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Shropshire Rocks (an introduction)

Planet Earth – how did it all begin

Scientists believe that about 13.8 billion years ago, something (or nothing) exploded in what they call The Big Bang. This enormous explosion hurled hot matter (atoms, elements, gases) out into the emptiness of space.

As this matter got further away from the centre of the explosion it started to cool and form spinning clouds of dust and gas. As they span, they started to flatten out and form discs, with the heavier atoms and elements (like iron) attracted to the centre of this spinning cloud of dust. This formed a hot, bright glowing cloud at the centre, the birth of a new star. As all these materials collided they heated up and got squeezed, and these dust particles were attracted to one another through gravity. Any thing, no matter how big or small it is attracts other things, with bigger heavier things having more “pulling power” than smaller lighter things. So dust particles collided to make bigger dust particles to make lumps, and those lumps attracted other chunks, and that may be how the first planets formed.

All this collision is a hot business. Early planets would have been collections of cold chunks of material, but when something hits one of these it releases a lot of energy in the form of heat, warming up this newly created junior planet, or planetissimal.

Spiral Galaxy

So the planets in our solar system were growing by devouring the leftover bits of debris from the dust cloud they were born out of, and heating up in the process. Most of the heavier elements were attracted to the centre of the spinning dust cloud. Mercury, the closest planet to the Sun is very iron rich , and the amount of iron found in the planets gets less as you move away from the sun. In the early few million years of the solar system, there were probably one or two extra planets in addition to the ones we know today, one in between Mars and Jupiter, where the asteroid belt is today and another rogue planet orbiting between Mars and Earth. The planet between Mars and Jupiter probably disintegrated and became the asteroid belt. Jupiter has a very strong gravitational field and the tug of war between that planet and the sun probably ripped the planet to pieces not long after it was formed.

Our planet was starting to settle down and warm up. You might be aware that today scientists think that the earth is made up of layers, but this almost certainly wasn’t the case about 4600, 000, 000 (that’s four thousand six hundred million years ago, or 4.6 billion years ago). The inside of the earth would have been a mixture of whatever it had swept up from the dust cloud. This constant bombardment from asteroids and comets would have caused the Earth to warm up, and eventually the inside of the Earth got hot enough to melt iron. All the iron and other heavy elements like uranium and nickel collected together at the centre forming a metallic ball, the Earth’s core. This left all the lighter elements like oxygen, silicon and carbon in the surface layers of the earth.

Having this huge metallic ball in the centre of the planet created a new thing, a magnetic field. Just like a magnet the earth started to project a magnetic field around itself and this was enough to deflect fierce solar winds that battered the planet and allow the first atmosphere to form around the planet. The surface of the earth would have been very hot, covered in volcanic eruptions, probably no water and bombarded by deadly solar radiation and meteors. Any atmosphere that formed would have to have come from one of these sources and the most probable is volcanoes. Today when volcanoes erupt they don’t just spew out hot lava and ash they also release millions of tonnes of gases into the atmosphere and it would have been the same back at the dawn of the earth. Volcanoes erupt gases like carbon dioxide, sulphur gases, water vapour and other nasty unbreathable things, so the first atmosphere would have been pretty poisonous.

An atmosphere, any atmosphere is a useful thing for a planet to have, because it can start to act as a defence mechanism for solar radiation and meteor bombardment. But nothing could withstand the onslaught that was about to happen.

About 4 billion years ago, that rogue planet crashed into the surface of the earth vaporising millions of tonnes of rock and metal from the earth and destroying itself in the process. All the molten material that was thrown into space didn’t get far though; it was captured by the gravitational attraction of the earth and it cooled into the Moon.

Have you ever wondered why the astronauts that landed on the moon jumped rather than walked? That’s because the moon has less gravitation pull than the Earth and this is because when that rogue planet collided not much of the core of the earth was vaporised. It was mostly light, rocky material that went to create the moon, so it only has a tiny iron core in comparison to the core of the Earth.

The formation of the moon through this catastrophic collision would have stripped away the fledgling atmosphere and disrupted the protective magnetic field around the earth. It was probably this collision that tilted the axis of the earth slightly away from the vertical and gave us seasons.

