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■ HARAPPAN'S DRAINAGE SYSTEM

◇ Well laid-out streets and side lanes associated with an efficient and well-planned drainage system are other notable features of Harappan settlements.

◇ The sewage chutes and pipes were separate from drains for collecting rain water.

◇ Drains and water chutes from the second storey were often built inside the wall, with an exit opening just above the street drain.

◇ At Harappa and Mohenjodaro, terracotta drain pipes directed waste water into open street drains made of baked bricks. These connected into large drains along the main streets, which emptied their contents into the fields outside the city wall.

◇ The main drains were covered by corbelled arches made of brick or stone slabs.

◇ There were rectangular soakpits for collecting solid waste at regular intervals.

■ GEOMAGNETIC STORMS

◇ The varying conditions in the magnetosphere, known as space weather, are largely driven by solar activity.

◇ If the solar wind is weak, the magnetosphere expands; while if it is strong, it compresses the magnetosphere and more of it gets in.

◇ Periods of intense activity, called geomagnetic storms, can occur when a coronal mass ejection erupts above the Sun and sends a shock wave through the Solar System. It takes just two days to reach the Earth.

◇ At the Earth's surface, a magnetic storm is seen as a rapid drop in the Earth's magnetic field strength.

◇ Ring Current: Ring current is the name given to the large electric current that circles the Earth above its equator during magnetic storms.

■ AURORA

◇ Aurora is the name given to the luminous glow in the upper atmosphere of the Earth which is produced by charged particles (solar wind) descending from the planet's magnetosphere.

◇ Positive ions slowly drift westward, and negative ions drift eastward, giving rise to a ring current. This current reduces the magnetic field at the Earth's surface.

◇ Some of these particles pe*****te the ionosphere and collide with the atoms there.

◇ This results in an excitation of the oxygen and nitrogen molecular electrons. The molecules get back to their original state by emitting photons of light which are the aurorae.

◇ The charged particles follow magnetic field lines which are oriented in and out of our planet and its atmosphere near the magnetic poles. Therefore, aurorae mostly are seen to occur at high latitudes.

PLANNING OF HOUSES in HARAPPA

◇ People lived in houses of different sizes, mostly consisting of rooms arranged around a central courtyard.

◇ Doorways and windows generally faced the side lanes and rarely opened onto the main streets.

◇ The view from the lane into the courtyard was blocked off by a wall.

◇ There are remains of staircases that may have led to the roof or a second storey.

◇ The fact that some of the houses at Mohenjodaro were two stories high or more is also suggested by the thickness of their walls.

◇ Floors were usually made of hard-packed earth, often re-plastered or covered with sand.

◇ The ceilings were probably over 3 m high. Roofs may have been made of wooden beams covered with reeds and packed clay.

◇ The doors and windows of houses were made of wood and mats.

◇ Many houses or groups of houses had separate bathing areas and toilets.

■ THE OUTER CORE

◇ The outer core, surrounding the inner core, lies between 2900 km and 5100 km below the earth's surface.

◇ The outer core is composed of iron mixed with nickel (nife) and trace amounts of lighter elements.

◇ The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to the inner core.

◇ The density of the outer core ranges from 9.9 g/cm3 to 12.2 g/cm3 .

◇ The temperature of the outer core ranges from 4400 °C in the outer regions to 6000 °C near the inner core.

◇ Dynamo theory suggests that convection in the outer core, combined with the Coriolis effect, gives rise to Earth's magnetic field.

■ THE MANTLE

◇ It forms about 83 per cent of the earth's volume and holds 67% of the earth’s mass.

◇ It extends from Moho’s discontinuity to a depth of 2,900 km.

◇ The density of the upper mantle varies between 2.9 g/cm3 and 3.3 g/cm3.

◇ The lower mantle extends beyond the asthenosphere. It is in a solid state.

◇ The density ranges from 3.3 g/cm3 to 5.7 g/cm3 in the lower mantle.

◇ The mantle is composed of silicate rocks that are rich in iron.

◇ Regarding its constituent elements, the mantle is made up of 45% oxygen, 21% silicon, and 23% magnesium (OSM).

◇ Temperatures range from approximately 200 °C at the upper boundary with the crust to approximately 4,000 °C at the core-mantle boundary.

◇ Because of the temperature difference, there is a convective material circulation in the mantle (although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile).

■ MOST ABUNDANT ELEMENTS OF THE EARTH'S CRUST

ELEMENT - APPROXIMATE % BY WEIGHT

◇ Oxygen - 46.6%

◇ Silicon (Si) - 27.7%

◇ Aluminium (Al) - 8.1%

◇ Iron - 5.0%

◇ Calcium (Ca) - 3.6%

◇ Sodium (Na) - 2.8%

◇ Potassium (K) - 2.6%

◇ Magnesium (Mg) - 1.5%

■ THE CRUST

◇ The crust is the outermost layer of the earth making up 0.5-1.0 per cent of the earth’s volume and less than 1 per cent of Earth’s mass.

◇ Density increases with depth, and the average density is about 2.7 g/cm3 (average density of the earth is 5.51 g/cm³).

◇ The thickness of the crust varies in the range of 5-30 km in case of the oceanic crust and as 50- 70 km in case of the continental crust.

◇ The continental crust can be thicker than 70 km in the areas of major mountain systems. It is as much as 70-100 km thick in the Himalayan region.

◇ The temperature of the crust increases with depth, reaching values typically in the range from about 200 °C to 400 °C at the boundary with the underlying mantle.

◇ The temperature increases by as much as 30 °C for every kilometre in the upper part of the crust.

◇ Outer covering of the crust is of sedimentary material and below that lie crystalline, igneous and metamorphic rocks which are acidic in nature.