New record holder: The oldest rock on earth

It is a sensation for science: In northern Canada, geologists have stumbled upon the oldest rocks ever discovered. They belong to the Nuvvuagittuq greenstone belt on Hudson Bay and are over four billion years old.

An international team of researchers has now dated the rocks in northern Canada to 4.28 billion years. This would make it just 300 million years younger than our solar system. Now the scientists are investigating whether the ancient rocks are a remnant of the very first Earth’s crust, which once separated from the mantle. If so, the discovery could help unravel some of the secrets of Earth’s very early history. Perhaps the rocks will reveal something about where and when life began? The researchers also hope to be able to read in the rocks how the atmosphere changes and when the first continent of our Earth was formed.

By the way, the name Greenstone Belt comes from the colour of its metamorphic rock. It is the minerals contained in the rocks that give them a greenish colour in some places.

Age Ranking of the “Ancient Ones

They are all old and wrinkled. But the chunks of the oldest rocks are separated by millions of years. Until recently, when the rock from the Nuvvuagittuq rock belt was dated at a proud 4.28 billion years, another rock in northern Canada was considered the oldest rock on earth: the so-called Acasta gneiss in the northwest of the country. This rock is also 4.03 billion years old. With its great age, the old gneiss surpasses a formation of banded iron in Greenland: this now ranks third on the age scale of rocks. The rocks in Greenland are “only” 3.9 billion years old!

Cycle of the rocks

No rock on earth is made to last forever. It weathers on the surface, is transported away and deposited again. When two plates collide, sedimentary layers are compressed and folded into high mountains. The rock of submerging plates melts in the earth’s interior and forms the source of volcanoes. Lava spewed out by a volcanic crater cools in turn and solidifies back into rock.

It is an eternal cycle that ensures that even the hardest rock is transformed again and again and new things are created from it. Of course, this transformation does not happen overnight, but over millions of years. The “players” in this cycle are three groups of rocks, each of which is formed under different conditions:

When magma cools, the hot mass solidifies into magmatic rock. This can happen both on the Earth’s surface and in its interior. Where layers of eroded rock debris accumulate, on the other hand, the sediments are compressed under the weight of their own weight. This pressure causes them to solidify into sedimentary rock. High pressure and great heat in the earth’s interior, in turn, cause rocks to transform and form another. Geologists then speak of metamorphic rock.

These three rock types are closely connected: Each type can change into any other. This rock cycle will go on and on as long as the earth exists

Metamorphic rocks

It happens inside the earth: strong pressure and high temperatures cause the components of the rock, the minerals, to react with each other and transform. In this way, new rock is formed. Because the Greek word for transformation is “metamorphosis”, geologists also speak of metamorphic rocks.

A correspondingly high pressure occurs when two earth plates collide and one plate dives under the other. The rock is then squeezed together, as if in a huge press. A frequent result of such rock metamorphosis is blue slate. Its source rock is basalt or a rock with a similar composition to basalt.

Great heat also causes rocks to transform. For example, in the vicinity of a magma earth, it is baked like in an oven. Marble, for example, is nothing other than limestone that has been heated very strongly in the earth’s interior; during this process, new minerals form and the rock becomes harder. Sandstone also transforms at high temperatures, because its quartz grains then stick together: the original sedimentary rock becomes the harder quartzite.

In contrast to complete melting through volcanism, the rock remains solid during metamorphosis. However, if the temperature continues to rise, the rock eventually becomes liquid magma. When this mass cools down, it becomes magmatic rock again. The cycle of the rock is in full swing.

What is rock?

In some places it peeps out from under a thin cover of plants, elsewhere it rises up as a steep rock face: the bare rock. It is the building material that makes up the earth’s crust and mantle. Rock, however, is not a uniform mass. Similar to a cake batter – only much harder – it is a mixture of different ingredients: the minerals.

Rock therefore consists of different minerals. Depending on their composition, the minerals combine to form certain types of rock. Granite, for example, is a rock that consists of the minerals feldspar, quartz and mica. The fact that granite is made up of different minerals is evident from the fact that it is speckled: it contains lighter and darker parts that owe their different colours to three different minerals. The darker parts come from the mineral mica. The quartz mineral often appears whitish to grey. The third mineral, feldspar, can take on all kinds of colours, even pink. Unlike the hard granite rock, the softer sandstone consists almost entirely of quartz. For this reason, sandstone looks more uniform than the speckled granite.

Almost all minerals arrange themselves according to a certain lattice pattern into uniform shapes, the crystals. For example, the mineral rock salt grows into a cube. However, the regular arrangement also results in other shapes with smooth surfaces, as can be clearly seen in a rock crystal. This consists of particularly pure and therefore transparent quartz. If, on the other hand, liquid is enclosed in the quartz, it turns a milky, cloudy colour. Geologists then speak of milky quartz.

The Earth Ages

Since its formation, the earth has changed a lot: Mountains, oceans and continents have formed and passed away, animal and plant species have spread and become extinct. Most of these changes happened very slowly, over many millions of years. But every now and then there were drastic events: Within a few thousand years, environmental conditions changed drastically.

For the scientists who study the history of the Earth, these drastic changes are like a new chapter in a book: they divide the Earth’s history into different sections called eons.

At the beginning, 4.5 billion years ago, the Earth was completely uninhabitable. It emerged as a hot ball of glowing molten rock surrounded by hot, corrosive and toxic gases. This sounds like a description of hell – and from the Greek word “Hades” for hell also comes the name of this time: Hadaic. It ended about four billion years ago with the first major change: the earth had cooled down so much that the surface became solid – the earth got a crust.

