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Lithosphere | Geography

Lithosphere | Geography
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LITHOSPHERE AND TECTONIC PLATES

  • Scientists believe the Earth began its life about 4.6 billion years ago. The continents probably began forming about 4.2 billion years ago as the Earth continued to cool.
  • It was not until the turn of the 20th century that scientists determined that our planet is made up of four main layers
  • The inner core
  • The outer core
  • mantle
  • Crust
  • The core is composed mostly of iron and is so hot that the outer core is molten, with about 10% sulphur.
  • The inner core is under such extreme pressure that it remains solid.
  • Most of the Earth’s mass is in the mantle, which is composed of iron, magnesium, aluminium, silicon and oxygen silicate compounds.
  • At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner.
  • The crust is much thinner than any of the other layers and is composed of the least dense calcium and sodium (Na) aluminum – silicate minerals.
  • Being relatively cold, the crust is rocky and brittle, so it can fracture in earthquakes.
  • The crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere.
  • The layer below the rigid lithosphere is a zone of about 50 – 100 km down, is especially soft and plastic and is called the asthenosphere.
  • The asthenosphere is the part of the mantle that flows and moves the plates of the Earth.
  • A heavy load on the crust like an ice cap, large glacial lake, or mountain range can bend the lithosphere down into the asthenosphere which can flow out of the way.
  • The load will sink until it is supported by buoyancy.
  • The crust is composed of two basic rock types granite and basalt.
  • The continental crust is composed mostly of granite.
  • The oceanic crust consists of a volcanic lava rock called basalt.
  • Basaltic rocks of the ocean crust are much denser and heavier than the granitic rock of the continental crust. Because of this, the continents ride on the denser oceanic plates.
  • The Earth’s outermost layer, the lithosphere, is broken into 7 large, rigid pieces called plates
  • The African
  • North American
  • South American
  • Eurasian
  • Australian
  • Antarctic
  • Pacific plates
  • Several minor plates also exist including the Arabian, Nazca and Philippines plates.
  • These plates are all moving in different directions and at different speeds from 2 cm to 10 cm per year.
  • This theory of Plate tectonics explains “how the earth works”.
  • The place where the two plates meet is called a plate boundary.
  • Boundaries have different names depending on how the two plates are moving in relationship to each other.

Convergent Boundary

  • When two plates collide, the edge of one dives beneath the other and ends up being destroyed in the mantle.
  • Places where plates crash or crunch together are called convergent boundaries.
  • Plates only move a few centimetres each year, so collisions are very slow and last millions of years.
  • Even though plate collisions take a long time, lots of interesting things happen.
  • For example, an oceanic plate has crashed into a continental plate.
  • The continental plate “front ends” bends and the edge of the continental plate has folded into a huge mountain range, while the edge of the oceanic plate has bent downward and dug deep into the Earth.
  • A trench has formed at the bend. The folding and bending makes rock in both plates break and slip, causing earthquakes.
  • As the edge of the oceanic plate digs into Earth’s hot interior, some of the rock in it melts.
  • The melted rock rises up through the continental plate, causing more earthquakes on its way up and forming volcanic eruptions wherein finally reaches the surface.
  • An example of this type of collision is found on the west coast of South America where the oceanic Nazca Plate is crashing into the continent of South America.
  • The crash formed the Andes Mountains, the long string of volcanoes along the mountain crest and the deep trench off the coast in the Pacific Ocean.
  • When two plates collide with each other mountains, volcanoes and earthquakes are formed.
  • For example
  • The Rockies in North America
  • The Alps in Europe
  • The Pontic Mountains in Turkey
  • The Zagros Mountains in Iran
  • The Himalayas in central Asia were formed by plate collisions.
  • Each year, thousands of people are killed by earthquakes and volcanic eruptions in those mountains.
  • In 1883 an eruption of Krakatau volcano in Indonesia killed 37,000 people.
  • In 1983 an eruption caused mudslide in Columbia killed 25,000 people.
  • In 1976, an earthquake in Tangshan, China killed an astounding 750,000 people.

