Above the molten outer core sits the mantle – the sausage meat of my scotch-egg planet. The mantle consists primarily of rock-forming minerals made up of magnesium oxide and silicates. These rock-forming minerals have names like olivine, garnet and pyroxene. Although the mantle is solid, on timescales of millions of years it moves ever so slowly. Nearly 85 per cent of the Earth’s volume is made up of the mantle. It is at least 1,750 miles thick and is hot, but not as hot as the core of our planet. The deepest part of the mantle, next to the outer core, reaches a temperature of 4,000–5,000 degrees Celsius, while the top of the mantle is considerably cooler, at 200–600 degrees Celsius.
The temperature gradient between the top and bottom of the mantle generates movement where the hot minerals and rocks deeper down very slowly rise towards the surface while the cooler material closer to the surface moves inwards. It can take hundreds of millions of years for a piece of material to move from the top to the bottom of the mantle. A human lifespan is a blink of an eye compared to the eons over which the dynamics of the Earth’s mantle play out. Despite this, the slow convection is important for our planet’s dynamics and drives plate tectonics that can lead to earthquakes and some types of volcanic eruptions. These are the most dramatic home-grown events our planet experiences, but before discussing them it will be helpful to consider the outermost layer of the Earth, the breadcrumbs covering the scotch egg: the crust.
The Earth’s crust is fractured and consists of fifteen tectonic plates. These are large slabs of rock, and they are divided into seven major and eight minor plates. The crust ranges from being 3 miles thick under the oceans to an average of 20 miles thick on continents, although in some places where there are large mountains such as the Himalayas, it can be as thick as 60 miles. Our neighbouring planet Mars is not as fractured as Earth and it has no plate tectonic activity at all. Venus is about the same size as Earth, and up until recently scientists thought that its crust was much thicker and that it too was devoid of plate tectonics. However, using recent observations from the Magellan spacecraft, researchers have revealed that Venus’s crust is of a similar average thickness to Earth’s. The scotch-egg structure of our planet may be typical of rocky planets, and that means Earth may eventually share the same fate as Mars. In time, as our planet cools, the crust will harden and the movement of tectonic plates will cease. But for the foreseeable future our fractured crust will continue to be dynamic, continents will shift and earthquakes will be a risk in many parts of the world.
New crust is continually being formed in areas where two tectonic plates move apart on deep-sea ridges. As the plates move apart, molten rock from the mantle, called magma, moves up, before cooling and forming new crust. In areas where two plates are being pushed together, one plate is pushed down, slowly melting, before joining the mantle and starting its long journey towards the core. The tectonic plate that is pushed up creates land. This is why the continental crust where land occurs is thicker than the plate under the sea.
Continental crust gradually gets worn away via processes that geologists call weathering and erosion. Repeated freezing and thawing of water in fissures in rocks can break them apart, wind and rain can wear them down over eons, and acids in water can also contribute to weathering. Life, too, can contribute to weathering and erosion, with plant roots, fungi, bacteria and even some animal species contributing to the breakdown of rocks. Over long time periods erosion can reduce the heights of tall mountain ranges. Think of the Purnululu range that I visited for my fiftieth birthday.
Tectonic plates are continually shifting and changing size and shape depending upon whether the rate of loss to the mantle or the rate of gain from the formation of new crust is faster. If our crust was thicker, convection in the mantle would fail to generate the plate tectonics that fracture plates or drive them apart, and it is possible there would be no, or very little, land on our planet. One consequence of our planet’s thin crust is that the pressure on tectonic plates can build up as they rub up against one another, and this can lead to earthquakes as the pressure gets too much and the tectonic plates suddenly jerk. The ground shakes as the plates shift and settle into new positions. Tectonic activity has been a key feature of the history of land on our planet, and played an important role in the spread and diversification of life that led to our existence. For example, 200–300 million years ago there was only one supercontinent, Pangea. Plate tectonics broke it apart and, slowly, the continents that are familiar to us emerged. The break-up of Pangea split apart populations of animals and plants, and in many cases as time ticked by these populations diverged, evolving to become completely different species on separate continents. In more modern times, the movement of tectonic plates can be catastrophic for humans. The largest earthquake in the twenty-first century occurred in the Indian Ocean on 26 December 2004. The resulting tsunami killed over 225,000 people.
Fifty-four million years ago a tectonic plate that contained what is now Europe and North America split in two. The reason for the split is unclear, but it may have been the weight of the two continents being pulled towards the centre of the planet by gravity, along with the build-up of magma at a weak point in the centre of the plate. When the ancient plate split it caused enormous volcanic activity, throwing vast quantities of hot rock, ash and dioxides of carbon and sulphur into the atmosphere. Such fractures of a plate are, mercifully, rare, but they do not always generate massive volcanic activity. Sixty-three million years ago, another plate split into the ones where modern-day Seychelles and India are found. Volcanic activity in the region 6 million years earlier meant that little magma had built up and was insufficient to cause a huge eruption like that seen between what is now modern-day North America and Europe 9 million years later. Comparisons such as these reveal that local history matters. Each earthquake, tectonic plate fracture and volcanic eruption is different. Yet they are driven by the same processes, such as the movement of rocks in the mantle, new crust formation in the deep sea, hardening of the crust, and subduction, where one plate is forced beneath another as they collide. These processes interact, generating a range of possible geological outcomes. Earth scientists have developed a remarkable understanding of these processes and are starting to gain astonishing insights from other planets, but accurately predicting what will happen in the future is currently hard: we cannot predict earthquakes or volcanic eruptions in advance with much confidence.
At the time when the first life evolved, Earth’s atmosphere was made from the gases ejected from volcanic eruptions. Hydrogen sulfide, methane, carbon monoxide and carbon dioxide were abundant. If life had not evolved, our atmosphere’s composition today might resemble that found on Venus, consisting of 95 per cent carbon dioxide and only a percentage or two of nitrogen, oxygen and other molecules. Not that we’d be around to measure it. Although the exact composition of the atmosphere early in Earth’s history is not known precisely, scientists agree that the percentages of carbon dioxide and methane have dropped significantly over time, while nitrogen and oxygen have increased. Oxygen didn’t start to increase until between 2 and 2.5 billion years ago. It was produced as a by-product of photosynthesis in species of cyanobacteria, but it could not accumulate in the atmosphere in any appreciable amount until other reactive elements, such as iron, had oxidized. Nitrogen started to increase in abundance from about 4.4 billion years ago. It is so abundant in the atmosphere today because it does not easily combine with other elements to form the crystals and rocks that make the ground beneath our feet. Nitrogen is consequently common in the atmosphere and relatively rare in the planet. Oxygen, in contrast, is found in rocks because it oxidizes with many other elements so easily, and it also forms water.