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Columnar jointing
The volcanic activity builds remarkable geological landforms, such as volcanic domes. Columnar jointing is also a natural curiosity looking like a large pipe organ. Such structures are observed all over the world, the most popular are the Giant's Causeway in Irish and the Devil Tower in Wyoming in the United States. In France, the Central Massif is the place offering the most of columnar jointing: from Saint-Flour in the Cantal to Agde in the Hérault department. The reason of this specific shape is the cooling process of the igneous rock (basalt, phonolite, trachyte, andesite, etc.) from a lava flow. When a rock loses heat, the volume decreases inducing shrinkage fractures. To reduce the loss of energy, the thermal contraction is delimited by hexagonal fractures, the only regular geometric shapes covering the whole matter – which is not the case of circles – with the smallest perimeter for the same surface area. The cooling is produced by both upper and lower surfaces of the lava; prismatic columns perpendicular to isotherms are thus formed. The regularity of prisms depends on the constant and slow temperature decreasing. In general, we observe three layers of prisms from the same lava flow: regular columns called colonnades in the bottom, irregular columns called entablature in the middle part, and more or less straight colonnades in the top. By analogy, we find these polygonal shapes in clay soils as described by desiccation cracks in summer, or in the arctic landscapes due to freeze-thaw cycles.
The land of the Moon
The Earth’s dynamics are largely dependent on its sole natural satellite: the Moon. The latter turns around the Earth every 27.3 days. However, the synodic period, i.e., the duration for a complete revolution from a terrestrial reference, is 29.5 days, which is the same as for the Moon’s rotation period. We call this phenomenon tidal locking, explaining why we always observe the same side of the Moon from the Earth. These two astronomical objects affect each other. For example, the Moon produces tidal movements that played an important evolutionary role, paving the way for the species to evolve to leave the water and live on land. It is also responsible for the Earth tide with the displacement of solid Earth’s surface of about tens of centimeters. The Earth-Moon interaction is also important for the reproduction of several marine species, especially for synchronizing the gamete dissemination in corals. Its origin has stimulated the scientific curiosity for a long time. Several hypotheses were proposed: common origin with the Earth, fission of an Earth fragment, captured foreign body. Finally, many scientific evidences lead us to conclude that is the result of an impact between the early Earth and a drifting star the size of Mars, named Theia, 4.5 billion years ago. The suspended dust resulted from the collision, a mixture of the Earth and Theia, would gradually aggregate to form the Moon. This corroborates with the Earth and Moon similarities of the chemical compositions. The proximity of the Moon stabilizes the tilt of the Earth, which gives a certain regularity in the seasons. It is continuously moving, however, away from the Earth, about 4 cm per year. Today, the Moon has the same apparent diameter as the Sun, which explains that the majority of solar eclipses visible on Earth are total eclipses.
Snowflakes
During winter season, heavy snowfall can cover landscapes in a white blanket of snow. Snowflakes present great surprises for those who want to take a closer look. The snow is composed of a multitude of small crystals with various shapes resulting from an atmospheric process with different steps from the water condensation around a condensation nuclei, such as atmospheric dusts. The hexagonal prism is the first general structure for all these snowflakes, related to the ice structure and the covalent bound O–H between two water molecules H2O. The growth is slower on the faces compared to the edges of the crystal, which provide a plane shape. Then, flake’s branches are structured by the faster vertex development compared to edge one. Overall, the shape is determined by two main factors: air temperature and water saturation. Flat crystals are shaped at temperatures between −10 and −20 °C, more or less jagged depending on humidity condition; outside of this range of temperature, we find column crystals. The first classification was performed at the beginning of 1930’s by Wilson Bentley, American farmer, who inventoried and photographed several thousand of different shapes of crystals. Apart from the artistic optical quality of snowflakes, snow also plays a role in acoustical insulation and makes winter landscapes quieter than other seasons.
Halo
Have you ever seen luminous circles in the sky? These are not evidence of an alien life; these bright curves – also called halo – are formed by refraction and diffraction of sunlight or moonlight. One condition is required to shape these photometeors: very small ice crystals of about tens micrometers need to be located in the high atmosphere. Cirrostratus are clouds composed by this type of crystals. As for other high-level clouds (cirrus and cirrocumulus), cirrostratus are in the top of the troposphere, about 6,000 to 12,000 m from the ground. They are, however, thinner allowing passing sunlight and moonlight through them. The tiny ice crystals are the key for refraction and diffraction of light. The cold temperatures between −5 and −25 °C shape hexagonal crystals. The angle of refraction is 22° when light rays enter the lateral face of the hexagonal prism. This leads to a smallest halo (22° halo) most often visible. The angle, however, reaches 46° when light rays enter the base of the prism, leading to a bigger less bright and common halo (46° halo) compared to the previous halo. Other rare structures are possible: Sun pillar, sundogs, upper tangent arc, and circumzenithal arc. Cirrostratus and halos occur with warm front, generally indicating a possible future rain or snow event within the next 24 hours.
