The serious study of Earth science did not begin until the 19th century and since then, there have been many momentous discoveries. The age of the Earth has been pushed back from a few thousand years to around 4.5 billion years. Earthquakes and volcanoes are no longer the signs of angry gods. We understand how mountain ranges form. We believe that the following are the top 10 greatest discoveries in Earth science. What do you think?
1 – The Earth has a Core
Until the study of seismic waves, knowledge of the Earth’s structure was limited to observations made at or near the surface. When there is a large earthquake, the shock waves can be detected at seismic stations around the world. Studying how these travel through the Earth enables scientists to ‘see’ the inside of our planet.
British scientist Richard Oldham discovered that there were 3 different types of shock wave from earthquakes and that the P-waves and S-waves can pass through the whole Earth, arriving at different times at different seismic stations dotted around the World.
In 1906, whilst studying waves from a particular earthquake, he realised that the data suggested the Earth had a core. Earthquake waves usually increase in speed as they get deeper, however, he noticed that at a certain depth, they are suddenly slowed down. By studying data from hundreds of other earthquakes, he was able to give an approximate size of the core and deduce that the core was molten as S-waves were unable to get through.
2 – The Core has Two Parts
Waves from an earthquake don’t reach every seismic station. The Earth’s core creates shadow zones where no waves from an earthquake can be detected. There is an S-wave shadow zone that begins at an angle of about 104 degrees from the Earthquake whereas the P-wave shadow zone is smaller, they cannot be detected directly in an area of 104 to 140 degrees from the earthquake.
Working almost 30 years after Oldham, Danish seismologist Inge Lehman was examining the records of a 1929 New Zealand earthquake. She noticed some P-waves appeared to have been reflected off a solid boundary within the core as they had been picked up in the P-wave shadow zone. No P-waves should be detected here, so she suggested in a 1936 paper that the Earth’s core had two parts – a liquid outer core and a solid inner core.
She was such a well-respected seismologist that her suggestion was adopted by Earth scientists without hesitation. It wasn’t until 1970 that her findings were confirmed when more sensitive seismometers had been developed. We now know that the inner core is mainly solid iron and is the likely origin of the Earth’s magnetic field.
3 – Continental Drift
German meteorologist and geophysicist Alfred Wegener had the idea that the continents had once been joined together. He called this super-continent ‘Urkontinent’ (primal landmass) which is now known as ‘Pangaea’ (all Earth). It wasn’t until 20 years after his untimely death in 1930 that his hypothesis was starting to gain acceptance.
One of the problems was the mechanism. Earth scientists of the era had no idea how the continents could have moved, Wegener had proposed several ideas but he had overestimated the speed at which they travel – 250cm per year as opposed to the actual 2.5 cm (give or take).
A further problem was that it was widely thought that he had based his matching of continents on coastlines which of course had been changed by the processes of erosion and deposition. Wegener was smarter than that and had matched the continental shelves using the 200m depth isobath (undersea contour line) to avoid that objection.
He also saw similarities between certain structures and fossils (like glossopteris seen in the photo above) on continents now separated by thousands of miles. When the continents were closed together, he could see that these were more than similarities, they were exact fits.
Continental drift is well worth its place in the top ten greatest discoveries in Earth science. Although Wegener did not live to see it, he was the founder of one of the greatest scientific revolutions of the 20th century, akin to that within astronomy – the Big Bang versus the Steady State theory.
4 – Sea Floor Spreading
The existence of a mid-Atlantic ridge had been suggested as early as 1850 and was confirmed about 20 years later by engineers looking at the possibility of laying the first transatlantic cable. Fast forward 70-odd years and sonar measurements showed its true extent – it ran the entire length of the Atlantic and extended round the Cape of Good Hope into the Indian Ocean. It was dubbed the ‘Great Global Rift’ and it appeared that the central valley was geologically active.
During the 1940s, American geologist Harry Hess discovered that the floor of the Pacific ocean was mountainous. The discovery of the Great Global Rift inspired him to look back at his data. This led him to propose that the movement of the continents was a result of sea-floor spreading. Molten magma from beneath the crust could squeeze up between the two sides of the Great Global Rift pushing them apart. As this hot magma cooled forming new rocks, he reasoned that the sea floor should be older the further you are from a ridge.
This was found to be the case and, with other evidence such as the magnetic striping of the ocean floors, offered compelling evidence to back up Continental Drift.
5 – Plate Tectonics
If new material was continually being added to the plates of the Earth, why was the Earth not getting bigger? The answer to this was that the plates were disappearing back down into the Earth – subduction. This is the essence of plate tectonic theory which was built on Wegener’s theory of continental drift.
Along with firm evidence for sea floor spreading, a worldwide pattern of earthquake zones and volcanic zones had been recognised. These marked the edges of a series of large plates that form the crust of the Earth. They float and move on the Earth’s mantle, a region of intensely hot rocks in a plastic state.
There are 3 types of plate boundary – constructive, destructive and conservative. Constructive boundaries are where the plates move apart and new material is added. Destructive boundaries see plates subducted back into the Earth where they re-melt and return to the mantle. Conservative boundaries are where plates slide past one-another. No material is either added nor subducted.
Using these three types of plate boundary can explain mountain ranges, volcanoes and earthquakes.