The earth cooled further so that liquid water could collect on the crust: Oceans were formed. And in these seas, life began about 3.8 billion years ago – but initially only in the form of the simplest bacteria. The Greek word for origin or beginning is in the name of this period: Archaic. An important climatic change about 2.5 billion years ago marked the transition to the next epoch: primitive organisms began to influence the environment. They produced oxygen, which until then had been almost non-existent in the atmosphere.

The early unicellular life forms became more complex over time, forming cell nuclei. Later, some also began to work together permanently in associations – this eventually became the first multicellular organisms. However, they did not yet have solid shells or skeletons, so that hardly any fossils have survived from this period. The epoch owes its name to this period before the emergence of fossils: Proterozoic.

The Proterozoic ended 550 million years ago with an explosion of life: within a short time, an enormous variety of species developed from the primitive forms of life. These species were much more complexly built – and some also already had hard shells, which were preserved as fossils for the first time. Therefore, for scientists, the history of life only really becomes visible from this point onwards. And this epoch is also named after the Greek term for “visible”: Phanerozoic.

This age of life has lasted for 550 million years until today. However, the development of life did not proceed evenly: After the explosive spread of life, there were two devastating mass extinctions. These mark further important cuts in the Earth’s history, so that scientists divide the age of life, the Phanerozoic, into three sections called eras.

The oldest era of the Phanerozoic began 550 million years ago with the mass emergence of new species. It is called the Palaeozoic Era. At first, life took place only in the oceans. Then plants colonised the land, and later the animal world followed suit: First, amphibians developed, which were already able to venture a little on land, and finally also reptiles, which became independent of the water and conquered the land. The Palaeozoic ended about 251 million years ago with the greatest mass extinction of all time: More than 90 percent of all animal and plant species became extinct, especially in the oceans. The reason for this has not yet been conclusively explained. Scientists suspect that an ice age was to blame, possibly as a result of a meteorite impact.

When the surviving animal and plant species had to get used to their new environment, the Mesozoic era began. It is above all the age of the dinosaurs: giant lizards evolved and dominated life for almost 200 million years. But the Mesozoic also ended with a drastic event: about 65 million years ago, a large meteorite struck the Earth. It threw so much dust and ash into the air that the sky darkened and the climate changed for a long time. The dinosaurs and many other species became extinct.

Small mammals in particular, which were best able to adapt to climate change, benefited from this. They had already evolved in the Earth’s Middle Ages, but had remained in the shadow of the dinosaurs. Now they were able to spread rapidly, conquer a wide variety of habitats and continue to evolve. Humans are also descended from this group. This most recent age continues to this day and is therefore also called the Earth’s New Era or Cenozoic Era.

This rough classification of Earth history is oriented towards very drastic changes in life: explosive proliferation or mass extinction. In between, however, there were further upheavals due to various other influences – changes in the oceans and continents due to continental drift, climate change between ice ages and warm periods, composition of the air and much more. Invariably, the new conditions favoured some species and disadvantaged others. Thus, the three sections of the Phanerozoic (Age of Life) can still each be divided into several periods.

Oceanic and continental crust

The earth’s crust is not the same everywhere. The land masses of the Earth consist of continental crust, the ocean floor of oceanic crust. One of the differences is that the continental crust contains mainly silicon and aluminium in addition to oxygen. The oceanic crust, on the other hand, also has a high proportion of magnesium. But that is far from the only difference:

Oceanic crust forms at the bottom of the sea, where magma rises and solidifies along the mid-ocean ridges. As crust constantly grows back here, the two lithosphere plates are pushed outwards. Towards the coasts, the oceanic crust thus becomes older and older. Some of the oldest pieces are around 200 million years old. They lie in the Atlantic off North America and east of the Mariana Trench in the Pacific. However, the oceanic crust, which is about five to eight kilometres thick, does not get any older: because it is heavier than the continental crust, it submerges when it collides and is melted again in the Earth’s interior.

Although the continental crust is lighter, it is thicker than the oceanic crust: on average, it reaches 40 kilometres, under mountains even up to 80 kilometres in depth. When exactly it formed is still a mystery even to scientists. The oldest known rock on Earth provides clues: it was found in northern Canada, is over four billion years old and is probably a remnant of the very first crust.

The outermost shell of the earth

Like an egg from an eggshell, the Earth is also surrounded by a hard shell. This outermost layer surrounds the Earth’s mantle and is called the Earth’s crust. If you compare the earth to a peach, the earth’s crust is – relatively speaking – as thick as its skin. Under continents, it reaches an average depth of 40 kilometres, under the oceans even only about seven kilometres.

Below this lies the outer part of the Earth’s mantle, which reaches down to a depth of about 100 kilometres. It is also solid, but consists of heavier rock. The earth’s crust and this outermost part of the mantle together are also called the “lithosphere”. This solid layer of rock is broken into plates of different sizes that drift very slowly on the hot, viscous mantle.

Where the molten rock from the hot mantle penetrates upwards, the earth’s crust can break open. Lava then flows out and becomes new crust. This mainly happens where the plates of the lithosphere adjoin each other, such as at the mid-ocean ridges.

In Iceland, for example, these plate boundaries are clearly visible: Cracks and furrows run through the earth’s crust here, where the Eurasian and North American plates drift away from each other. There is also a plate boundary in the Mediterranean region. Because the African plate is pressing against the Eurasian plate here, there are many volcanoes and earthquakes in Italy.

The crust is covered by the soil. The soil of the land masses forms from weathered rock and the remains of animals and plants. The seabed, on the other hand, develops from deposits such as clay and the sunken remains of marine organisms. On the coasts, the seabed also consists of deposited debris that has been eroded from the mainland and washed into the sea.