How long is time?
Time is a fundamental concept in geosciences. Depending on the scientific field, we can use different scales to measure time, either the minute or hour for meteorology, or the millions or billions of years for geology or astrophysics. But how to define and measure these units? It is easy to understand how to define some of them using the notion of planetary movements. The duration of Earth’s rotation around the Sun characterizes the year, the Moon’s rotation around the Earth define the month, and the Earth’s rotation on itself, the day. But did you know that the Earth’s rotation on itself is less than 24 hours, but exactly 23 hours, 56 minutes, and 4 seconds. This results from the Earth’s rotation around the Sun, changing the relative position of the Sun. The Earth covers 360° for 365.24 days, that is to say, almost 1 ° per day. Moreover, various parameters modify the real planetary movements, such as the axial precession observed for a spinning top, or the decrease of rotation speed by the progressive tidal friction. The mean solar day can be subdivided by hours, minutes, and seconds (1/24, 1/1,440, and 1/86,400 of mean solar day, respectively). Physicists also largely worked to solve the temporal equation. The quartz clock is one of the first well-known clock: the frequency of vibration is produced by hitting a quartz crystal of a specific shape. In 1967, the General Conference on Weights and Measures adopted the atomic clock to define the time in the International System. The frequency is given by the vibration of electrons change in energetic levels. The second is now clearly defined as the duration of 9,192,631,770 vibrations of a stable caesium-133 atom.
Lord of the rings
As observed for Saturn, the planetary rings never failed to fascinate. A famous advertising for Perrier in the 90’s made them dance around a planet. These rings are composed of various fragments of rock and ice of different sizes from one micrometer to several meters. We distinguish dusty rings (with small particles) – Jupiter, Neptune, and a part of Saturn's (G and E) rings –, from main rings with large and dense particles (Uranus and the major part of Saturn’s rings). It is difficult to define exactly the planetary rings, and to distinguish them apart from natural satellites. Indeed, some rocks located in Saturn’s rings are of the same order of magnitude as Deimos, the smallest satellite of Mars. The flat disk results from the planet rotation: every particle is located in the equatorial plane. One of the key principles for understanding the planetary rings is the Roche limit: this is the limit of stability for a satellite below which it disintegrates due to the tidal forces. Several scenarios explain the origin of the rings: satellite particles that fell within the Roche limit, particles from the impact between a foreign body and the planet itself, or captured foreign body. The dynamics of the rings is complex and poorly understood. However, the rings are younger than the planet and their evolution is relatively short. Although the four giant planets of the Solar System have rings, the most popular are those of Saturn: more than 60 satellites, and seven rings (A to G). The overall thickness of the ring complex is about a kilometer (but only several meters locally), and they spread over several hundreds of thousands of kilometers. The mass of all the particles could form a satellite of 250 km in diameter. A special feature of Saturn is the presence of small satellites (Pan and Minas) in the Roche limit, while the rings extend beyond this limit. This allows us to observe complex interactions between these two types of structures, including matter exchanges. For example, a satellite can play a “cleaning” role of a ring circumference caking the material, whereas two other satellites (Pandora and Prometheus) limit the external rings. But these planetary rings are not exclusive to the Solar System: a recent study observed rings 200-times larger than Saturn orbiting around the exoplanet J1407b (located at 434 light years, several decades heavier than Jupiter); they are spread over 120 million of km. The amount of material orbiting is close to the Earth mass.
