The Terran epoch : from \(500 \text{M}\) years SE
At the end of the Hadean epoch, earth entered the Archean epoch, which is characterised by milder conditions. Although still different from the ones we can experience today, they are also not markedly different, and the turning point can be seen as the start of a new geological period, characterised by a much lower temperature: the infall of meteorites had decreased to an extremely low rate; short-lived radioactive elements had decayed; primordial accretion heat’s dissipation rate had shrunk. This makes for “livable” conditions, both in the biological and sense, but also in the more colloquial one, which can be related to our notion of environment: conditions stable enough to act as credible surroundings for macroscopic, complex entities (rather than some sort of original chaos).
The natural history of earth has been periodized into eons that relate to specific conditions on earth, but cannot be easily generalized into necessary steps, even for a select sub-set of earth-like planets. In that sense, the Archean, which on earth is simply deemed to end when the biosphere develops to a material extent, does not really have a geological identity that could be transferrable to other planets, where life may have developed earlier, or not at all. For that reason, we won’t use that periodization, and instead refer to a Terran epoch, starting half a billion years into the solar era, and lasting to this very day.
The conditions that prevail during the Terran epoch are suitable to see the emergence of the geosphere. With that term, I want to denote the outside layer of an earth-like planet. On earth, it is made up of three components.
The lithosphere is the outermost layer of the "rocky" earth. It includes the crust as well as a thin part of the upper mantle called the litospheric mantle. Those two are given different names due to their chemical and mineralogical discrepancies, but mechanically they really form one unit. The lithosphere is largely a solid phase, although it can also be home to liquid, molten rock, and includes some underground gases, trapped in its entrails.
The lithosphere can be 50 to 250 kilometers thick, and weighs around \(2\times 10^{22} \text{kg}\).The hydrosphere is the sum total of all water on a planet. Why should we single out this specific compound? On earth, it plays a specific role, uniquely being present in solid, liquid and gaseous forms, albeit in lopsided fashion: the hydrosphere is largely a liquid phase. Besides, liquid water is seen as the necessary condition for the emergence of complexity, given as a solvent for all chemical, and later biochemical, reactions. This mostly liquid hydrosphere is unique in the solar system today, although Mars, Venus and many of the gas giants’ moons likely had liquid oceans at some point in their history. Solid ice is still widespread, especially of course within the ice giants themselves, although the absence of liquid phase would make the use of the hydrosphere moniker questionable.
Could there be a hydrosphere equivalent with another liquid than water? That is possible, perhaps even likely. Some planets may have oceans of magma, or even be covered in it, but it most definitely doesn’t fulfil the complexity-fostering criteria. Ammonia, argon, cyanides, acid, alkanes, may also be prevalent in a liquid state on some exoplanets, but it’s not clear they do either.
On earth, the hydrosphere weighs \(1.4\times 10^{21} \text{kg}\). Only 1.75% is in ice form, and a small amount (0.001%) is in the atmosphere. Of the remaining 98.25%, more than 96.5% is in oceans, seas and bays.The atmosphere is the gaseous layer that lies above the lithosphere and hydrosphere. It is 98% nitrogen gas (\(\text{N\_2}\)) and 21% oxygen gas (\(\text{O\_2}\). Technically it extends up to 10,000 km above the ground, but three quarters of its mass is within the first 11 km. It weighs about \(5.5\times 10^{18} \text{kg}\).
The total mass of the geosphere, \(2.15\times 10^{22} \text{kg}\), is then only 0.36% earth mass (which is \(6\times 10^{24} \text{kg}\)). We’re then isolating a thin slice of the full astronomical objects that is a planet, to focus entirely on the dynamics of the environment that’s accessible from its surface.
Each of these three spheres can contribute, and in the case of earth, do contribute, to the geosphere’s dynamics. As the mesospheres from each of our levels, it can then act as a refuge against rising entropy where mild conditions, representing the right mix between chaotic hell and dead desert, are able to nurture complexity.
