search engine

Thursday, September 25, 2008

EarthquakesCBC News Online May 13, 2008An earthquake is one of the most destructive events in the natural world. More than three million people were killed in earthquakes in the 20th century alone. Although scientists are able to predict which regions are most likely to be hit, it is impossible to predict when a major quake will occur.

What is an earthquake?
The ground beneath our feet seems rock-solid, but our planet's surface is in fact a dynamic grid of slowly moving sections, known as tectonic plates. Normally, this motion is imperceptible to humans, showing itself only on geological time scales (North America and Europe, for example, are drifting apart at the rate of just five centimetres per year). The movement causes stress to build in the crust. If the stress rises beyond a critical threshold, a portion of the crust will give way, shifting suddenly and violently. This sudden motion usually occurs along a fault or fracture – a zone of weakness in the earth's crust. The result is what we call an earthquake.
The Giza Necropolis stands on the Giza Plateau, on the outskirts of Cairo, Egypt. This complex of ancient monuments is located some 8 km (5 mi) inland into the desert from the old town of Giza on the Nile, some 25 km (15 mi) southwest of Cairo city centre. Great Pyramid of Giza is the only remaining monument of the Seven Wonders of the Ancient World.This Ancient Egyptian necropolis consists of the Pyramid of Khufu (known as the Great Pyramid and the Pyramid of Cheops), the somewhat smaller Pyramid of Khafre (or Chephren) a few hundred meters to the south-west, and the relatively modest-size Pyramid of Menkaure (or Mykerinos) a few hundred metres further south-west, along with a number of smaller satellite edifices, known as "queens" pyramids, causeways and valley pyramids. The Great Sphinx lies on the east side of the complex, facing east. Current consensus among Egyptologists is that the head of the Great Sphinx is that of Khafre. Associated with these royal monuments are the tombs of high officials and much later burials and monuments (from the New Kingdom onwards), signifying the reverence to those buried in the necropolis.
Of the five, only Menkaure's Pyramid is seen today without any of its original polished limestone casing, with Khafre's Pyramid retaining a prominent display of casing stones at its apex, while Khufu's Pyramid maintains a more limited collection at its base. Khafre's Pyramid appears larger than the adjacent Khufu Pyramid by virtue of its more elevated location, and the steeper angle of inclination of its construction – it is, in fact, smaller in both height and volume. The most active phase of construction here was in the 25th century BC. It was popularised in Hellenistic times when the Great Pyramid was listed by Antipater of Sidon as one of the Seven Wonders of the World. Today it is the only one of the ancient Wonders still in existence.
Due largely to 19th-century images, the pyramids of Giza are generally thought of by foreigners as lying in a remote, desert location, even though they are located in what is now part of the most populous city in Africa [1]. Consequently, urban development reaches right up to the perimeter of the antiquities site, to the extent that in the 1990s, Pizza Hut and KFC restaurants opened across the road [2]. The ancient sites in the Memphis area, including those at Giza, together with those at Saqqara, Dahshur, Abu Ruwaysh, and Abusir, were collectively declared a World Heritage Site in 1979 [3].

[edit] Construction of the Great Pyramid
Main article: Egyptian pyramid construction techniques
There have been varying scientific theories regarding the pyramid's construction techniques. Most construction theories are based on the idea that the pyramids were built by moving huge stones from a quarry and dragging and lifting them into place. The disagreements center on the method by which the stones were conveyed and placed. A recent theory proposes that the building blocks were manufactured in-place from a kind of "limestone concrete".
In addition to the many theories as to the techniques involved, there are also disagreements as to the kind of workforce that was used. One theory, suggested by the Greeks, posits that slaves were forced to work until the pyramid was done. This theory is no longer accepted in the modern era, however, Archaeologists believe that the Great Pyramid was built by tens of thousands of skilled and unskilled workers who camped near the pyramids and worked for a salary or as a form of paying taxes until the construction was completed. The worker's cemeteries were discovered in 1990 by archaeologists Zahi Hawass and Mark Lehner. Egyptologist Miroslav Verner posited that the labor was organized into a hierarchy, consisting of two gangs of 100,000 men, divided into five zaa or phyle of 20,000 men each, which may have been further divided according to the skills of the workers.[1]
The base of the Great Pyramid forms a nearly perfect square, with only a 19-cm (about 7.5-in) difference between its longest and shortest sides, out of a total length of about 230 m (756 ft). This huge square is also almost exactly level. When newly completed, the Great Pyramid rose 146.7 m (481.4 ft)—nearly 50 stories high. The pyramid’s core probably includes a hill of unexcavated rubble, making it impossible to determine its exact number of blocks. Researchers estimate that 2.3 million blocks were used to build the Great Pyramid, with an average weight of about 2.5 metric tons per block. The largest block weighs as much as 15 metric tons.
The work of quarrying, moving, setting, and sculpting the huge amount of stone used to build the Great Pyramid was most likely accomplished by several thousand skilled workers. Thousands more unskilled laborers and supporting workers—bakers, carpenters, water carriers, and others—were also needed for the project, so that a total of as many as 35,000 men and women were involved in the project. Many archaeologists and engineers now believe that the pyramid builders were not slaves, as was previously thought, but paid laborers who took great pride in their task. Most were probably farmers, contracted to work for a limited period. Specialists, who were permanently employed by the king, filled the positions that required the most skill—architects, masons, metalworkers, and carpenters.
In building Khufu’s pyramid, the architects used techniques developed by earlier pyramid builders. They selected a site at Giza on a relatively flat area of bedrock—not sand—which provided a stable foundation. After carefully surveying the site and laying down the first level of stones, they constructed the Great Pyramid in horizontal levels, one on top of the other.
Most of the stone for the interior of the Great Pyramid was quarried immediately to the south of the construction site. The smooth exterior of the pyramid was made of a fine grade of white limestone that was quarried across the Nile. These exterior blocks had to be carefully cut, transported by river barge to Giza, and dragged up ramps to the construction site. Only a few exterior blocks remain in place at the bottom of the Great Pyramid. During the Middle Ages (5th century to 15th century) people took the rest away for building projects in the city of Cairo.