield volcanoes are characterized by low-explosivity lava-fountaining that forms cinder cones and spatter cones at the vent. Famous shield volcanoes can be found for example in | ( Shield Volcano.) |
| Cinder Cones | · or scoria cone is a steep conical hill of volcanic fragments that accumulate around and downwind from a volcanic vent · The rock fragments, often called cinders or scoria, are glassy and contain numerous gas bubbles "frozen" into place as magma exploded into the air and then cooled quickly · Many cinder cones have a bowl-shaped crater at the summit. Lava flows are usually erupted by cinder cones, either through a breach on one side of the crater or from a vent located on a flank · Cinder cones are built from lava fragments called cinders. The lava fragments are ejected from a single vent and accumulate around the vent when they fall back to earth. · Cinder cones grow rapidly and soon approach their maximum size. They rarely exceed 250m in height and 500m in diameter. · The shape of a cinder cone can be modified during its (short) life. When the position of the vent alters, aligned, twin or secant cones develop. Nested,buried or breached cones are formed when the power of the eruption varies. | ( Cinder Cone. ) |
| Spatter Cones | · hot erupting lava contains just enough explosive gas to prevent the formation of a lava flow, but not enough to shatter it into small fragments the lava is torn by expanding gases into fluid hot clots, ranging in size from 1cm to 50cm across, called spatter. · spatter falls back to Earth the clots weld themselves together and solidify forming steep-sided accumulations. These accumulations focused on an individual vent are called spatter cones. | (Fig) Spatter Cone. |
| | · the "system" of those volcanoes is not "simple". Caldera complexes for instance have often got a large caldera with many subsidiary vents and deposits, some of which could be considered "volcanoes" in their own right. · volcano that consists of a complex of two or more vents is reckoned as a compound or complex volcano. · Examples of Complex Volcano McDonald Island (Australian Territory, Indian Ocean), Long Island(PNG), Guntur (Java, Indonesia), Dieng (Java, Indonesia), Dukono (Halmashera, Indonesia), Unzen (Japan), Asama (Japan), Iwate (Japan | |
Answer Questions B.
How earthquakes occur
Defining an Earthquake
Earthquake is one of the most destroying natural disasters. Unluckily it often happens in several regions. Recently a horrible earthquake has shaken West Sumatra . It has brought great damages. Why did it occur? Do you know how an earthquake happens?
Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves. It make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs.
During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus is called the epicenter of the earthquake.
Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves. It make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs.
During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus is called the epicenter of the earthquake.
Teacher use this model to explain how the earthquakes occur.
How earthquakes occur?
1. When rock strata are subjected to stress, they begin to deform or bend
2. All rocks have a certain rupture strength, which means that they will continue to bend, rather than break, as long as the stress imposed on them does not exceed this rupture strength. When the stress finally becomes too great, the rocks suddenly move along a plane (the fault)
3. That may or may not have existed before the deformation began. That sudden movement snaps the rocks on each side of the fault back into their original shape
4. and produces an earthquake.
A Short Report on suitability of the model based on students’ interview record.
This report illustrates, by means of a suitable model, how an earthquake occurs and what types of damage may result. The report is intended to help students and others visualize what causes earthquake shaking and some of the possible results of the shaking. By studying the suitable model, students will come to understand that earthquakes result from faulting in the Earth and that the potential consequences of earthquakes are numerous and serious.
Earthquake-like seismic waves can also be caused by explosions underground. These explosions may be set off to break rock while making tunnels for roads, railroads, subways, or mines. These explosions, however, don't cause very strong seismic waves. You may not even feel them. Sometimes seismic waves occur when the roof or walls of a mine collapse. These can sometimes be felt by people near the mine. The largest underground explosions, from tests of nuclear warheads (bombs), can create seismic waves very much like large earthquakes. This fact has been exploited as a means to enforce the global nuclear test ban, because no nuclear warhead can be detonated on earth without producing such seismic waves.
Types of Faults
A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.
Most earthquake faults, unlike California 's San Andreas fault , are too deep for seismologists to watch.
A strike-slip fault is a fault on which the two blocks slide past one another. These faults are identified as either right-lateral or left lateral depending on whether the displacement of the far block is to the right or the left when viewed from either side. The San Andreas Fault in California
Answer Question C.
Model a Solar Eclipse
Materials
- a torch
- a globe
- a pencil
- a platicine
To Do and Notice
- Do this experiment in a dark room.