The mere definition of the lithosphere enables its dynamics. In older planets, or smaller ones, the whole mantle has gone rocky, curtailing geological activity for good. The fact that the earth’s mantle is a viscous material made of molten rock makes it relevant to distinguish between the rigid, brittle layer close to the surface and give it a name. The phenomenon that grounds most of the solid earth’s geology is plate tectonics. This denotes the fact that the lithosphere is split into rigid plates which “float” on the top part of the viscous mantle is called the asthenosphere. The source for the tectonic forces themselves is the convection in the mantle due to the temperature gradient there, together with the earth’s rotation and other factors. It consists in the plates moving and interacting with each other, following two basic movements:
When two plates try to get away from each other, we see mantle upwelling: the crust is stretched apart and hot mantle material can come up in the form of magma, generating new crust as it cools down.
The total surface of the planet has to stay roughly constant, so any new crust needs to be compensate by crust disappearing somewhere else. That is what happens when a plate is recycled back into the mantle by sliding under a neighboring plate, in a subduction movement. In such areas, the crust reacts to the mechanical stress, getting buckled up and tightened, which results in mountain ranges
Areas where plates interact are prone to experience seismic events: earthquakes, which result from the breaking of rocks in the crust or in the mantle (which is generally viscous, but also behaves like a non-newtonian fluid: it reacts like a rigid solid under sudden shocks). When molten rock finds its way from the mantle, through the lithosphere, to the surface, we experience volcanism. That magma is called lava after it reaches the surface. Such events can be violent and explosive (eruptions) or steady and continuous (effusion). Eruptions are mostly observed in subduction areas, in the form of cinder cones, composite volcanos or shield volcanos, and on scales ranging from hundreds of meters to hundreds of kilometers. Effusive volcanism is generally observed on large scales. So-called igneous provinces, associated with super-mantle plumes (“pipes” where magma continuously flows up), cover millions of square kilometers. Even larger, wide swathes of sea floors can also see a constant magma inundation, that renews the oceanic crust.
Plate tectonics and its consequences are not the only phenomena that shape the solid phase of the geosphere. To a small degree (most of the time), impacts from extraterrestrial bodies also play a role.
The dynamics of the liquid and gaseous phases are of even greater import. The dynamics of the hydrosphere can be summed up by the concept of the water cycle. Every year, about 40,000 cubic kilometers of water move across the surface of the continents to the oceans (about one thousandth percent of the hydrosphere), profoundly shaping the surface of the continents. In short, evaporation by the sun lifts off water into the atmosphere, and the counterforce to this process is gravity that forces it back down. It then all starts with the oceans (and seas, and bays, which are distinct geographical terms for parts of the same geological system). As a liquid, water naturally gathers around a local minimum in gravitational potential, and conforms to the shape of its container. Water gathers over a mafic/basaltic part of the crust called oceanic crust that is denser and thinner than its granitic counterpart, the continental crust, and therefore sits lower on the mantle.
After absorbing solar energy, liquid water from the oceans undergoes evaporation, turning into vapor and joining the atmosphere. That vapor can experience condensation, effectively going back to a liquid state, but this time as suspended droplets, forming clouds. When conditions are met, precipitation sees water falling back to the ground, as rain, snow or hail. Once again subject to gravity, water, if liquid, can either infiltrate the crust and go underground, or run off at surface level. Should it instead become part of ground-level ice or snow, it stays there until melting occurs, leading in turn to infiltration or runoff. In cold climates, ice can become a permanent fixture (on geological scales), in the form of an ice cap, or a glacier that moves downslope, eroding bedrock in the process. Infiltrated water can stay in underground reservoirs, with a way out is either through transpiration directly to the atmosphere, or discharge into the ocean. Lastly, the water that runs off gathers into flowing bodies, from tiny streams to huge rivers, ending up in overground reservoirs like ponds or lakes, or into the oceans. As we will see, the water systems plays a crucial role in shaping the lithosphere, with the erosion of rocks as well as their transport over the surface of the earth.
