The Four Realms of Earth - Part 1

prasanthi's version from 2012-04-01 15:35


ICSE Class VII - Geography


* Important Facts
§ The four realms of the earth are:
i. Lithosphere - the solid portion of earth
ii. Hydrosphere - all the water bodies on earth
iii. Atmosphere - the blanket of air around us
iv. Biosphere - the living world
§ The outermost solid layer of earth, made up of rocks and soil, is called the Lithosphere. It is also called the crust of the earth.
§ About 71% of the total area of lithosphere is covered by water. The remaining 29% is occupied by land.
§ The large masses of land are called continents, while the vast water bodies are called oceans.
§ The dead remains of plants and animals is called humus.
§ The rate of decrease in temperature of air by 10 C for every 165 m increase in height is called normal lapse rate.
§ The narrow boundary that separates the stratosphere from the troposphere is known as tropopause.
§ The narrow boundary that separates the mesosphere from the stratosphere is known as stratopause.
§ The narrow boundary that separates the thermosphere from the mesosphere is known as mesopause.
§ Electrically charged particles are called ions.
§ Thermosphere is also known as ionosphere because of the presence of ions.
§ Carbon dioxide keeps the earth warm by absorbing the heat radiated from the surface. This is called the green house effect.


* Continents
§ There are seven continents on earth. They are:
i. Asia
ii. Africa
iii. North America
iv. South America
v. Antarctica
vi. Europe
vii. Australia
§ Majority of the landmasses are in the Northern Hemisphere.
§ Asia, Europe and North America lie almost entirely north of the Equator.
§ Australia and Antarctica lie completely south of the Equator.
§ Africa and South America lie of both sides of the Equator.


* The interior of the earth
§ The earth is divided into three concentric layers. They are:
i. Crust
ii. Mantle
iii. Core
§ The crust is the outermost layer of the earth's surface.
§ The thickness of the crust varies from 5 to 8 km beneath oceans to an average of 35 km under the continental landmass.
§ The main elements that make up crust are silicon (Si), aluminum (Al) and magnesium (Mg).
§ The main elements of continental landmass are silicon and aluminum. Thus it is called sial.
§ The main elements of Oceanic crust are silicon and magnesium. Thus it is called sima.
§ The layer below crust is called mantle.
§ The mantle is 2900 km thick and is mostly made up of silicon, iron and magnesium.
§ The mantle consists of two layers:
i. Upper mantle - average temperature is 8700 C
ii. Lower mantle - average temperature is 22000 C
§ The mantle will be is a semi-solid state.
§ The innermost layer of the earth is called the core.
§ The thickness of the core is about 3500 km.
§ The core is made up of nickel and iron.
§ The core is divided into two layers:
i. Outer core - has a temperature of 37000 C and is in molten, liquid state.
ii. Inner core - has a temperature of 50000 C and is still in solid state.
§ The inner core is still in solid state, despite having temperature of about 50000 C, due to the extreme pressure exerted by the weight of overlying rocks.


* Rocks
§ Geologically speaking, rocks include all the solid materials of the earth's crust, whether it is hard like granite or soft like clay.
§ The natural inorganic substances found in the earth's crust are called minerals.
§ Minerals contain one or more elements and have definite chemical compositions and physical properties.
§ Quartz, feldspar and mica are some common rock-forming minerals.
§ Diamond is the hardest mineral.
§ Some rocks contain metals in large quantities. Such rocks are called ores.
§ Rocks are classified into three types. They are:
i. Igneous
ii. Sedimentary
iii. Metamorphic
§ Igneous rocks
→ The term 'igneous' is derived from Latin word ignis meaning 'fire'.
→ The hot molten material in the interior of earth is called magma.
→ When magma reaches earth's surface through cracks and crevices or through volcanic eruptions, it is called lava.
→ Cooling and solidification of molten magma either inside or on the surface of the earth results in the formation of igneous rocks.
→ Igneous rocks are hard and crystalline in nature and are formed either below or above the surface of the earth.
→ When cooling of magma takes place below the surface of the earth, it results in the formation of intrusive plutonic rocks. Granite and dolerite are good examples of such rocks.
→ When cooling of magma takes place above the surface of the earth, it results in the formation of extrusive or volcanic rocks. Basalt and obsidian are good examples of such rocks.
→ Igneous rocks are also called primary or basic rocks.
→ Igneous rocks make up a large percentage of the earth's crust.
§ Sedimentary rocks
→ Running water, moving ice, winds and waves constantly break down rocks on the earth's surface into smaller fragments. These small pieces of rock are called sediments.
→ Sediments are of different sizes, ranging from gravel to sand, clay and silt.
→ Sediments are usually transported by rivers and deposited on the floors of lakes, seas or oceans. Due to heavy pressure they get compressed and are cemented together resulting in the formation of sedimentary rocks.
→ Sedimentary rocks are also called stratified rocks as their deposition takes place in layers or strata.
→ Sedimentary rocks constitute about 70% of all the rocks exposed on the earth's surface.
→ The dead remains of plants and animals are called Fossils.
→ Fossils are found in sedimentary rocks.
→ Coal is a sedimentary rock.
→ Petroleum and natural gas are found in the layers of sedimentary rocks.
→ Petroleum and natural gas are called fossil fuels.
§ Metamorphic rocks
→ Metamorphosis means 'change of form'.
→ When igneous or sedimentary rocks are subjected to extreme heat and pressure, they undergo a complete change in their form and characteristics. Such rocks are called metamorphic rocks.
→ Marble, slate, gneiss and quartzite are some examples of metamorphic rocks.
→ Marble is formed from limestone. Slate is formed from shale. Gneiss is formed from granite. Quartzite is formed from sandstone.
§ Significance of rocks
→ In ancient times, rocks were used to make tools and weapons
→ Rocks are used in construction
→ Rocks contain minerals, which are the raw materials used in many industries
→ Rocks contain fossil fuels which are the main sources of energy
→ Rocks determine the type of landform, drainage and soil of the area
→ Some rocks are sources of precious stones used in jewellery
§ Soil
→ Soil is the uppermost layer of earth's crust. It is a mixture of fine powdered rocky material and humus.
→ The process of soil formation is very slow. It takes thousands of years to get 2-3 cm thick layer of soil.