6 – The Layers of the Atmosphere
Leon Teisserenc de Bort was a French meteorologist who set up a private weather station in the late 19th century. Years of careful observation using instruments mounted on balloons led him to the conclusion that the Earth’s atmosphere was made up from two layers.
He found the air temperature steadily decreased up to a about 10 miles (16 km) after which it remained pretty much constant. He named the lower layer the ‘troposphere’ and the upper layer the ‘stratosphere’.
Our weather occurs entirely in the troposphere, directed by ‘jet streams’. Each day, 17 satellites, 1000 weather ballons and over 10,000 ground based weather stations monitor this in an effort to understand and predict what will happen.
7 – Magnetic Field Reversals
Bernard Brunhes, a French geophysicist discovered geomagnetic reversal. In 1905, whilst studying basalt lava flows, he discovered one that was magnetised in a direction opposite to that of the present-day magnetic field.
A systematic study was carried out by Japanese geologist Motonori Matuyama in the 1920s. He studied Japanese basalts and was able to correlate magnetic field direction to stratigraphical position of the rocks.
At the time, neither of these two earth scientists knew how important their discoveries would be.
In the 1960s, studies of magnetic reversal of sea floor basalts provided evidence that the newly developed plate tectonics theory was viable. A number of geologists working on palaeomagnetism of the Atlantic’s floor discovered that many reversals had taken place. These appeared as ‘stripes’ on the sea bed and ran essentially parallel to the central rift of the mid-Atlantic ridge. The pattern repeated on the opposite sides of the ridge which could only be satisfactorily explained by sea floor spreading.
8 – Periodic Ice Ages
The geological term ‘erratic’ describes a rock which is in the wrong place. For example, the Norber erratics on the southern slopes of Ingleborough. These are a group of millstone grit boulders lying on a limestone platform. We know now how they got there (carried by ice from millstone grit outcrops further north and left behind when it melted at the end of the last ice age) but that wasn’t always the case.
From the mid 18th century, Earth scientists and engineers had seen erratics in the Alps. Locals often said they had been left there by the glaciers which were once more extensive. But to scientists, that seemed too far-fetched to be true. But slowly the idea was accepted but these variations in glaciers and ice fields were always seen as being local.
The first recorded person to suggest worldwide ice ages was Danish-Norwegian Geologist Jens Esmark. In a paper of 1824, he suggested climate change as being the cause for these glaciations. The problem was finding evidence and a credible mechanism to drive the ice ages. One of the ideas circulating was the idea that the Earth’s orbit was somehow responsible.
Scottish scientist James Croll appears to be the first to have formalised the orbital concept in 1875 and added the suggestion that the effects would be amplified because the high albedo of the ice would reflect energy from the Sun back into space. His calculations indicated ice ages would be cyclic in nature with the last one ending about 80,000 years ago. Observations placed the end of the last ice age somewhere between 6,000 and 35,000 years ago so his theory didn’t work.
Milutin Milanković was a talented Serbian mathematician and scientist who, during his time as a PoW during WWI, turned his mind to the ice age problem. He looked at the cumulative effects of changes in the orbit, precession and axial tilt of the Earth and came up with a theory now known as the ‘Milanković Cycles’ that worked better than Croll’s model. There are still some aspects of it that need ironing out but it has become accepted mainstream science.
9 – Global Warming
Our next greatest discovery in Earth science concerns the atmosphere. Records show the average global air temperature has risen over the past hundred years by almost a degree celsius. Different groups use slightly different methods but their data always presents the same pattern. The decade 2000 to 2009 was the warmest since records began and the 2010s are set to beat that.
Some scientists claim that the warming is perfectly natural and is nothing to worry about. Others believe that it is linked to the burning of fossil fuels leading to an increase in the greenhouse gas, CO2.
David Keeling first measured atmospheric CO2 levels from the top of Mauna Loa in 1958. His measurements show that there are around 3.3 billion tonnes of CO2 being added to the atmosphere every year.
The shrinking of glaciers throughout the planet is well documented and much modelling is being done to predict how long they will last and to work out if it is the CO2 released from fossil fuel burning that is the root cause. Whilst the Earth naturally releases greenhouse gases, the quantities are minute compared to the quantities released by human activity.
10 – Geological Change
Our final ‘greatest discovery’ was thanks to 3 Scottish scientists, James Hutton, John Playfair and Charles Lyell who developed and promoted the idea of ‘Uniformitarianism’. Thanks to their work, the science of geology was truly born together with the realisation that the Earth was more than a few thousand years old.
Prior to this, it was believed that all of the features on the Earth were created by catastrophes, like the mythical biblical flood. Through the study of rocks in Scotland, James Hutton concluded reality was very different.
He accepted that catastrophes did happen but recognised that they were localised events. Geological processes, he said, were continuous long term slow changes and ‘the present is the key to the past’. In other words, the processes we see today eroding and creating new rocks have operated throughout geological time. The publication of John Playfair’s book Illustrations of the Huttonian Theory of the Earth popularised Hutton’s theory.
25 years later, Charles Lyell produced one of the great geology books of all time – his 3 volume Principles of Geology which supported and extended Hutton’s theory thanks to his careful fieldwork.