The dynamics of the atmospheric systems are evidenced by the existence of different climates and of weather. We’ve seen that the retention of an atmosphere is far from a given. A rocky planet needs to be far enough from its sun (lest the gas is blown away by solar winds, as on Mercury), big enough for its gravity field to keep the atmosphere close (unlike our moon), not so extreme (temperatures and internal activity) that it could just be expelled in outer space, and have a strong magnetic field (to shield it from the bulk of solar activity). But even assuming that an atmosphere is there, the existence of significant dynamics is not a given. Earth’s rapid rotation about its axis, which determines day and night, spreads the incoming solar energy more or less uniformly around circles of latitude, and strongly affects circulations patterns, both in the atmosphere and hydrosphere. The earth’s rotation around the sun, coupled with its axis’s tilt with respect to the ecliptic plane results in different seasons; another factor of periodic modulations, albeit on a yearly scale this time.
Our atmosphere is divided into several layers:
The troposphere is the lowest layer. It is warmed up by heat (infrared radiation) from the surface, and temperature decreases with altitude. It is characterised on a large scale by convection movements, and is the locus of weather (winds, clouds, precipitation…). It is the part of the atmosphere that is most involved with the water cycle. The transfer of water vapor from the oceans results in tremendously large thermal exchanges, which is of course significant driver of atmospheric circulation. In turn, air movements (winds) agitate oceanic water, creating waves that eroded the shorelines, forming beaches, lagoons and related coastal features. Winds can also not only erode the lithosphere, but also carry considerable amounts of sand and dust, sometimes shaping whole landscapes, like sand dunes and deserts.
The stratosphere is warmed by the absorption of ultraviolet light rays. It is home to a local temperature minimum, above which the temperature drops with altitude.
The thermosphere, about 100 kilometers high, is heated by UV rays as well as X-rays. The air there is evolving from a gas phase and an ionized medium.
The exosphere is the outermost layer, where the atmosphere gradually fades into space. The temperature still rises with altitude, but in a medium with super-low density, it is a very different notion as nearer the surface. Atoms regularly escape into space, as the atmosphere is very slowly “peeled off” the earth.
Having briefly outlined the large-scale dynamics of the geosphere, we can focus a bit more on individual objects. On rocky planets, individual, solid objects on the brittle surface are called by the generic name of rocks. This moniker excludes biogenic materials (that are directly and exclusively generated by living beings, like guano or seashells) and anthropogenic materials (directly and exclusively generated by human activities, like plastics or ceramics). Rocks can then only be made of materials that occur naturally as a result of geological processes.
Rocks are solid aggregate of minerals and mineraloids. There have been different definitions of minerals, but they all hinge on some notion of regularity. Nowadays, the requirements are a well-defined crystalline structure, and a fairly well-defined chemical composition (which leaves some stoichiometric leeway). Mineraloids form a motley crue of materials, including obsidian (a volcanic glass), anthracite or black coal), opal (an amorphous form of silica), tektite (meteoritic glass), etc.
Minerals, on the other hand, having a fairly stable chemical composition, are normally defined on that basis:
Native element minerals are chemically homogeneous crystals with only one element. A well-known case is that of native metals, where metallic elements are assembled by metallic bonds, for instance gold, silver, platinum, palladium, iron, aluminum, zinc, nickel copper, tin, lead... Native metalloid elements include silicon, arsenic and antimony. Their bonding schemes are more versatile and polymorphous, involving metalling as well as covalent bonding. Lastly, although most nonmetals are gaseous under terrestrial conditions (only one is liquid: bromine), some can exist in native mineral form: phosphorus, sulfur, selenium, iodine, and most famously carbon, whose two stable crystalline forms we’ve encountered already (diamond and graphite). Nonmetal native minerals are example of network covalent bonding, or sometimes of looser intermolecular forces holding atoms together.
Alloys are intermetallic compounds where different metallic (or metalloid) elements are bound metallically together. Naturally occuring alloys include brass (copper-zinc), bronze (copper-tin), electrum (gold-silver)...
Ionic minerals represent the overwhelming majority of solid materials on planets. As we’ve seen, these compounds associate a metal cation with a nonmetal anion (metalloids can play either role). Minerals are classified based on the anion.
Sulfides have a single-element sulfur anion (\(\text{S}^\text{2-}\)). Similarly we can encounter oxides, carbides, phosphides, halides (for halogens), selenides, tellurides, etc.