Divergent boundary

  • Places, where plates are coming apart, are called divergent boundaries.
  • Earth’s lithosphere is pulled apart it breaks along parallel faults.
  • The block between the faults cracks and drops down into the soft, the asthenosphere.
  • The sinking of the block forms a central valley called a rift.
  • Magma seeps upward to fill the cracks. In this way, new crust is formed along the boundary.
  • Earthquakes occur along with the faults and volcanoes form where the magma reaches the surface. Divergence can occur on the continent and as well as on the oceanic floor.
  • Divergence on the continent causes rift valleys and are 30 to 50 kilometres wide.
  • Examples include the East Africa rift in Kenya and Ethiopia and the Rio Grande rift in New Mexico.
  • Divergence across the ocean floor causes rift valleys with only a kilometre or less wide.
  • Divergence along the Mid Atlantic ridge causes the Atlantic Ocean to widen at about 2 centimetres per year.
  • Most of the world’s active volcanoes are located along or near the boundaries between shifting plates. Such volcanoes are called plate boundary volcanoes.
  • The peripheral areas of the Pacific Ocean Basin, containing the boundaries of several plates are dotted with many active volcanoes that form the so-called Ring of Fire.
  • The Ring provides excellent examples of the plate – boundary volcanoes including Mount St. Helens.
  • Geographically these tectonic plates are grouped as seven continents and five oceans.

The Continents and its features

  • A continent is defined as a large unbroken land mass completely surrounded by water, although in some cases continents are connected by land bridges.
  • The seven continents are
  • Asia,
  • African
  • North America
  • South America
  • Antarctica
  • Europe
  • Australia
  • The island groups in the Pacific are often called Oceania.
  • These continents occupies nearly 29% of the Earth’s total area.
  • Mountains, Plateaus and plains are significant features of those continents.
  • A mountain is a landmass that stands significantly above its surrounding.
  • The majority of mountains are created when tectonic plates collide, causing folding and uplifting of rocks along the plate boundaries.
  • The uplifted land is then eroded into peaks and valleys.
  • A plateau is another feature of the Earth’s surface.
  • They are broad uplands that rise abruptly from the surrounding area.
  • The majority of them are created when tectonic plates pulled, causing faulting and slipping down of rocks along the plate boundaries.
  • Tibet, the highest plateau of the world, Colorado plateau, Decan Plateau, and East African Plateau are noteworthy.
  • A plain is a vast, low-lying area of the continents. They are formed in different ways.
  • Plains made through the actions of rivers are called alluvial plains
  • The Ganges and the Brahmaputra are the largest river plains in the world.
  • The plains made through wind sedimentation are called loess.
  • The plain formed where the Yellow River flows in China is loess.
  • The plains formed through the actions of waves are called coastal plains.
  • The eastern coastal plain of India is a good example.
  • Mountains, plateaus and plains are all part of the lithosphere and they are made up of many types of rocks. Some rocks are white and some others are black.
  • A few rocks are brittle and others are hard; some are like sand, are permeable, others, such as clay are non-permeable.
  • Rocks are composed of many minerals such as silica, aluminum, iron and magnesium.
  • The nature of the rock is determined by the presence of its minerals.
  • Rocks can be classified into three types based on their formation.
  • Igneous rocks
  • Sedimentary rocks
  • Metamorphic rocks

Igneous rocks

  • The earth is about 4,600 million years old.
  • The oldest rocks that have been found were created by volcanic eruptions over hundreds of millions of years.
  • These rocks are still made every time a volcano erupts.
  • These rocks are formed from molten rock that has slowly cooled underground or erupted to the surface of the earth through a volcano.
  • While molten rock is underground, it is called magma.
  • Molten rock that has erupted from a volcano, by contrast, is called lava.
  • These rocks are usually very hard. Igneous rocks have crystals.
  • The size of these crystals may depend on how quickly the molten rock has cooled.
  • If it cooled slowly, the crystals are large; if it cooled quickly, they are small. Today igneous rocks represent 95 per cent of the Earth’s crust.

Sedimentary rocks

  • Sedimentary rocks are formed by the accumulation and cementation of mud, silt, or sand derived from weathered igneous rock fragments.
  • Sedimentary rocks represent less than 5 per cent of the Earth’s crust but 75 per cent of the Earth’s land surface.