So the earth started all over again, forming a new, but still toxic atmosphere and calming the ravaged surface of the planet, with the continual collisions of comets and asteroids now pounding both the earth and its new satellite.

In light of this, it’s a wonder that life ever happened. The volcanoes that give out all those harmful (to us) gases also spewed out the one thing that life needs – water.

The first life may have formed as early as 3.8 billion years ago, pretty soon (geologically speaking) after the birth of the planet. Geologists call this first 2 billion years of earth history the Hadean, named after the Greek mythological idea of hell. A pretty accurate description of the earth at those times, hot violent and reeking of sulphur.

Some say life came spontaneously into being from a soup of molecules and a lightning strike, others that it was brought from elsewhere on one of the many comets that were pounding the planets surface, but however it began, once it started it would never stop.

The first life would have been simple single celled organisms adapted to live in a radically different environment to the one we know today. They would have been tough enough to withstand the high levels of solar radiation that battered the planet and probably fed off of sulphurous rocks as some bacteria still do today. One of the side effects of intense radiation is that it causes cells to mutate and change and in single celled organisms that hadn’t invented sex yet these mutations would be quickly seen.

One of these mutations would be the use of a green pigment that uses sunlight and carbon dioxide to make a food source. We know this process as photosynthesis and it is going on all around us today in plants and trees, and in the oceans. These simple celled organisms used photosynthesis to make their own food and as a by-product they produce something that we humans would come to be grateful for later – oxygen.

Oxygen is a wonderful thing. It loves combining with other elements rather than be on its own, a real party animal. So when these algae started producing Oxygen it reacted like crazy with anything around it, hydrogen to make water, carbon and calcium to make limestone, iron and other metals to make oxides. Only when all the available things for oxygen to react with had been used up, then oxygen started to build up in the atmosphere.

This new atmosphere would probably have been poisonous to many of the early inhabitants of the earth, and slowly they would either have died out or evolved to take advantage of the new situation. Oxygen in the upper atmosphere reacts to form ozone, one of the most important filters of solar radiation and this would also have been a blow for the first life forms, which were used to higher levels of solar radiation.

2.5 billion years ago and the surface of the earth was now relatively stable. Oceans had built up from volcanic water and melted comet ice and now true continents started to emerge. How these continents are made and why the earth is so different from many other planets in the solar system is our next stop.

Earth Interior
Plate Tectonics
Have you ever noticed how the coastlines of Africa and South America look as though they would fit together? Or wondered why some areas have earthquakes and volcanoes while others have none?

There is an idea that explains both of these strange observations and it is called Plate Tectonics. The thin skin at the surface of the earth is called the crust and this is broken up into chunks that move around slowly.

Tectonic plates
Below this is a layer called the mantle, which is hot solid rock that can move, a bit like plasticine. In the middle of the planet is the core, which has lots of iron and also lots of uranium and other radioactive elements. There are nuclear reactions going on all the time in the core and these provide a source of heat that has got to be got rid of somehow.

The radioactive core heats up the plasticine mantle and just as when you put a pan of water on the stove – heat rises up and warms the surface and the cooler water or mantle sinks back down again to be reheated. This circulation is called convection and there are huge convection currents in the mantle driven by the heat from the radioactive core. These convection currents drag the thin crust, across the surface of the earth.


At the beginning of the earth story, there was only one type of crust that was made of pure melted mantle, which contains lots of iron minerals and this cools to make a very heavy and dense rock. Over time these rocks melted and were reheated and this changed them slightly. Lighter minerals melt at lower temperatures and rise up, leaving iron rich minerals behind, and these lighter minerals like quartz make up most of the rocks that are found in continents.

So now we have two types of rock, iron rich crust that is found beneath the oceans and is dense and heavy and quartz rich rock that is light and makes up continents. The plates then are either mostly continental, which means that they carry a continent on them, like Europe or North America, or they are oceanic like the Pacific plate, which means that they are mostly under the oceans.


