When there is a sudden breaking of rocks within the earth or an explosion, wave like energy travels through the earth and is recorded on seismographs. These are called seismic waves and they are of two types: body waves and surface waves categorized into different kinds. What we will be discussing are body waves; there are two types of body waves: primary waves (P waves) and secondary waves (S waves).
Both kinds travel through the Earths interior in all directions and arrive before the surface waves emitted by an earthquake. These waves are of a higher frequency than surface waves. The first kind of body wave is the P wave or primary wave.
This is the fastest kind of seismic wave, and, consequently, the first to ‘arrive’ at a seismic station. It can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air. They are also known as compressional waves, because of the pushing and pulling they do. The second type, on the other hand, is the S wave or secondary wave; much slower than a P wave and can only move through solid rock, not through any liquid medium. S waves move rock particles up and down, or side-to-side–perpendicular to the direction that the wave is traveling in.
Both kinds of body wave helps determine the nature of the Earths structure since their movement gives scientists ideas and clues about the nature of Earth’s interior. The specific movement of S waves led seismologists to conclude that the Earth’s outer core is a liquid. The conclusions of scientists about the Earths internal structure were collected and drawn together from observing and studying how these waves travel through the Earth. Measuring the time it takes for both types of body waves to reach seismic stations can give scientists idea about the epicenter of an earthquake. Since P waves travel faster than S waves, theyre always detected first. The farther away from the epicenter, the larger the time interval between the arrival of P and S waves. A crack or fracture in the earth’s surface is called a fault; motion along a fault normally occurs and this movement can cause earthquakes or–in the process of mountain-building–can release underlying magma and permit it to rise to the surface, thus giving birth to a volcano.
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A fault marks the boundary between two plates. There are three main types of faults. A divergent fault occurs when two plates are moving away from each other. When two plates come together, the result is a convergent fault and a fault that occurs when two plates slide past each other is known as a transform fault. 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. They were able to come up with a list of about 26 types of volcanoes on Earth (Simkin and Seibert, 1900).
For purposes of simplicity, we will discuss three forms; a volcanos form is largely determined by the composition of the erupting magma and the type of erupted products.
Including their shapes which are determined to a large degree by the explosivity of the eruptions, and to the abundance of water and its degree of interaction with magma. Cinder cones are simple volcanoes which have a bowl-shaped crater at the summit and rarely exceed 250m in height and 500m in diameter. They usually are created of eruptions from a single vent or opening. During the eruption, blobs (“cinders”) of lava are blown into the air from vents; break into small fragments that fall and pile up around the opening to the volcano. The piles, usually made of lave and not ashes, accumulate around form an oval-shaped small volcano. A great example of a cinder cone is Paricutin in Mexico.
... . Composite Volcanoes are formed when both lava and tephra erupt from a central vent ... ; lava gradually builds forming a low, broad dome shaped mountain. Cinder volcanoes are formed from the build up of accumulated tephra forming a cone shaped mountain ...
It was born in February 20, 1943 in a corn field and grew to 300 feet in 5 days. Another well known cinder cone is in the middle of Crater Lake in Oregon, USA. Shield volcanoes are large volcanic forms with broad summit areas and low-sloping sides (shield shape).
They are built by many layers of runny lava flows. Lava spills out of a central vent or group of vents and 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. Shield volcanoes may be produced by hot spots which lay far away from the edges of tectonic plates.
The eruptions of shield volcanoes are characterized by low-explosivity lava-fountaining that forms cinder cones and spatter cones at the vent. A good example of a shield volcano is the Island of Hawaii (the “Big Island”).
The Big Island is formed of five coalesced volcanoes of successively younger ages, the older ones apparently extinct. The main volcanoes are Mauna Loa and Kilauea. Composite volcanoes — sometimes called strato-volcanoes are the Earth’s grandest mountains. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases often exceeding 2500m in height, 1000sq.km in surface, and 400km3 in volume.
Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone. Although strato-volcanoes are 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 in California), elongated along a fissure (like Hekla).
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Some of the most conspicuous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Hood in Oregon, Mount St. Helens in Washington, Mt Pinatubo and Mt Mayon in the Philippines, and Mt. Etna in Italy.
The man who first proposed the theory of plate tectonics, one of the most important and far-ranging geological theories of all time, was a brilliant interdisciplinary scientist, Alfred Wegener. Harry Hammond Hess was very influential in setting the stage for the emerging plate-tectonics theory in the early 1960s. He believed in many of the observations Wegener used in defending his theory of continental drift, but he had very different views about large-scale movements of the Earth. Hess stresses on a ground-breaking hypothesis that later would be called seafloor spreading. The theory was strengthened further when dating studies showed that the seafloor becomes older with distance away from the ridge crests. Finally, improved seismic data confirmed that oceanic crust was indeed sinking into the trenches, fully proving Hess’ hypothesis, which was based largely on intuitive geologic reasoning. His basic idea of seafloor spreading along mid-oceanic ridges has well withstood the test of time.
Allan Cox was interested in dating rock specimen and together with Richard Doell and Brent Dalrymple made a successful method of determining ages of rocks collected from all over the world, and eventually establishing a timescale or calendar showing the complicated and irregular schedule of polarity changes in the Earth’s past. This was important because it gives credence to the idea of seafloor spreading apart. This finding was the first convincing evidence for movement of large areas of the Earth’s solid crust, and it led quickly to the postulates of plate tectonics. Drummond “Drum” Matthews, with his outstanding career in geology and geophysics, contributed to the fundamental understanding of the structure and evolution of the earth’s crust which further gave authority and weight to the continental drift theory. Archie Carr on his part, contributed to the theory through his widely acknowledged expertise on the Earth ecology — the old and new ecosystem. References Cas, R.A.F. and J.V.
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