Metamorphic rocks

  • Metamorphic rocks are igneous or sedimentary rocks that have been altered by heat and / or pressure either because they have been buried and folded deep in the crust or because they have come into contact with molten igneous rock.
  • Metamorphism can result in the formation of completely new minerals.
  • It can also destroy original structures such as sedimentary layering or fossils.
  • Intense pressure causes the realignment of minerals, forming new layers.
  • About one per cent of rocks in the crust are metamorphic.
  • In a Rock Cycle, weathering is the first step for a number geomorphic and biogeochemical processes and is fundamental to many other aspects of the hydrosphere, lithosphere and biosphere.

The Rock Cycle

  • The rocks of the Earth’s crust are constantly being created, worn down and redeposited in a slow cycle. The rock cycle beings with the weathering of igneous rocks.
  • The Rock Cycle is a group of changes.
  • Igneous rock can change into sedimentary rock or into metamorphic rock.
  • Sedimentary rock can change into metamorphic rock or into igneous rock.
  • Metamorphic rock can change into igneous or sedimentary rock.
  • Igneous rock forms when magma cools and makes crystals. Magma is a hot liquid made for melted minerals.
  • The minerals can form crystals when they cool. Igneous rock can form underground, where the magma cools slowly. Or, igneous rock can form above ground, where the magma cools quickly.

Weathering

  • Weathering is the breakdown and alteration of rocks and minerals at or near the Earth’s surface into products that are more in equilibrium with the conditions found in that environment.
  • Most rocks and minerals are formed deep within the Earth’s crust where temperatures and pressures differ greatly from the surface.
  • The physical and chemical nature of materials formed in the Earth’s interior is characteristically in disequilibrium with conditions occurring on the surface.
  • This disequilibrium, these materials are easily attacked, decomposed, and eroded by various chemicals and physical surface processes.
  • The products of weathering are a major source of sediments for erosion and deposition.
  • Many types of sedimentary rocks are composed of particles that have been weathered, eroded, transported and terminally deposited in basins.
  • “Weathering also contributes to the formation of soil by providing mineral particles like sand, silt, and clay.
  • Elements and compounds extracted from the rocks and minerals by weathering processes supply nutrients for plant uptake.

Products of Weathering

  • The process of weathering can result in the following three outcomes on rocks and minerals:
  • The complete loss of particular atoms or compounds from the weathered surface.
  • The addition of specific atoms or compounds to the weathered surface.
  • A breakdown of one mass into two or more masses, with no chemical change in the mineral or rock.
  • The residue of weathering consists of chemically altered and unaltered materials.
  • The most common unaltered residue is quartz.
  • Many of the chemically altered products of weathering become very simple small compounds or nutrient ions.
  • These residues can then be dissolved or transported by water, released to the atmosphere as gas or taken up by plants for nutrition.
  • Some of the products of weathering, less resistant alumino-silicate minerals, become clay particles. Other altered materials are reconstituted by sedimentary or metamorphic processes to become new rocks and minerals.
  • These rocks may have gone through several cycles and may have undergone metamorphism or sedimentation before they finally became soil.
  • It takes a hundred years to form one centimetre of soil from the weathering of rocks. In a few places, this soil is only a few centimetres deep; in other places, it is 20 to 30 centimetres in depth.
  • Soils are the unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.
  • Soil particles are generally classified according to size; that is,
  • Sand as large particles
  • Silt as medium
  • Clay as fine
  • Particles larger than sand are classified as gravel
  • Are large enough to be identified as an individual rock.
  • Soil is essential for fixing the roots of plants and also provides the necessary nutrients, suitable temperature, and moisture for its growth.
  • Soil maintains its fertility with the remains of decayed plants and animals.
  • The growth of plants depends on the fertility of the soil. All living organisms depend on these plants. But human interference makes the soil infertile.
  • If this continues, ultimately it will affect the “soil-plant-organism” links and create negative impacts in the rock cycle.
  • To enrich the soil, humans add natural and chemical fertilizers.
  • Chemical fertilizers usually damage soil, partially because they eradicate the natural microorganisms.
  • Essentially, all life depends upon the soil.
  • There can be no life without soil and no soil without life; they have evolved together.

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