Friday, December 31, 2010
Tuesday, December 28, 2010
Glaciers produce may kinds of debris and drifts. As the ice progresses, a thick layer of fine clay builds up underneath. The pressure may be enough to keep the water liquid at the base even though the temperature is below 0 degree C. This lubricates the flow and leaves mounds of clay called drumlins. T the snout of the glaciers, where ice is melting, it deposits the rest of its load of sediment and rock as a terminal moraine that can block in a subsequent lake. The moraine in from of New Zealand’s Franz Josef glacier is 430 meter high.
If an ice sheet retreats, as happened at the end of the last ice age, it leaves the country side coated with what is known as boulder clay or till- a completely unsorted rock mixture ranging from the finest clay to house sized boulders. Sometimes a block of ice is left behind in the clay and when that melts it leaves a deep pond or kettle hole.
Melting can result in stratigraphical puzzles with, for example, big blocks of ancient rock sitting randomly on top of much younger material. These are known as erratic and their rock type often gives clues to the path taken by the ice. Erratic are often deposited in areas of different rock type. They perch precariously if they were dropped by rapidly melting ice. Ice in snow fields high in mountains compacts and begins to flow, leaving a corrie, cirque at the head of the valley. Back to back cirques leave jagged arêtes and pyramidal peaks.
Friday, December 24, 2010
Ice sheets once covered huge areas of both the Northern and Southern hemispheres. Today they have retreated and are restricted to Polar Regions and the highest mountain areas, but the landscapes carved during those earlier icy times still remain.
We tend to regard ice as a solid yet, under pressure, it can flow in the same sort of way that rocks flow within the Earth’s mantle. The structure of ice is very similar to that of rock, too. As snow compacts, air is squeezed out and it slowly turns from a white crumbly texture into a blue crystalline substance, the crystals being of ice rather than minerals. They are lubricated by a microscopic film of water that is kept as a liquid by dissolved salts.
Ice can truly transform the Earth’s landscape. It occupies a greater volume than water so, as it freezes in crevices and joints, it acts like a wedge, gradually breaking off pieces of rock or even boulders. Meanwhile, snow accumulates around the mountain peaks, either triggering avalanches or compressing into ice.
Eventually, the ice begins to flow, making a scooping action as it starts to move down the mountain side. At very high latitudes ice covers everything in a sheet that may be hundreds of meters thick. Within it, there may be faster flowing ice streams as the ground underneath falls away, is lubricated by mud or is even warmed by volcanic activity. as it thins, the sheet can part around rocky outcrops, or nunataks, to form valley glaciers and then reunite on the other side into what are called piedmont glaciers.
The flow rate of ice sheets and glaciers can be very slow- between a few and a few hundred meters a year. So, to maintain the same flow as even a small mountain stream, a glacier has to fill the whole valley. As it goes, it grinds the rocks underneath it into fine flour, and boulders embedded within its deep striations in the sides of the stepped valley that is being carved by it.
Tuesday, December 21, 2010
Rainwater is a weak acid; it contains dissolved carbon dioxide and humic acids and is capable of dissolving away rock such as limestone.
In limestone regions, streams flowing over what is known as limestone pavement often suddenly disappear underground down a swallow hole. They continue to flow underground sometimes through great cave systems Streams tend to follow a step like path, seeking out the weakest passage along bedding plains and down vertical joints in the lime stone. In the early stage of development of an underground cave system 9phreatic stage), water completely fills the passage and dissolves out a near circular tunnel. As the volume of water increases, the stream widens and cuts down into the bottom of the tunnels; the stream is now free flowing (vadose stage). Eventually, it may open up and follow a new and lower set of passageways, leaving empty, dry caves above it.
The solution of limestone to calcium bicarbonate is reversible. As the saturated water drips from the ceiling or splashes on the floor it evaporates and calcium carbonate precipitates out again, forming stalactites and stalagmites. These may eventually join up to form columns. Sometimes a part of the roof of the passage or cave collapses, opening a pothole or chimney.
Caves are also formed when the sea erodes into the weaker parts of a cliff. Melt water can carve out ice caves in glaciers, and molten lava draining from flow tubes can leave tunnels behind.
Tuesday, December 14, 2010
The earth is the only planet in the Solar System on which water exists in three forms - ice, liquid and vapor. The reason it is anything other than ice is because of sunshine. The Sun not only warms the land but evaporates water from the sea and powers weather systems so that it rains back down on the hills. The winds that whip up waves are also indirectly caused by solar power. The force of the water in a waterfall or a crashing wave represents a tremendous power.