What makes the desert beautiful, is that somewhere it hides a well
The deserts are not all made of sand dunes. Some of them are simple huge surface of bare soil or stones. Similarly, deserts are not all hot, as shown by Antarctica. But then, how to define a desert? The simplest criterion is the amount of precipitation received in the surface: less than 250 mm per year for the semi-deserts, less than 100 mm per year for the deserts, and less than 50 mm per year for the hyperarid deserts. Precipitation may, in some areas, be absent for several years or even decades successively. This is the case of the Atacama Desert in Chile. But, actually, the water balance deficit is a more accurate definition: water inputs (precipitation) below the water outputs (evaporation and evapotranspiration). These deserts are mainly located in the intertropical area, specifically around latitudes 30° North and South (the Sahara in North Africa, the Gobi in China, the Sonora in Mexico and the United States, the Kalahari in Africa South and the Great Victoria Desert in Australia). This results from the general circulation of the atmosphere, and more particularly from the convection Hadley cell. Hot and humid trade winds converge to the equator, resulting in an air upward. Upstairs, the mass of air becomes cold and dry, allowing to dry regions at latitudes ± 30°. In such environments, the only species need to resist to long dry periods using a multitude of ecophysiological and behavioral adaptations (xerophile species). Animals as well as plants must face up to water (water storage in the camel humps, leaf reduction for Cactaceae, deep root system for Acacia) and thermal stress (resistance of wide temperature range for the beetle Adesmia metallica, limitation of contact area with the ground for lizards). Some plants also reduce their life cycle to a few days following a rainfall event: they are called ephemerophytes, and allow the desert bloom.
Sea salt
The legends holds that the salt content in oceans would be either produced by salt mills from depths, or escaped from amphorae of salt following the shipwrecks. Nevertheless, the salinity hardly varies over the time. Only geographical variations are observed: from 10 g of salt per kg of sea water in the Baltic Sea, to four times higher in the Red Sea. It reaches 35 g of salt per kg of seawater salinity on average. This includes a large number of components, such as chlorine, sodium, sulfate, magnesium, calcium or potassium. Most of these elements derived from the weathering of continental rocks. The transport of them from the continents to the oceans is provided by rivers. Sodium chloride (NaCl) – salt used as a condiment – constitutes 86% of the mass of sea salt. We distinguish the sodium ion (Na+) from the chloride ion (Cl). The first ion is exclusively provided by the continents. However, the second one (nearly two times more concentrated) is partly explained by the active submarine volcanism during primitive periods of the Earth. The emission of hydrochloric acid (HCl) released a large amount of chloride ions in the global ocean. Unlike other ions, such as calcium (Ca2+), sodium and chloride ions are not involved in biological cycles, which explains their high stability in solution in the oceans. Paradoxically, the freshwaters feed in salt the ocean waters. 
The geology of the Massif Central
We all learned that the age of a mountain depend on the height of its tops, eroding gradually over time. If this principle is correct, the example often given – Alps vs Massif Central – is somewhat less true. The morphological history of the Alps is simple because this mountain range is relatively young. However, the case of the Massif Central is little bit complex. Indeed, the Variscan range, including the Massif Central and the Armorican Massif, was born in the Devonian (380 Ma) after the collision between two continents: the Gondwana in the South and the Euramerica in the North (Variscan orogeny). At that time, the Massif Central reaches heights comparable to those of the current Alps. During the Mesozoic breakup of the supercontinent Pangea (250 Ma), the Massif Central is gradually dismantled and eroded to a peneplain. The sediments of the Quercy or the Rouergue reflect a rise of the sea level in the margins of the massif during the late Jurassic (150 Ma). The present-day landform observed cannot be dated from the Paleozoic. It was during the Alpine orogeny (Oligocene, 30 Ma), that the current landform was born. At that time, the collision between the African and European plates gave the Alps range, and reactivated the ancient Variscan faults allowing the uplift of the Massif Central. This tectonic activity has resulted in a mantle upwelling, promoting volcanism observed from 15 Ma (Limagne) to 3,500 years (Chaîne des Puys), latest geological episode of the Massif Central.
Aurora
Borealis in the Northern Hemisphere and australis in the Southern Hemisphere, aurorae are visible in the night sky at high latitude (between 65 and 75°). Their names come from Aurora, goddess of the morning light, in Roman mythology. This phenomenon results from the interaction between the particles emitted by the Sun and the molecules of Earth's upper atmosphere. The surface of the Sun is the center of many activities (prominences, coronal holes, flares…) which are responsible for the emission of charged particles (electrons, protons, and ionized atoms) into space as solar winds, moving at several hundred kilometers per second. After a few days, these particles reach Earth’s orbit and are deflected by the magnetosphere surrounding the Earth. Some of them are still able to come into contact with the gas molecules of the Earth's ionosphere (between 70 and 1,000 km) at the poles, which are areas of convergence of magnetic field lines. This allows them to acquire a higher level of energy, creating instability. The return to the ground energy state releases photons, generating the light forms up to 1,000 km long. The color depends both on the nature of the excited atoms (nitrogen, oxygen or hydrogen) and their altitude: yellow-green, red or blue-violet. These lights have also been observed on other planets in the Solar System, like Jupiter, Saturn or Uranus.