- Place the globe on the table.The globe represent as the earth.
- Place the torch in front of globe so that is roughly aligned to the centre of the globe. The torch represent as the sun.
- Roll the plasticine into a ball ( as the moon)with a diameter of 4 cm. Stick the pencil through its centre.
- Switch on the torch.
- Move the plasticine it is between the globe and the torch.
- Slowly move the moon so that the shadow travels across the model earth. The moon's shadow moves across the earth from west to east.
8. Slowly move your model moon farther away from the model earth. Notice that at some point the dark inner shadow disappears. The inner, darker part is called the umbra. The umbra is opaque to sunlight. The eclipse is total only in the regions swept by the umbra.
What's Going On?
The ratio of the large object to the small object and the distance between them is a fairly accurate model of the earth-moon system. Anyone on earth within the path of the small dark shadow, called the umbra, will experience a total solar eclipse. The path of totality is quite narrow, however, often something like a hundred miles (160 km) across, which explains why most eclipse chasers have to travel significant distances. Within the lighter, larger shadow, called the penumbra, the sun is only partially blocked, and anyone within the penumbral path will see a partial eclipse.
Solar eclipse
A solar eclipse occurs when the Moon passes between the Sun and Earth, so that the Sun is partially or totally covered. Solar eclipses can only occur during new moon, when the moon is between the Earth and the Sun. Whether it will be a total or partial eclipse depends on where the moon is in its orbit.
Since the moon's orbit around the earth is not perfectly round but oval, it's distance from the Earth varies from about 221,500 to 252,000 miles. Whenever its orbit brings it closer to the Earth, it appears larger then the Sun, and when the three align, a total eclipse occurs.
A solar eclipse occurs only when the Moon is at the new phase. The New Moon cannot be seen until it moves across the face of the Sun.Like a tree, a building or yourself, the Moon has a shadow in sunlight. The shadow of the Moon is 374,000 km long (the Earth's shadow is about four times longer) and its length fluctuates by about 6,400 km due to the variation of the distance of the Moon from the Earth along its orbit.
Since the average distance of the Moon from the Earth is 384,000 km, then to have a total solar eclipse, the Moon's distance from the Earth must be a bit closer than average.The shadow of the Moon consists of two overlapping parts that are conical in shape .The inner, darker part is called the umbra. The umbra is opaque to sunlight. The eclipse is total only in the regions swept by the umbra.
The path of the umbra across the Earth is called the totality path. The totality path is usually about 100 km across, but under the most favorable conditions, when the Moon is at its nearest distance to Earth and the Earth is at its farthest distance from the Sun, the umbra can have a diameter of about 270 km.
The outer and larger part of the lunar shadow is called the penumbra. The eclipse is partial only in the regions covered by the penumbra. The penumbra extends for about 16,000 km and it may be over 3,000 km wide. Many people watch the partial eclipse, and there are those who are lucky enough that the totality path passes across their homes.
The shadow of the Moon is not simply a large dark spot on the Earth. Due to the combined effects of the motion of the Moon, the rotation of the Earth about its axis, and the orbital motion of the Earth, the lunar shadow races across the surface of the Earth at supersonic speeds sweeping a large area of the Earth.
Answer Questions D
What are the tides?
Tides are the periodic rise and falling of large bodies of water. Winds and currents move the surface water causing waves. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Ocean levels fluctuate daily as the sun, moon and earth interact. As the moon travels around the earth and as they, together, travel around the sun, the combined gravitational forces cause the world's oceans to rise and fall. Since the earth is rotating while this is happening, two tides occur each day.
When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season. When the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides.
How tides are formed
Roughly every 12 hours, the oceans on each side of the globe rise a little and then fall back.These tides are caused by the varying gravitational pull between the spinning Earth and the Moon and the Sun
Types of tide
| Type of tide | Explanation | Picture |
| Neap tide | · During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons. · Neap tides occur when these gravitational forces act at right angles to each other resulting in a lower than normal tidal range · comes from an old English word for "low" or "to nip". | The Bay of Fundy at low tide |
| Spring tide | · When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon. · Spring tides occur when the gravitational forces of the Sun and Moon reinforce each other resulting in a higher than normal tidal range · Spring tides are nothing to do with the season, but it is believed the name is derived from a medieval word to "leap up". | The Bay of Fundy at high tide |
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