Worldwide, hydroelectric power accounts for as much energy production as nuclear power, and could provide a lot more. Every meter of the North Atlantic coastline of Europe receives an average of 50kW of power in the form of waves. Water can quite literally, in geologically short timescales, move cliffs and mountains, wearing them down, grinding them up and washing their remains away.
The line where land meets sea stretches for hundreds of thousands of kilometers around the world. Water may appear to be a soft chisel but it never fails to find the weakest points in rocks, splitting off boulders and cutting caves and arches through the headlands.
Waves break onto a shore in a circular motion, throwing sand or stones up the beach then dragging them back. If there is a current along the coast, the sand or stones zigzag their way with the current gradually stripping the beaches and building a long spit downstream. Over geological timescales sea level has varied by hundreds of meters, leaving raised beaches half way up present day cliffs, drowning valleys once occupied by glaciers and turning river valleys into natural harbors. In some places cliffs are being washed away into the sea faster than humans can defend them.
Monday, December 13, 2010
Rocks that are deeply buried have nowhere to go if they fault, so instead they form folds. They can be broad, gentle folds those under southeast England, where the top of the fold or anticline has eroded away leaving the North and South Downs exposed and the London basin full of sediments. Such gentle folds are the comparatively minor knock on effects of the formation of the Alps. There, the collision of Africa with Europe compressed the sediments so much that folds piled northwards one on top of another in great over folds, or nappies: a vertical cliff can expose a repeating sequence of layers.
The life cycle of a mountain range
In a wide sedimentary basin, deposits accumulate layer by layer, sinking under their own weight and hardening as they are compressed these sediments laden troughs which are known as geosynclines are the potential birth place of mountain ranges of they occur between two colliding continental plates. Colliding continents begin to uplift the sediment, deforming it by folding produces symmetric anticlines and synclines. Continuing pressure may cause uneven folding and therefore asymmetric anticlines and synclines which eventually produce a recumbent fold the anticline is now in effect above the syncline and the rock layers on one side of the anticline are inverted. Further pressure may break the inverted layer, resulting in an over thrust fold. A nappe is formed when this layer disappears due to stretching and fracturing as uplift and folding continues.
Tall mountain ranges are produced by large scale faulting, the intrusion of magma domes and extrusive volcanic activity, but most importantly by large scale folding. As soon as mountains are formed weathering processes break up the rock surface and water and ice erode incisions into the mountainsides. Landslides, glaciers and rivers carry material away. The mature landscape stabilizes as rocky peaks become gently rounded hills, rivers widen and slow, and vegetation stabilizes the soil.
Friday, December 10, 2010
A relief map of the world reveals the structure of global mountain systems: the great backbone of both North and South America from the Rockies to the Andes, where the Pacific has pushed underneath spouting volcanoes; the high t peaks of the Alps and Himalayas where continental land masses have collided; and the ridges and wrinkles that mark ancient oceans long since squeezed out of existence. With the oceans drained, other even larger features become visible. The ocean ridge system, where new crust is formed, consists of long mountain ranges. Isolated groups of volcanoes such as the Hawaiian chain stand out as great underwater mountains composed of millions of cubic kilometers of basalt. The ocean trenches, where crust is swallowed, plunge up to 14000 m beneath the sea, and are flanked by volcanic atolls. Thought the ancient wrinkles reveal a long history, continuing activity shows that the Earth is still a dynamic planet.
A normal fault is where one block slides down the fault face compared with the other block. A strike slip fault is where one plate grates alongside another. The movement in this case is not vertical but horizontal. Features called horsts and grabens result from blocks moving between two faults.
The most famous strike slop fault in the world runs from San Francisco in the north to the hills behind Los Angeles. It is the San Andreas Fault and, with its branches and tributaries, has been on the move almost continuously for thousands of years. Hardly a day goes b without a few tremors along its length. Quakes brought San Francisco to halt in 1989 and Los Angles in 1994 but the last big ones were in 1906 and 1857 respectively.
Thursday, December 9, 2010
Forty million years ago upwelling in the mantle was splitting Africa apart. It lifted the Atlas Mountains and split open the Red sea. The crack continued down east Africa forming the Great Rift Valley. The stretching was at its greatest 3.5 million years ago when volcanoes erupted. In Kenya the volcanic material filled up the valley as fast as it was created. On the western branch of the rift, that did not happen and deep lakes fill the valley.