Time and space
- The universe began 14 000 million years ago. At the start, it was smaller than an atom and has been expanding since
- Around 5000 million years ago, a great swirl of dust and gas came together to form the solar system.
- About 99.9% of that material became the sun.
- Gravity pulled the remaining dust and gas particles together into grains. These grains were pulled to make smaller clumps, the clumps were slowly pulled together by gravity to make planets
- At first, the Earth was so hot that it was a liquid. Gradually , the Earth's surface cooled and it's oceans and atmosphere were formed from volcanic gases
- Human have been on the planet for around 1/5 million years
Time and Space 2
- The Earth is an enormous, layered ball with a radius of 6400 km. Its rocky Crust averages only 10-40 km deep. The Mantle extends to about halfway to its centre. The Core is made mostly from iron, solid at the centre, liquid above.
- The Sun is a ball of extremely hot gases. Its diameter is 109 x larger than the Earth. The Earth and the Moon orbit around the sun once each year. They are 150 million km from the Sun. Saturn is 10 x further away from the Sun than the Earth is.
- Outside our Solar System, the nearest star is just over 4 light-years away. This is 30 thousand times the distance from Saturn to the Sun. The Sun belongs to a spiral galaxy called the Milky Way
- Our Galaxy is shaped like a disc with a bulge in the centre. Its arms rotate through space, once every 200 million years. The distance from the centre to the outer edge in 10 thousand times the average distance between stars. The Sun is just one of the 100 000 million stars in the Milky Way
- The Universe is everything that astronomers observe, directly or indirectly. It contains thousands of millions of galaxies. Galaxies cluster together to from wispy webs. Most of the Universe is empty and cold.
Gradually, geologists learned to work out the history recorded in rocks. They use clue like
- Deeper is older- in layered rocks, the youngest rocks are usually on top of older ones
- Fossils are time markers- many species lived at particular times and later became extinct
- Cross-cutting features- if one type of rock cuts across another rock type, it is younger. For example, hot magma can fill cracks and solidify as rock
But these clues can only tell you which rocks are older, not the exact age
Some rocks are radioactive. Scientists today can estimate their age by measuring the radiation that they emit (give off). This is called Radioactive Dating. The Earth's oldest rocks were made 3900 million years ago.
Most geologists (about 200 years ago) believed that the Earth began hot. They compared it to a drying apple that wrinkles as it shrinks. They thought that mountains were formed through the shrinking of the Earth's surface.
A new theory of mountain building was needed in 1900, when radioactivity was discovered (the heating effect of radioactive materials in inside the Earth prevents cooling). Alfred Wegener saw that the shapes of South America and Africa fit together like a jigsaw. Wegener thought this meant that the continents are moving and they had once been joined together. He looked for evidence recorded in the rocks- the theory is called continental drift
Continental Drift 2- Evidence
Scientists started making maps of the ocean floor. They found a chain of mountains under most oceans (this is now called an Oceanic Ridge). In 1960, a scientist suggested that the seafloor moves away from either side of an oceanic ridge. This process, Seafloor Spreading, could move continents.
Beneath a ridge, material from the Earths solid mantle rises slowly. As it approaches the ridge, pressure falls and some material melts to form magma. Movements in the mantle pull the ridge apart. Hot magma erupts to form new rock.
In 1963, a research student explained the symmetrical "stripe" pattern found in rock magnetism across and oceanic ridge. If hot magma rises at a ridge and cools to make new rock, then the rock will be magnetised in the direction of the Earths field at the time (it changes).
Seafloor drilling in 1969 provided further evidence of seafloor spreading. Sediments further away from oceanic ridges are thicker. This shows that the ocean floor is youngest near oceanic ridges, oldest farthest away.
The plate tectonic explanation for the Earth's outer layer
- The Earth's outermost zone, or lithosphere, consists of crust plus the rigid upper mantle
- It is made up of about a dozen giant slabs of rock, and many smaller ones called tectonic plates
- Currents in the Earth's solid mantle carry the plates along
- The ocean floor gradually grows wider at oceanic ridges by seafloor spreading. These are called constructive margins
- Ocean floor is destroyed where plates dip down beneath an oceanic trench. These are called subduction zones or destructive margins.
- The rigid plates are moved slowly, sometimes moving apart, sometimes pushing together, and sometimes sliding past each other
The movement of the plate tectonics causes continents to drift. It also explains parts of the rock cycle, mountain building, most earthquakes and most volcanoes
Plate Tectonics 2
Mountains- collisions between tectonic plate cause mountains to form, 3 ways this happens-
- An oceanic plate dives back down into the Earth, causing volcanic peaks on the surface
- Pushing movement at destructive margins can cause rocks to buckle and fold, forming a mountain chain
- An ocean closes completely and 2 continents collide in slow motion. The edges of the continents crumple and pile up, making a chain (e.g.- Himalayas)
Volcanoes are simply vents in the Earths surface that erupt magma and molten rock. The magma then forms lava or ash as it releases it's gases. They are common at plate boundaries where the crust is being stretched, compressed of uplifted. You may know if a volcano is about to erupt when- There is a change in the amount and type of gases, there is a local earthquake or the sides of the volcano swell with magma
Plate Tectonics 3
There are 3 ways plates move against each other-
- They move apart, Stretching movement e.g- oceanic ridges
- They push together, squashing movement e.g- Himalayas
- They slide past each other e.g- San Andreas Fault, California
Earthquakes mostly happen at previous breaks, called faults. The shunting of the Earth's plates causes greats forces to build up along fault lines. Eventually, the forces are so great that the rocks locked together break, and allows plate movement. The ground shakes. Earthquakes are common at all moving plate boundaries. The most destructive happens at sliding boundaries on land, or undersea causing tsunamis
To be ready for an earthquake or a tsunami, governments can
- Educate people
- Organize public drills
- Enforce building regulations- reduce the chances of buildings collapsing
- Prepare emergency plans and trained staff
Meteors and Craters
There is a huge crater in the USA. The 1st scientists to see it thought it was caused by a volcano. But in 1902, a scientist found that the crater rim contains many fragments of iron. He new that meteorites contained iron and concluded that a violent impact has caused the crater.Also-
- Quartz dust particles ( only produced by huge pressures) were found.
