When first formed Earth= completely molten (hot liquid). Estimated that Earth is 4500 million years old as it has to be older than its oldest rocks
Studying rocks tells us more about the Earths structure and how it has changed because of:
- Erosion: Earths surface is made of rock layers and the oldest is usually at the bottom. Layers are compacted sediment, produced by weathering and erosion and the erosion changes surface over time.
- Craters:The moons surface is covered with imact craters from meteors. Meteors also hit the Earth but craters have been erased by erosion
- Mountain formation: If new mountains were being formed, the Earths surface would have eroded down to sea level.
- Folding: Some rocks look as if they've been folded in half. Required huge force over a long time
- Earths age can also be found by studying: Fossils of plants and animals in sedimentary rock and radiactivity of rock. A rocks radioactivity decreases over time.
The Structure of the Earth
Thin Rocky Crust:
- Thickness varies between 10km and 100km
- Oceanic crust lies beneath oceans
- Continental crust forms continents
- Extends almost halfway to the centre of the Earth
- Has a higher density, and different composition than rock in the crust
- Very hot, but under pressure
- Accounts for over half fof the earths radius
- Made of nickel and iron, and has a liquid outer part and solid inner part
- The decay of radioactive elements inside the Earth releases energy, keeping the interior hot
- Continental drift theory was proposed by Wegener
- Saw that continents had a jigsaw fit, with mountain ranges and rock patterns matching up
- Also fossils of same animals on different continents
- He said that different continents had seperated and drifted apart
- Also claimed that when two continents collided they forced each other upwards to make mountains
However geologists didnt accept his theory because:
- He wasnt a geologist so was seen as an outsider
- Supporting evidence was very limited
- Could be explained more simply e.g a land bridge
- Movements of continents was not detectable
Evidence from seafloor spreading finally convinced the scientific community that Wegener was correct
Earths crust is cracked into large pieces called tectonic plates. These plates...
- float on Earths mantle so they are less dense
- can move apart, move towards or slide past eachother
- The lines where plates meet are called plane boundaries. Volcanoes, earthquakes an dmountain ranges usually occur and these plate boundaries
- Earthquakes that are near the sea can result in a tsunami
- A geohazard is a natural hazard e.g floods and hurricanes.
- Some have warning signs that give authoriteis time to evacuate area or take other measures.
- Some other geohazards strike without warning so precautionary measures have to be taken
- buildings in earthquake zones are designed to withstand tremors
- authorities often refuse planning permission in areas prone to flooding
- The mantle is fairly solid just below Earths crust and further down it's liquid.
- Convection currents in the mantle cause magma tomrise. The currents move the solid part of mantle and tectonic plates
- Where the plates are moving apart, magma reaches the surface and hardens, forming new areas of oceanic crust and oushing existing floor outwards
- New crust is continuously forming at the crest of an oceanic ridge and old rock is pshed out causing seafloors to spread a few cm each year
- Earth has a magnetic field and it changes polarity every million years
- Combined with seafloor spreading, this produces rock stripes of alternating polarity and geologist s can see how quickly crust is forming by the width of the stripes
- Occurs at constructive plate boundaries where plates are moving apart
- When oceanic and continental plates collide, the deanser oceanic plate is forced under the continental plate- this is subduction
- Oceanic platesmelts and molten rock can rise to form volcanoes. this occurs at destructive plate boundaries
- Mountain ranges form along colliding plate boundaries as sedimentary rock is forced by the pressue created in a collision
Plate Tectonics 2
Earthquakes occur most frequently at plate boundaries:
- The plates slide past each other or collide
- Pressure builds up as plates push on each other
- Eventually stored energy is released and waves of energy spread from the epicentre
Plate movement is crucial in the rock cycle:
- Old rock is destroyed through subduction
- Igneous rock is formed when magma reaches the surface
- Plate collisions can produce high temperatures and pressure causing the rock to fold
- Sedimentary rock becomes metamorphic rock
Waves from Earthquakes
- Earthquakes produce wave motions on the surface and inside the Earth. These waves can be detected by instruments on the Earths surface such as a seismograph
- There are two types of waves genreated by earthquakes and they are primary and secondary waves (P and S waves)
- Travel faster than S waves
- Can travel through both liquids and solids
- Can travel through the lwuid region of the outer core of the Earth
- Can travel through solids
- Can't travel through the liquid region of the outer core of the earth
Waves from Earthquakes 2
- P waves are longitudinal waves and S waves are transverse waves.
- P and S waves travel at different speeds in different rocks of different density.
- If the rock has a high density then the waves will travel faster.
