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Setting the Stage

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© Alasdair Skelton

In a paper entitled Age of the Earth which was published in Science in 1955, Clair Patterson and colleagues estimated that the Earth is 4.5 billion years old. Their estimate was based on comparing proportions of different lead isotopes (meaning forms of the same element with different masses) on Earth and in meteorites. This works because radioactive decay of uranium affects the proportions of lead isotopes and because we know how fast uranium decays.

We know very little about the first half billion years of Earth’s history because its surface was covered with a magma ocean and rocks were neither formed nor preserved, meaning that we have no geological record of this time. Research by Paola Sossi and colleagues which was published in Science Advances in 2020 suggests that Earth’s atmosphere at that time was akin to the present-day atmosphere of Venus (which is mostly carbon dioxide). This unpleasant chapter of Earth’s history is called the Hadean Eon, aptly names after Hades and underworld.

Artist’s impression of the Hadean Eon by Susanne Hjerp

With the first rocks, we enter the Archean Eon, four billion years ago and towards the end of this 2-billion-year-long eon, we find the oldest rocks in Sweden. These underly the northern lands of Sápmi. Measurements of small variations of Earth’s gravitational pull allow us to distinguish between denser rocks from ancient oceans and lighter rocks from ancient landmasses. From these measurements we can map the late Archean world. There were no continents at this time. Instead, Earth’s surface might have been a worldwide ocean with many thousand small volcanic islands. Gravity measurements from Sápmi allow us to make maps and reconstructions of this ancient world.

Map of a part of Sápmi towards the end of the Archean Eon by Susanne Hjerp

Reconstruction of a part of Sápmi towards the end of the Archean Eon by Susanne Hjerp

© Alasdair Skelton
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