Formation of the Earth

1. The history of the Universe condensed into one year

The Universe is about 13.8 billion years old. This span is so vast that it defies imagination. To make it tangible, the astronomer Carl Sagan popularized a now-classic analogy: compressing the entire history of the Universe, from the Big Bang to today, into a single calendar year.

On this scale:

• 1 month ≈ 1.15 billion years
• 1 day ≈ 38 million years
• 1 minute ≈ 26,000 years
• 1 second ≈ 440 years

The Big Bang occurs on January 1 at midnight, and we are on December 31 at 11:59:59 p.m. Here are the main stages of this cosmic year:

• JANUARY
• January 1Beginning of the expansion of the universe — the Big Bang.
• APRIL
• April 1Our galaxy is formed.
• SEPTEMBER
• September 9Birth of our solar system.
• September 14The Earth is formed.
• OCTOBER
• October 1Life appears.
• October 9Bacteria appear.
• NOVEMBER
• November 1Invention of sex by micro-organisms.
• November 12Appearance of fossil plants and photosynthesis.
• November 15The first nucleated cells begin to thrive.
• DECEMBER
• December 1The atmosphere becomes rich in oxygen.
• December 16The first worms emerge.
• December 17Invertebrates appear.
• December 18Oceanic plankton is in place.
• December 19Appearance of the first fish and first vertebrates.
• December 20Plants colonize the land.
• December 21The first insects.
• December 22The first amphibians.
• December 23The first reptiles.
• December 24The first dinosaurs.
• December 26The first mammals.
• December 27The first birds.
• December 28Extinction of the dinosaurs.
• December 29The first cetaceans and first primates appear.
• December 30The large mammals are on Earth.
• December 31 — 10:30 p.m.Appearance of the first humans.

2. The formation of sedimentary and volcanic rocks

Sedimentary rocks and volcanic rocks are the two great families most often encountered at the Earth's surface. Although produced by very different processes, they are linked by the great rock cycle that ceaselessly transforms one into the other over geological time.

Sedimentary rocks

Sedimentary rocks form through the accumulation and consolidation of materials at the Earth's surface, on the bottom of seas and lakes or in deserts. Their formation always follows the same cycle:

1. Erosion — breakdown of pre-existing rocks by water, wind, frost, chemistry or living organisms
2. Transport — by rivers, wind, marine currents, glaciers
3. Deposition — when the transport energy decreases, particles settle in successive layers (the strata)
4. Diagenesis — under the effect of pressure and circulating pore water, the sediments compact then cement to form a coherent rock

Three families are distinguished according to the origin of the particles:

Sedimentary rocks cover about 75 % of the continental surface and make up most of the everyday geotechnical ground in France. They record the history of the Earth in their strata and fossils.

Volcanic (and plutonic) rocks

Igneous rocks form through the cooling and crystallization of a magma — a molten rock from the upper mantle or from partial melting of the crust. The cooling rate determines the crystal size and therefore the classification of the rock:

According to their chemical composition (silica content), we distinguish:

• Basic rocks (basalt, gabbro, ~50 % SiO₂) — dark, dense, rich in iron and magnesium. They make up most of the oceanic crust.
• Acidic rocks (rhyolite, granite, ~70 % SiO₂) — light-coloured, light, rich in quartz and feldspar. They form most of the continental crust.

Present-day volcanism occurs along the oceanic ridges (basic effusive volcanism), at subduction zones (acidic explosive volcanism) and above hot spots (Hawaii, Réunion, Iceland).

3. The formation of the Paris Basin

The Paris Basin is one of the largest sedimentary basins in Europe and the most important in mainland France. It covers about 140,000 km² in the heart of the country, drained by the Seine and its tributaries. Its geological structure directly governs the geotechnical conditions of the Île-de-France region.

Origin and structure

The basin formed through subsidence (gradual sinking) of the Hercynian basement starting in the Triassic (about 250 million years ago). On several occasions during the Secondary and Tertiary eras, shallow seas covered the area, successively depositing sedimentary layers that stack up to 3,000 m thick at the centre of the basin.

The overall structure is made of concentric rings: the youngest layers crop out at the centre (Paris) and the oldest at the periphery (Burgundy, Lorraine, Normandy, Sologne). In cross-section, the image is that of plates nested one inside another, slightly tilted.

The main geological layers (most recent to oldest)

Geotechnical importance for Paris and the Île-de-France

A detailed knowledge of the geology of the Paris Basin is essential for any construction in the Île-de-France. It makes it possible to anticipate:

• the cavities left by old underground quarries (coarse limestone, gypsum) — risk of sinkholes (fontis)
• the swelling clays of the Sparnacian and Ypresian — seasonal shrink-swell
• the superimposed aquifers (Fontainebleau sands, Lutetian, Chalk) — uplift pressures and drainage
• the lateral variability of the layers — transitions from clay to sand, compressible lenses

Parisian geotechnical engineers systematically consult the BRGM 1:50,000 geological map and the cavity databases (Inspection Générale des Carrières — IGC) before any project.