Think back to the idea of circulating currents in the mantle. Where one of these rising currents hits a plate it starts to pull it apart and eventually a tear can open up in the crust. Instead of a gaping rip being left in the earth surface, the mantle below it melts and forms a chain of runny volcanic eruptions along the rift. As the two plates are pulled further apart over time more and more molten rock pours up into the gap and builds new oceanic crust and makes a new ocean.

Plate divergence

By this mechanism Europe is moving away from North America at about the same rate as your fingernails grow. The process has also created the one of the highest and longest mountain chains in the world – but we don’t see it because it is deep beneath the Atlantic Ocean.

The earth has an iron core, which gives the planet a magnetic field with a North and South Pole. You might think that North has always been North, but that’s not the case. Several times over the last 50 million years the earth has changed its polarity, with north becoming south and vice versa. This is a pretty radical change and it has been recorded like ticker tape on the floor of the Atlantic Ocean.

The new rock forming there is rich in iron and as it solidifies it picks up the natural magnetism of the earth, which you can measure. Scientists measuring the magnetic signature of the Atlantic Ocean discovered a pattern of positive and negative stripes and realised that these suggested that the poles have reversed in the past.

Magnetic Stripes
So if new crust is being created at these mid-ocean ridges, is the earth getting larger to take this into account, or is old crust being destroyed somewhere else to make room? Well, now that we can measure the Earth from space we are pretty sure that it’s not getting any fatter, so that leaves us with old crust being destroyed somewhere.

If crust were being destroyed, this would be a very violent process and there are places around the world that suffer very badly from earthquakes and volcanoes.

>If a piece of old, cold dense oceanic crust were being forced under a lighter piece of continental crust you would expect a lot of earthquakes from the grinding of one huge plate against another. You might also expect to find volcanoes as the slab of crust that is pushed down into the earth reacts with the mantle and heats it up. This mantle melting happens through water and ocean sediment being carried down into the earth on the slab of ocean crust. The water and impurities make the mantle melt and it punches up through the crust above it and makes a volcano.

These violent volcano and earthquake combinations are found all around the Western and Northern edges of the Pacific Ocean and give it its nickname of the Ring of Fire.

This process of one enormous piece of oceanic plate being forced under another plate is called subduction and it generates some of the most powerfully destructive earthquakes and explosive volcanoes seen anywhere on the face of the earth.

Plate subduction

When two continental plates collide, they are both light so neither one will be subducted. What happens here is that over millions of years an enormous chain of mountains is thrown up, like the Himalayas. These were created when the Continental plate of India crashed at high speed (geologically speaking) into the Asian plate. This huge mountain range has grown over the last 45 million years and is still getting higher every year as the Indian plate continues to push further North into Asia.

Continental - continental collision

This process generates massive earthquakes as chunks of crust try to slide out of the way of the enormous squeezing pressure, which can get so high it actually melts solid rock.

India plate motion

Sometimes the convection in the mantle causes the plates to neither pull apart nor crash together, but to grind past each other, with crust not being created or destroyed. This is happening in California along the world’s most famous tear in the earth’s crust – the San Andreas Fault.

Here, the Pacific plate, of which California is a part, is moving Northwards against the stationary North American plate. This is not a smooth easy journey because the edges of the plates are rough rock and stick against each other. When they stick, enormous pressure builds up to a point where something snaps in a dramatic fashion, creating powerful earthquakes. These earthquakes are almost impossible to predict. You might get bulges or dips in the ground around the area where the pressure is building up, or you might get lots of small quakes before a big one hits. But then again, an earthquake may happen with no warning at all.

San Andreas Fault

Through this process of moving plates and recycling rocks the earth has remained constantly active throughout its 4.6 billion-year journey. Have you ever wondered why the Moon has so many craters yet we don’t see or find many on Earth, which is (relatively speaking) right next door? Were we lucky in that meteors or comets just don’t hit the earth? No, this planet has been pounded by as many meteorites as the moon has; it’s just that the recycling of the crust that happens on earth, through plate tectonics, doesn’t happen on the moon. Everything that ever hit the surface of the moon is there for all to see. Yet on our planet they have been mostly subducted into the earth, been overgrown by plant life or covered over by sediment.


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