Layers of athe Earth
The Earth is made up of a number of layers.At the top is the crust – the thinnest layer .Next is the mantle,then the outer and inner cores.The Outer Core is probably liquid and the inner core solid.
|Sl No.||Layer||Average Depth (km)|
|1||Inner Core||5125 – 6371|
|2||Outer Core||2865 – 5125|
|3||Mantle||21 – 2865|
About 500 meteorites reach Earth every year. Many fall in the sea and in unpopulated areas and are never seen. The Hoba meteorite, the largest in the world, was found in Namibia in 1920. It measures 2.73 x 2.43m and is 82 per cent iron and 16 per cent nickel. It weighs more than 60 tonnes. Second largest is the Tent, found in Greenland in 1894 and now known by its original Eskimo name, Ahnighito. This meteorite weighs about 57.3 tonnes and is on display in the New York Museum of Natural History.
|Tertiary Pliocene||Pilocene||5,000,000—1,800,000||Ape-like human ancestors|
|Miocene||23,000,000—5,000,000||Apes and whaLes appear|
|Oligocene||38,000,000—2,000,000||Cats and dogs appear|
|Palaeocene||65,000,000—54,000,000||First large mammals|
|Mesozoic||Cretaceous||146000,000—65,000,000||Dinosaurs extinct, first flowering plants|
|Jurassic||208,000,000—146,000,000||First birds and mammals|
|Triassic||245,000,000—208,000,000||First dinosaurs and flying reptiles|
|Mississippian||360,000,000—325,000,000||First winged insects|
|Siturian||440,000,000—410,000,000||First land plants and insects|
|Ordoviciari||500,000,000—440,000,000||First corals and molluscs|
|Cambrian||544,000,000—500,000,000||First fish and shelled creatures|
|Precambrian||Proterozoic||2,500,000,000—544,000,000||Earliest fossils, first jellyfish|
|Archaic||3,800,000,000—2,500,000,000||First living cells|
|Hadean||4,500,000,000—3,800,000,000||Environment unable to support life|
|4,500,000,000 or earlier||Formation of Earth|
The largest meteorite craters
Many astrobtemes or collision sites on Earth have been aLtered by weather over miLLions of years. Scientists are not aLways certain whether crater-Like structures were caused by meteorites or are the craters of extinct voLcanoes. Those Listed beLow are all agreed to be meteorite craters, but new evidence based on photographs from space may reveaL other even Larger ones.
|Sl No.||Crater/location||Diameter (km)|
|1||Vredefort, South Africa||300|
|2||Sudbury, Ontario, Canada||250|
The 10 degrees of hardness
The Mohs scale, named after German mineraLogist Fiedrich Mohs (1773—1839), is used for comparing the reLative hardness of mineraLs. Each mineraL on the scale can be scratched by the harder ones beLow it.
|IMobs scale No.||Substance|
Rock or mineral?
There are three categories of rock — igneous, sedimentary and metamorphic.
Igneous rocks originate deep in the Earth. They erupt from voLcanoes as magma and cool or soLidify as they rise to the upper layers. BasaLt is an igneous rock and so is granite, a very hard rock often used in buiLding. Pumice stone is a soft igneous rock that is ejected from volcanoes. As it cooLs, it often fiLLs with so many air bubbLes that it fLoats in water.
Sedimentary rocks can be formed by deposits in water (aid occasionaLLy by wind). Sandstone is a common example — Uluru (Ayers Rock), AustraLia, is the Largest known rock (see box). Organic sedimentary rocks are formed by Living pLants and animaLs — coaL comes from pLant matter and Limestone from the calcium from billions of plants and animals. Chemical sedimentary rocks occur when chemical processes take place and minerals are deposited.
Metamorphic rocks are igneous or sedimentary rocks that have changed as a result of high temperatures or pressures. Slate used on roofs is a familiar example.
Minerals are naturally occurring substances with a definite chemical composition. Most mineral names end in “ite”. Many have a practical use or contain a chemical compound or element that can be extracted and used commercially. Bauxite, for instance, is the main source of aluminium. Gems are minerals that are highly prized for their rarity or appearance, eg diamonds, sapphires, emeralds and rubies.
The rock formerly known as Ayers Rock, Northern Territory, Australia, is believed to be the world’s largest free-standing rock. It is made of sandstone and measures 335m high, 3.6km long and 2km wide, It was originally called after South Australian premier Sir Henry Ayers, but it is now known by the name given to it by local Aborigines, to whom it is sacred.