Oceans

Oceans cover 71 percent of the Earth’s surface. Since the early Earth was probably too hot for liquid water to condense, all the water in today’s oceans probably came out of volcanoes and from falling comets. Today, the oceans average more than 3500 meter in depth and the deepest part, the Mariana Trench in the Pacific, goes down 11034 meter. The shallow shelves around many continents are, geologically, extensions of those continents. They support a wide range of marine life as well as concealing extensive oil deposits.

 They are bounded by the continental slope, down which sediments fall, producing graded beds of coarse to fine sediments fall, producing graded beds of coarse to fine sediment called turbidities.

The surface of the ocean provides an important source of moisture and warmth which controls the Earth’s climate. The deep ocean floor is relatively unexplored. It has been mapped by sonar to reveal ridges, canyons, volcanoes and sea mounts. Deep sea fishing and occasional visits by submersibles exploring the cold, dark and crushing pressure of the ocean deeps provide glimpses of bizarre life forms- giant invertebrates, fish that carry their own lanterns and entire communities that never see the Sun, depending for nutrients on hot submarine springs.

The Soil and Soil profile

Plants and hence all life depend on the soil. Rock breaks up chemically and physically to form different soil types. The physical break up produces sand and silt, but chemical decomposition is more complex. Chalk and lime stone dissolve away leaving little residue and therefore are overlain by thin soils. Silicates on the other hand slowly react with water to form clay minerals. What happens then depends on the flow of water through the soil. If, as in temperate climates, rainfall continues through most of the year, chemicals such as iron hydroxide will tend to wash out of the top 30 cm or so, leaving a pale Grey earth, or podzol, and re depositing the iron underneath as a darker layer that can develop into an impervious hard-pan, often eliminated by ploughing. In the tropics, high rainfall during the wet season mobilizes the iron but evaporation concentrates it near the surface. During the dry season plants draw water from further down, bringing iron and aluminum hydroxide to the surface and producing red late rite. If this continues, the late rite becomes hard and impervious, making the soil infertile.

Another important soil type is the black chernosem of the steppes. During the dry summers grass draws up calcareous solutions which make the humus black and insoluble so that iron hydroxides are not leached out. Intensive agriculture can eradicate such soil profiles but the physical nature of the soil is still important. If it is sandy, it is light and easily drained but holds little organic matter and nutrients soon wash away. Clay, by contrast, is heavy and waterlogged when wet, and hard when dry, but does retain nutrients. Vegetable matter can form soil too, such as peat, which retains water well. The most fertile soils are mixture of sand and clay- loam- with plenty of organic matter. Soils are valuable and vulnerable. If exposed to acidic pollution they can release aluminum salts that poison plants and water. If vegetation s is removed they get washed away altogether.

 The uppermost layer, or topsoil, is rich in organic matter, but short of minerals. This layer is penetrated by roots and has its own established ecosystem. Beneath is the subsoil, rich in minerals but short of organic materials. Underneath is layer of weathered rock and still deeper is the unweathered bedrock.

Landscaping by wind

On its own, wind has little effect on rocks. But let it transport sand particles and it can blast a desert landscape. Deserts tend to for m under consistently high pressure weather systems, or close to cold ocean currents which prevent evaporation into rain clouds. Clear skies expose rocks to intense heat by day and allow them to cool by night. The constant heating and cooling expands and contracts the surfaces of rocks, powering them to dust or making sheets flake off, producing smooth, rounded hills or inselbergs such as Ayers Rock in Australia.

Weathering processes produce a mixture of sand and rock and, although the popular image of a desert is of an endless expanse of sand, sandy deserts make up only 20 percent of the total. One reason is because wind whips up sand and dust and blows it away, leaving a layer of  heavier pebbles as a protective crust. The pebbles receive an intense blasting of sand which wears away the wind ward side. If the prevailing wind changes or the pebble overbalances, another side is presented and the resulting pebble has several flat faces and is known as a dreikanter.

 Where sand does dominate, it does not lie flat but builds into dunes such as barchans and seif dunes. If there is an obstacle such as a rock out crop or a bush, sand can build up in front and behind it, producing a long tail of sand in the lee of the obstacle.

Although it seldom rains in deserts, when it does occur the storms can be heavy. With little or no vegetation to retain it, the water produces flash floods which races down steep sided water courses, called wadis, scouring sand and rock as they go. Eventually this leads to a highly eroded landscape, a good example being the badlands of Arizona.

The Last Ice Age

In four main periods during the last ice age, a vast sheet of ice advanced south from the North Pole, covering Canada, Greenland, Siberia, Scandinavia and most of Britain including the North Sea.  Before its course was blocked by a moraine, the River Thames used to flow far to the north of London, past St Albans. The Great Lakes between Canada and the USA are the remains of great melt water lakes. As the ice retreated the lakes filled until they drained over higher ground to the south.

 When the ice retreated from the St. Lawrence, vast quantities of cold, fresh water flooded into the Atlantic, disrupting ocean currents and causing a brief refreeze since fresh water freezes at higher temperatures than salt water. The first ice ages that left their mark in the rock record occurred during the Pre Cambrian period. An ice age during the Ordovician affected what is now the Sahara desert. One in the Carboniferous period caught much of the Southern Hemisphere.

The most recent ice age began about 3.5 million years ago and is probably still in progress, though we are at present in a relatively mild spell. The temperature appears to have fluctuated between two relatively stable states about 35 times during the Earth’s history, triggered perhaps by the wobble of the Earth on its axis and variations in the Sun’s activity.

Deposition by Ice

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.

Landscaping By Ice

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.

Caves

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.