Hydroxides feature one or more hydroxide anions (\(\text{OH}^\text{-}\)).
Sulfites’ anion is \(\text{SO\_3}^\text{2-}\), and we see a similar configuration in selenites, tellurites, etc.
Sulfates’ anion is \(\text{SO\_4}^\text{2-}\). Similarly we have carbonates (\(\text{CO\_3}^\text{2-}\)), nitrates ((\(\text{NO\_3}^\text{-}\)), as well as borates, selenates, tellurates, phosphates, arsenates, etc. But the most important mineral in that group by far (and in any group) is that of silicates. Indeed those materials make up more than 95% of the earth’s crust. This is explained by chemical reasons: oxygen and silicon are the most prevalent elements in the earth’s crust by far, at 47% and 28% by mass; and by crystallographic reasons: the sulfate anion’s tetrahedral structure is extremely stable and versatile, in terms of periodic connection with other anions or in terms of cation integration.
Organic minerals are rare. There are lots of organic, non-living solids like amber (fossilized tree resin), jet (compressed fossilized tree) or shells, but since they originate from biological processes, they are not even considered as mineraloids, let alone minerals. However, rare minerals like evenkite (\(\text{C}_{24}\text{H}_{50}\)) qualify as organic crystalline strucutres that has appeared through strictly geological processes.
The fundamental structure of minerals explains their macroscopically evident characteristics:
Physical properties:
Density
Thermal properties (melting/boiling point, heat resistance...)
Radioactivity.
Chemical properties:
Taste and odor
Reaction to acid
Solubility
Crystalline properties:
Habit, the typical shape of individual or groups of crystals.
Hardness, the ability of a mineral to resist scratching. The Mohs scale of hardness ranges from 1 (talc) to 10 (diamond, the hardest mineral of all).
Cleavage and fracture, referring to to the way a mineral breaks along respectively flat and uneven surfaces. For example, mica breaks along one plane to produce thin, flat sheets.
Tenacity, a mineral’s resistance to breaking, bending, or deforming. It can be brittle (break easily), malleable (can be flattened or bent without breaking), sectile (can be cut into thin shavings with a knife), ductile (can be drawn into wires), or flexible (can be bent and then return to their original shape).
Optical properties:
Color: Color, one of the most noticeable properties of minerals (although it can vary with thermodynamic conditions or purity).
Luster, the way a mineral reflects light. It can be described as metallic (a shiny, polished surface), sub-metallic (a qualitatively similar, but duller aspect) or non-metallic (glassy, vitreous, pearly, silky, greasy, or earthy appearance).
Streak, the color of a mineral’s powder when it is scraped across an unweathered surface (it may not be the same as the mineral’s external color).
Diapheneity, the extent to which a mineral can transmit light (ranging from transparent to opaque).
Phosphorescence, is the ability of a mineral to emit light after being exposed to ultraviolet radiation.
Electromagnetic properties:
Magnetism
Electrical conductivity
Piezoelectricity, the ability to convert pressure or stress into an electric charge.
Rocks are consolidated mixtures of minerals and mineraloids. They first appeared when the crust solidified at the start of the Archean epoch, but haven’t stopped transforming since, as part of the rock cycle. There are three types of rocks according to their "immediate" origin.
Igneous rocks are the result of magma (molten rock) cooling down, either on the surface (following eruption or effusion), in which case it is called extrusive, or in the crust, in which case it is called intrusive/
Sedimentary rocks are produced when sediments (weathered fragments of other rocks) are buried, compressed and cemented together.
Metamorphic rocks are formed via the underground alteration of other rocks from heat, pressure, or chemical action.
The rock cycle, powered by tectonics, weather and water flow, ensures that physical materials circulate and are rotated between rock types:
All type of rocks can be uplifted to various parts of the globe, where they’ll get exposed to weathering, turned into sediments and transported by the hydrospheric and atmospheric systems (for instance, magma from a deep ocean trench can end up as an igneous rock on top of a mountain).