- When they looked at the layers of rock surrounding the crater, they found they were in reverse order to the layers of rock in the surrounding desert.
- These 2 observations support the impact explanation
Scientists have examined rock samples from the Earth, the Moon, Mars and meteorites. Radioactive dating shows that none of them are over 5000 million years, so the Solar System is probably this age
What caused the mass extinction of the dinosaurs,
- Luis Alvarez found a thin layer of Iridium Clay in Italian rocks. The iridium was in amounts that are common for meteorites or asteroids. The rocks below and above the layer told them the clay arrived there 65 MYA
- They suggested that this could be a theory for the extinction of the dinosaurs. But at the time, there was no evidence of a giant crater of the right age and no other clay had been found
- But in 1991, a 65 MYO crater was found in Chicxulub, Mexico and iridium clay was found in other parts of the world. The impact must have been so violent that it partially vapourised the ground and asteroid. Wind would have carried the material all around the planet. Over the following months and years it would have settled into a layer of dust on the ground. BUT there are two main problems
- Many animals and plants had started to die out before the asteroid had struck. There have been other major impacts which did not cause mass extinction
What caused the mass extinction of the dinosaurs,
A third of the land surface in India has layers of black rock called Basalt. It must have arrived there in layers of molten rock. There were hundreds of lava flows from a super volcano. And eruptions release a lot of poisonous gases. The eruptions which made India's basalt where at their most intense 65 million years ago. But they started before then. These eruptions could explain why extinctions began before the 65 million year mark. BUT there was also flood basalt events that did not cause mass extinction.
What are we made of?
Scientist can spread light into a spectrum and study the colours present. If they shine light through different chemical elements, then each element produces a unique pattern. Fine dark lines in the spectrum show where that element absorbs light. Scientists use this technique to identify what elements are present in a sample.
When astronamers first looked at the spectrum of sun light, they were amazed to find similar patterns to those seen in the lab. They looked at other stars and found exactly the same 92 elements everywhere.
Scientists once struggled to understand the sun, it could not be a great ball of fire. Any fuel or oxygen would have run out long ago. They found that atoms had a central core, called a nucleus. Joining small nuclei together releases energy and new elements are created, this is nuclear fusion. But this only happens in stars because only at extremely high temperatures do the nuclei have enough energy. The sun fuses hydrogen to make helium.
Heavy Elements are made in Stars
The most common element in the universe is hydrogen. In stars fusion continues to make bigger elements. When fusion stops, big stars explode as supernovae. Their debris, containing all 92 elements, is scattered through space.
When our solar system formed, it gathered debris from dead stars. Exept from hydrogen and helium, the chemical elemants that make up everything on earth come from the stars.
Are we Alone
There are two ways of working out the distances to stars.
PARALLAX - The earth moves from one side of the sun to the other, every 6 months. Seen through an earth based telescope, a near by star will shift its position against the background of more distant stars. The near a star is the further it shifts.
BRIGHTNESS - Imagine a large number of lights of different brightness. Some are much further away than others. It could be hard to tell the difference between an nearby torch and a distant search light. But if you know whether it is a torch or a search light, then you can judge its distance. So, if you know what kind of star it is, then you can use its apparent brightness to estimate its distance. The nearer a star the brighter it seems.
LIGHT YEARS - Proxima Centauri is not bright enough to see without a telescope. But it is the closet star outside the Solar System. Parallax measurement shows that it is 4.22 Light Years away. A light Year is the unit of distance used by astronomers. It is the distance travelled by light in a year.
Are We Alone 2
By the early 20 Century astronomers in America were starting to use some really big telescopes. Shapley worked in California investigating faint patches of light called NEBULAE. He could see that some nebulae are dense clusters of stars. After measuring their distances some of them seemed to be more than 100, 000 light years away. He suggested they were part of a gigantic star system - the MILKYWAY.
Some nebulae have a spiral shape, one of these is Andromeda. He argued his idea against Curtis, an astronomer, who claimed that the nebulae are star systems outside the Milky Way. On the night Shapley came off better but in a few years new evidence proved that Curtis had been right.
Hubble used a new telescope to find out how far away Andromeda is. He used the same method as Shapley. It seemed that Andromeda was a million light years away. It is another huge collection of stars held together by gravity, another galaxy. Scientists can only suppose, that in the time it took for their light to reach the telescope, the stars in these galaxies will have changed.
How Did the Universe Begin?
BIG BANG - Scientists now imagine that the Universe was once incredibly hot, tiny and dense. This explanation is called BIG BANG THEORY - an explosion of a single mass of material. This is currently the accepted scientific explanation for the start of the Universe.
THE AGE OF THE UNIVERSE - 50 Years ago, scientists used the speed and distance of galaxies to estimate how log ago all galaxies were in the same place. They had to assume that the galaxies had always moved away at the same rate, which may or may not be true. So it was only a rough estimate - it came out at somewhere between 10 000 million years and 20 000 million years.
Then in 2003, new observations of the cosmic microwave background radiation gave a much more precise answer. The Universe is about 13 700 million years old.
Other lines of evidence to support the BBT among them - a hot big bang explains why the early Universe was about 24% helium by mass and the oldest stars ( 12 000 million years old) are younger than the Universe.