- The boundaries between the different types of rock leads to changes in he speed of P waves and Swaves
- This causes refraction or reflection of the waves
- Measurements taken from seismographs at different points on the Earths surface can be used to give evidence about the structure of the Earth
Types of Wave
- Waves are regular patterns of disturbance that transfer energy from one point to another without transferring particles of matter
- There are two types of wave- longitudinal and tranverse
- Vibrates backwards and forwards about it normal position
- moves backwards and forwards in the sam eplane as direction of wave movement e.g sound waves
- Vibrates up and down about its normal position
- Moves up and down at right angles to the direction of the wave movent e.g light and water ripplrd and all elctromagnetic waves
All waves have several important features
- Amplitude- maximum distrubance caused by a wave, measured by the distance from a crest (or trough) of the wave to the undisturbed postition
- Wavelength- the distance between corresponding points on two adjacent disturbances
- Frequency- the number of waves produced (or passing a particular point) in one second. Frequency is measured in hertz (Hz)
Wave speed and Frequency
If a wave travels at a constant speed...
- increasing its frequncy will decrease its wavelength
- decreasing its frequency will increase its wavelength
Frequency is inversely proportional to wavelength
If a wave has a constant frequency...
- decreasing its wave speed will decrease its wavelength
- increasing its wave speed will increase wavelength
The wave equations
Wave speed (m/s) = Frequency (Hz) x Wavelength (m)
A tuning fork of frequncy 480Hz produces sound waves with a wavelength of 70cm when it is tapped. What is the speed of the wave?
480Hz x 0.7m= 336m/s
Distance (m) = Wave Speed (m/s) x Time (s)
The Solar System
The solar system was formed about 5000 million years ago:
- Solar system started as dust and gas clouds, pulled together by gravity
- This created intense intense heat. Nuclear fusion began and the sun (a star) was born
- The remaining dust and gas formed similar masses which were attracted to the Sun
The sun is massive compared with the planers and contains over 99% of mass of solar system. These masses include-
- Planets- 8 large masses that orbit the sun
- Moons- small masses that orbit planets
- Asteroids- small, rocky masses that orbit the sun
- Comets- small, icy masses that orbit the sun
- Dwarf planets- small masses (e.g Pluto) orbiting the sun
Planets, moons and asteroids all move in elliptical (slightly squashed, circular) orbits. Comets move in highly eliptical orbits. Earth takes one year to make a complete orbit
Sun is only 500 million years older than the Earth.
Suns energy comes from nuclear fusion
- Hydrogen atoms fuse together to produce an atom with a larger mass i.e a new chemical element.
- Binding energy stored in hdrogen atoms is released
All the chemical elements larger than helium were formed by nuclear fusion in earlier stars.
It's the nuclei of hydrogen atoms that fuse together during nuclear fusion.
The Speed of Light
- Light travels at very high but finite (limited) speeds.
- If the distance is great enough, the speed of light can be measured
- The speed of light through space (a vacuum) is 300 000 km/s (around one million times faster than sound).
- Light form Earth takes just over one second to reach the moon ( approx 384 400km away)
- Sunlight takes 8 minutes to reach earth.
- When we look at the sun, we see it as 8 minutes earlier.
- Vast space distances are measured in light years.
- One light year is the distance light travels in one year (approx 9500 billion km)
- The nearest galaxy to the milky way is 2.2 million light- years away
Measuring Distances in Space
Distances are measured in two ways-
- Relative brightness: The dimmer a star, the further away it is. However, brightness can vary so a star's distance is never certain.
- Parallax- if you hold a finger at arms length and close each eye in turn, your finger appears to move. the closer your finger, the more it seems to move. Parallax uses this idea to work out distance. Stars in the near distance appear to move against the background of distant stars. The closer they are, the more they appear to move. The further the star, the less accurate the measurement is.
- Radiation from stars tells us what we know about them.
- Types of Radiation that stars produce include visible light, ultraviolet and infrared.
- Light pollution is when when electric lights on earth make it difficult to see the stars.
- The Hubble Space Telescope orbits at a height of 600 km, so its not affected by this or other atmospheric conditions.
- If a source of light is moving away from us, the wavelengths of light are longer than if the source is staionary.
- Wavelengths of light from nearby galaxies are longer than scientists expect. Means that galaxies are moving away form us.
- Observations made by Edwin Hubble showed that almost all galaxies ae moving away from us and the further away they are, the faster they are movin away . He developed this into Hubble's Law, which states:
The speed at which a galaxy is moving away is proportional to its distance from us.
- If all galaxies are moving away from us, this must mean that the universe is expanding.
- If an electromagnetic wave appears to have a longer wavelength than it should have, then its been red shifted. Object must therefore be moving away. The more the wavelengths are red shifted, faster the object is moving away.
- in a red shift, the lines in the spectrum are displaced towards red end of the spectrum.
The Beginning and the End
- Big Bang theory says that universe began 14 000 million years ago
- Its hard to predict the future of the universe because of the difficulties of measuring the very large distances involved, motion of very distant objects and the assumptions made in interpreting observations
A knowledge of the quantity of mass in the universe is also required when predicting the future as:
- If there isnt enough mass the Universe will keep expanding
- If there is too much mass then the gravity will force the universe to collapse or implode