All types of rocks can be buried and subject to metamorphic conditions.
All types of rocks can be buried and melt, turning into magma.
Igenous rocks are cooled magma, so it is sensible that their diverse characteristics depend on the nature of the magma they stem from, and the conditions under which they cooled. First, it’s important to remember that the mantle is not made of magma, but of solid rock that acts as a viscous material on large spatial and temporal scales. Magma typically appears in subduction zones. At points where tectonic plates diverge, the resulting loss of pressure can cause hot mantle rock to undergo decompression melting and liquefy. Where plates are converging, the subducting plate gets its water content "squeezed out". This lowers the melting point for underground rocks, which then melts. The exact features of the melting process influence the magma’s chemical content ad well as it’s physical properties (viscosity, temperature...). Then, whether cooling happens gradually within the crust, or suddenly at the surface, while exposed to the atmosphere, leads to very different crystalline structures (fast cooling doesn’t allow large crystals to grow).
Sedimentary rocks are borne out of sediments that are the outcome of three processes:
Weathering produces the sediment material itself through the interaction of surface rocks with the hydrosphere and atmosphere. Such interactions include:
Mechanical weathering: loss of pressure (when overlying rocks move), freezing and thawing, and salt crystal formation.
Chemical weathering: hydrolisis, oxidation.
The end result can be:
Fragments of the source rock (according to size: boulders, cobbles, pebbles, gravel, sand or silt).
Recombined new minerals (mainly clays).
Dissolved portions of the source rock (mainly salts).
Transport moves the material to its final destination. The main agents of transport are rivers, glaciers, winds, currents... Transformation can continue during transport, for example from abrasion. But the main effect of transport is the sorting of materials according to size and density, which determines the makeup of future sedimentary rocks.
Deposition occurs at a site called the sedimentary environment (like a riverbed, a delta, a beach, glacier, dune, lagoon, reef, marine shelf, deep sea fan, abyssal plain...).
Sedimentary rocks form when sediments solidify by cementation (precipitations from the pore waters) and compaction (from the growing burden of younger sediments accumulating). Today, 75% of all rocks at the surface of the earth are sedimentary. However they represent only 8% of the rocks in the earth’s curst. We can classify sedimentary rocks as follows:
Clastic sedimentary rocks are composed of fragments from other rocks or of clay. They are further sorted according to average grain size:
Conglomerates (grain size > 2mm) are consolidated deposits of gravels and pebbles with interbedded sand and mud, that conflomerates in streams or on beaches.
Sandstones (grain size 0.06 to 2mm) represent about 30% of sedimentary rocks on earth. Most abundant in sandstone are quartz, feldspar and micas, which are both hard and chemically stable, and thus can be resilient to long transport channels.
Shale or mudstone consists of consolidated mud (clay or other fine particles), and represents the bulk of all sedimentary rocks on earth (60-70%). It is softer than sandstone and tends to form smooth hills and slopes during weathering.
Chemical sedimentary rocks do not form from rock fragments or mud. Instead, they form when another, non-sedimentary rock, reacts with the dissolved compounds that have been transported during the sedimentation process.
Limestones form when calcium-rich water triggers the precipitation of minerals to form carbonate.
Evaporites consist mostly of salts forming from the evaporation of seawater.
Metamorphic rocks largely form in two types of settings: at convergent plate boundaries, where the moving plates generate high pressure (regional metamorphism); and in the vicinity of igneous intrusions, where surrounding rocks are heated by the ascending magma (contact metamorphism). The physical structure of those rocks is the result of readjustment to new environmental conditions:
High temperature releases the water contained in minerals into the pore spaces, fractures and crystal boundaries. This speeds up chemical changes considerably, leading to new mineral assemblages.
High pressure tends to favor minerals with higher density, whose atoms are tightly packed.
Changes in the surrounding chemical environment (for instance from magma, or another type of rocks) and also leads to mineral content being imported into the rock from adjacent bodies, or vice versa, leading to the overall rock composition to change (that is called metasomatism).
Metamorphic rocks are often made of minerals we also encounter in igneous or sedimentary objects, but some of them only arise via metamorphic means.