by Dr. Arun Ahluwalia

Landmarks in Earth's History
4600 million years (henceforth called my.) Earth, Moon and planets formed; Earth's magnetic field formed in this period.
3500 my. ago first permanent crust crustified.
3500 my. ago atmosphere ( with some carbon dioxide) & seawater formed.
3300 my. to 2500 my. ago oldest sedimentary rocks (layered rocks !) formed; photosynthesizing blue green algae flourished & around 2500 my. ago abundant free oxygen in atmosphere.
2300 my. first large scale glaciation on earth.
2000-1000 my. ago rapid growth of continents by accretion of microcontinents & possible formation of a super – continent; southern continents combine into Gondwanaland.
700-600 my. ago major glaciation affecting every continent & around 600 my. ago first appearance of soft bodied, multicellular organisms.
550 my. worldwide emergence of marine animals with shells & skeletons; algal colonies declined. If factors on earth or outside led to biomineralization in life forms is a topic endlessly debated across the world; also volcanic episodes.
480 my. first freshwater fish and plants.
440 my. giant sea scorpions ( upto 3 metres in size).
420 my. first land plants ( without this could not ever think of all the coal which came from dense forests which came later around 340 my. & dominated Gondwanaland!).
400 my. ago age of fishes.
250 my. ago mass extinction of marine life.
270 my. Supercontinent Pangea stretches from 60 o N to the S pole & 250 my. ago this moved north to straddle Equator; many continents in warm arid climates; Asian microcontinents begin to move away from Australia & Gondwanaland.
220 my. ago Dinosaurs developed from Thecodont reptiles; also earliest bird came.
210-145 my. Dinosaurs dominant; birds spread & 180 my. ago Africa & South America begin to split from North America opening up the Central Atlantic; 150 my. ago Rocky Mountains began forming.
145-65 my. Massive flood basalts erupted through gaping fissures in W,C India Dinosaurs dominated land; flowering plants emerged.
120 my. India split from Africa & Antarctica & began moving north; Australia split from Gondwanaland which started breaking up.
100 my. S America & Africa began to split.
65 my. Mass extinction of marine & land life; dinosaurs (land) & ammonites (sea) disappeared.
40 my. Rockies uplifted; 50 my grasses emerge on land & grazing animals & monkeys emerge; whales & dolphins return to sea & 35 my first Apes emerge; lush green grasses cover land.
30 my Japanese islands split from Asia opening up Japanese Sea.
25 my northern North Atlantic opens between Greenland & northern Europe; Africa moves north to close the Tethys Sea & collide with Europe.
20 to 15 my. Alps formed.
20 my. India begins to collide with Asia initiating beginning of the mightiest & youngest mountain chains Himalayas; 11-10 my great Apes & Hominid (human like!) separated.
15 my basalt lavas outpoured in southern Siberia, Central Europe, E Africa & Antarctica.
3 my Antarctica isolated South America moves away; the last pieces of Gondwanaland break apart; climate cooled dramatically; first Hominids ( Australopithecus appeared).
2 my. major glaciation.
1 my Homo erectus disperses from Africa as far as China & Java & 5 lakh years back man appears in Africa & migrates to Europe & 250,000 years ago modern humans emerged in southern Africa.
After the peak of last glaciation 18,000 years ago, man reached every continent.

Human perception needs overstretching to undertstand Earth's long but not a quiet and enigmatic history. Catastrophic events repeat in regular cycles e.g. 150 major earthquakes & 20 volcanic eruptions a year look like a normal geological routine. Even for the geologists quite used to massive forces of the Earth ( e.g. volcanoes, earthquakes & mountain uplift), the movement of whole continents listed above was an enigma, not for want of evidence about their being once big & later moving & breaking up, but because till early sixties, we did not have a plausible explanation of pushing enormous landmasses around on Earth's surface. Now this mechanism is accepted as Plate Tectonics:

- layer of the Earth below the crust- the mantle is partially molten & heat sources from within the mantle ( radioactive elements) set up huge convection currents, which in turn induce flows across the upper part of the mantle;
· solid crust sitting on the mantle is affected by these flows & the solid crust is in fact made up of a series of rigid plates ( six major & many minor) which move in response to mantle currents beneath- hence the continental drift.
· where plates are moving towards each other, one will be pushed under the edge of the other & back down into the mantle; these destructive plate boundaries are where most of the Earth's geological activity takes place
· the plates which make up the ocean floor are constantly recycled in this way, while the continents are permanent and have been gradually growing through the addition of more crust at their edges over 4000 million years of Earth's geological history telescoped above.

Plate tectonics remains a theory, since the Earth's mantle is fairly inaccessible and the movements in it are very gradual. This theory has been remarkably successful in accounting for the past and present Earthly phenomena.

Like it has been, Earth will possibly remain active for hundreds or thousands of millions of years. Circulation in the outer core maintains the Earth's magnetic field. Umpteen times in the geological past, it is documented that magnetic polarity has reversed and many major extinctions of life have coincided with such reversals.

It is visualised that heat sources inside the Earth will eventually run out & Earth's mantle will stop circulating, will cool down & solidify & the Plate Tectonics will cease. The crust may ultimately become fused into one solid plate. Surface processes like weathering & erosion will continue, but the Earth would then become a dead planet like the moon has been for most of its existence. In such a distant scenario the effects would be profound: surface water would disappear; cooling of core would stop generation of the Earth's magnetic field; the ionosphere would be dispersed making life impossible.