Unit 2:
The Earth's physical characteristics.
All the waters of the world meet again, and frozen seas and the Nile will mix in the humid flight of the clouds.
Hermann Hesse: Wandering, 1920
Inner and Outer layers
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The earth is structured in a series of layers. Some are internal and others are external. The inner layers go from the center of the earth outward: the inner core, the outer core, the lower mantle, the transition zone, the upper mantle and, finally, the earth's crust.
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The outer layers are formed by what we see around us: the waters of the planet (hydrosphere), the gas that surrounds the Earth (atmosphere) and the set of living beings that populate it (biosphere). Although part of it is underground, we can add to these outer layers the lithosphere, which includes everything that is not water in the earth's crust, it is basically stone.
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The lithosphere is divided into large blocks: the tectonic plates, which coincide, more or less, with the continents, although not always. The tectonic plates form a kind of rocky puzzle, but their pieces do not always fit well. The plates, in fact, are constantly moving, but it is so slight that we do not notice it. When two tectonic plates collide or separate violently, mountains, volcanoes, earthquakes and tsunamis occur.
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Types of Terrain
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On the lithosphere:
1) Plains: They are terrains of low altitude (up to 200 meters on the level of the sea) and rather flat surface. They can be used to plant crops. Examples: The Great European Plain or The Great Plains of North America.
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2) Plateaus: They are high plains (more than 200 m above sea level). Examples: The Central Plateau in Spain or the Great Tibetan Plateau, in Asia.
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3) Mountains: If they are young they have suffered little erosion and they appear tall and pointed. If they are old they are called massifs, and they have suffered more erosion, so their summit is more rounded and they are lower in altitude. A set of mountains forms a mountain range.
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4) Depressions: They are large valleys, caused by subsidence of the land. They usually follow the course of a long river. In Spain we have the Ebro depression and the Guadalquivir depression. Between Europe and Asia we find the depression of the Caspian Sea.
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Massif
Young Mountains
Relief under water
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We can find the same forms of relief under water, only with a different name.
1) continental shelf: It is the part covered by the water closest to the coast. When you get into the sea, that slight downhill that increasingly covers you, that's the continental shelf. It reaches up to 200 m deep.
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2) continental slope: It is a very steep downhill that appears suddenly and reaches the abyssal plain.
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3) Abyssal plain: It is a plain that extends along the bottom of the sea, to depths of 2000 m or more. It may have relief, such as submarine mountain ranges, which are called oceanic ridges, or depressions, which are called oceanic trenches.
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The relief of the coast
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1) Beaches: flat surface along the shore that is formed by sedimentation of sand or stones.
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2) Capes: They are extensions of land that go into the sea. Example: The Cabo de Creus in Spain or Cape San Vicente in Portugal.
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3) Gulf: It is an area of sea that takes over the land. Example: the ​gulf of Cadiz in Spain or the Gulf of Lion in France.
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4) Estuaries: They occur when a river, at its mouth, forms a single wide arm of water and part of the sea water enters that mouth. Example: the Tagus Estuary in Lisbon.
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5) Deltas: They occur when a river, at its mouth, is divided into many ramifications. Examples: In Spain, the Ebro delta. In Russia, the Volga delta.
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6) Fjords: Glacial valleys that have been flooded by the sea. They are typical of cold places. Example: the fjords of Norway.
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7) Peninsulas: They are extensions of land surrounded by water except for a part called isthmus.
How the relief is formed
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The relief of the earth does not form overnight, it takes hundreds of years to form or change. Tectonic plates that collide or separate, volcanoes, earthquakes or erosion are agents that form or modify the relief. Therefore, if we could travel in time thousands of years, we would verify that the mountains are not as high as now, that some have disappeared, or that others have not yet been formed, and the same would happen with valleys, rivers, plateaus, etc.
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Formation: internal agents.
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1) Orogenesis: The tectonic plates collide or separate forming the relief.
2) Earthquakes: Are caused by shock or sudden movement of tectonic plates.
3) Volcanoes: They are produced by cracks in the earth's crust, through which magma from the core is filtered to the surface. Normally, they are located where there is contact between tectonic plates.
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Modification: external agents.
There are three processes:
1) Erosion: The rock material is worn by natural elements or by human action.
2) Movements of eroded materials: These worn pieces of rock can be moved by natural forces or by human action and form new units of relief.
3) Sedimentation: Worn pieces of rock can accumulate in some specific place, forming a new soil. This new soil is called sediment.
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In these three processes a series of external agents intervene:
1) Atmospheric elements:
-Heat: It causes the rock to expand, so it can break.
-Cold: It causes the rock to contract, so it can break.
-Rain, snow or hail: This causes the rock to wear or dissolve little by little, changing its appearance.
-Wind: it produces that some pieces of the rock detach or move from place. The wind gives rise to the accumulation of sediments.
2) Liquid water or ice: The presence of water produces, little by little, the appearance of valleys or depressions. Ice can cause cracks in the relief. Water also carries sediments.
3) Living beings: The roots of a tree, for example, can break or move the rocks. The action of the animals also transforms the relive little by little. Man can transform the surface of the earth enormously: construction of tunnels, mines, quarries, cities, railways, highways, etc.
The hydrosphere
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The hydrosphere is made up of all the water on the planet. It is divided into two groups:
1) Marine water: that is, oceans and seas. They make up 97.5% of the total.
2) Continental water: that is, lakes, glaciers, rivers and groundwater (aquifers). They form the remaining 2.5%. Almost everything is fresh water.
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The water cycle.
The amount of water on the planet is always the same, but it changes constantly.
When it is hot, the water (marine or continental) evaporates (evaporation). Hot air has two qualities: it absorbs a lot of moisture and ascends towards the sky forming clouds (condensation). When those hot, humid clouds rise very high, they cool down. Cold air has two opposite qualities to those of hot air: it can not contain moisture and descends towards the ground. Then, it rains, it snows or it hails, (precipitation). When the water falls on the earth, the cycle begins again.
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Fresh water
Marine water: oceans and seas
Rivers
Lakes
Glaciers
Marine Currents
Marine currents are movements of large bodies of water through the oceans. There are warm currents and cold currents. Those that originate in areas near the equator are warm, and those that originate near the poles are cold. When the warm ones move away from the equator, they cool and mix with currents of different temperatures. The cold ones get warmer when they move away from the poles.
This is an example of the action of weathering and erosion: collapse of a cliff after the passage of a tropical storm in La Gomera, Canary Islands.