by Ted Nield
An example of how the process of continent formation might have started can be seen today on Iceland. By processes of partial melting and melt extraction in the system of conduits under the island, magmas of granitic (classic SiAl) composition are being formed. This is why, although Iceland is always portrayed as a black, basaltic place befitting its position on a mid-ocean ridge, in fact up to 10 per cent of its rocks are of light, granitic type.
Iceland sits astride a hot, hyperactive stretch of the Mid-Atlantic Ridge, and has been forming for a mere sixteen million years or so. Just like the early crust of the Earth, because the amount of volcanic material erupted there is higher than average, Icelandic crust is twice as thick as normal ocean crust, which all helps the lighter ‘continental’ type rocks to separate out as the cooling magma circulates in the plumbing below. Because of their lower density, sialic rocks – once created – would then remain at the surface, gradually coalescing as they jostled and fused by continuous minor collisions forming the protocontinents.
As the Earth cooled further, oceans began to condense, hydrological cycles of evaporation and precipitation began to operate and the erosion and deposition of sedimentary rocks could really begin. The earliest evidence of erosion comes from rocks over four billion years old, in the form of those amazingly robust and persistent microscopic mineral grains that John Joly saw at the centre of his pleochroic haloes: zircons.
As we have seen, when rocks undergo melting, and elements of differing atomic weight separate out between solid and liquid phases – a process called fractionation – different isotopes of the elements (despite their identical chemical properties) behave differently according to their slightly different weights and measures. Some prefer life in the melting pot, while others tend to remain in the solid. If continental crust is continually fractionating from the Earth’s primitive material, isotope ratios within the different rocks generated will gradually come to differ from average, or ‘bulk Earth’, values. Thus, even in one single, tiny grain of zircon, distinctive fractionated isotope ratios remain as the telltale signature of early crustal processes that generated suites of continental rocks of which those tiny crystals are today the only surviving remnants. Truly, a whole world in a grain of sand.
Mark Harrison at the Australian National University in Canberra and his co-workers have been studying zircons that were eroded, reincorporated and then sealed within younger rocks about 4.4 billion years ago. These rocks come from the Jack Hills in Western Australia, and themselves constitute one of the oldest pieces of continental crust on the planet. Harrison and his colleagues have tested these grains for two isotopes of the element hafnium (Hf ).
The zircons contain very low concentrations of another element, lutetium, whose radioactive isotope 176Lu decays to hafnium; so the researchers believe that the ratios of 176Hf to 177Hf which they find in these grains are very close to the primitive ratios that prevailed when they formed – that is, at the original zircon-containing rock’s absolute age, determined independently using uranium-lead dating methods. Those ratios are characteristic of fractionated sialic crust. What this suggests is that continents were not only forming, but even being eroded and their detritus redeposited, within as little as 200 million years of the Earth’s accretion: that is, between 4.4 and 4.5 billion years ago.
In fact, the more geologists look into this question, the more it seems as though fractionation processes had produced continents of almost modern dimensions fairly soon in Earth history. There is little direct evidence of this because very little continental crust dating from the Archean (between 4.5 and 2.5 billion years ago) survives today. Barely 7 per cent of rocks in the present continents are that old; all the rest have now long been eroded and recycled as younger continental crust. So how is it possible to tell how big Archean continents were? The answer comes in the form of meteorites.
The stony meteorites are the leftover raw material out of which the early planets were formed. This means that the early Earth originally displayed the same overall concentration of uranium as meteorites still do. In geological processes, uranium tends to fractionate into continental crust; the more uranium there is in the continents, the less there will be in the mantle. Therefore, one measure of the total amount of continental crust in existence at any given time is the degree to which mantle rocks are depleted in that element. The surprising result of testing those few surviving mantle-derived rocks from the late Archean (2.4 billion years ago) is that they appear to be just as uranium-poor as modern mantle.
Some time in the Archean (probably earlier rather than later) the continents became ‘fully grown’. Continents may since have fused and split, danced and skated over the globe like the faces of a Rubik’s cube in maddening and almost untrackable ways; but their total volume has remained about the same for the greater part of Earth history, the product of a dynamic balance between the erosion of existing continental crust and the production of new.
And into a very dark corner of this hostile, water-enveloped world, deep below the turbulent surface of the first global ocean, life was born. Earth was a young, hot planet; spinning so rapidly on its axis that a day lasted no more than five violent, storm-racked hours. Its acid, anoxic ocean was raised in frequent massive tides by a Moon that had not yet wound away to its present distance, though it may well not have been visible in the feeble light filtering through the murk of gas and dust thrown up by meteorite impacts that would, from time to time, vaporize sea and rock alike.
Slumbering green
Julian Huxley FRS (1887–1975), son of the great Thomas Henry Huxley, in a wistful poem to a dancer whose performance had captivated him, lamented that he felt
Weary of plodding science, where the vision
Must for achievement clothe itself in clay …
Nowhere do you find the vision obscured more often, or more thickly, than in the dreaded conference abstract: a tight little knot of compressed and overwrought jargon with which scientists announce early findings to a room of critical colleagues they are hoping to impress. But they are not all like that.
‘Earth agglomerates and heats. Volcanoes evolve carbon oxides, methane and pyrophosphate. Convection cells, stacked in the planetary interior, begin the cooling process. An acidulous Hadean ocean condenses from the carbon oxide sky. Stratospheric smogs absorb a proportion of the Sun’s rays. The now cool ocean leaks into the crust and convects …’
The author of this was Professor Mike Russell, then a research fellow at the Scottish Universities Environmental Research Centre in Glasgow and currently ‘distinguished visiting scientist’ at NASA’s Jet Propulsion Laboratory in Pasadena, California. Russell is a world-renowned expert on the early Earth and the planetary geology that can give rise to life. Together with his colleague Allan Hall, he has written widely about how life may have originated; but in order to do that, any scientist has to ask what sounds like an unanswerable question: namely, what exactly is life? For Russell and Hall, the best way to approach this loaded question is to stick to what can be observed and measured. Instead of asking what life is, Russell asks what life does. And to put it simply, life exists to make a mess.
‘A living cell assimilates nutrients, uses energy and generates waste. It consists mainly of carbon-based (“organic”) molecules that also contain hydrogen and other elements. Their defining structural feature is a mainly waterproof container, the cell membrane. Inside is a watery solution with a high concentration of organic molecules as well as some inorganic salts.’
To tackle this question of what life ‘does’, scientists need to understand natural sources of energy and what forms of energy are involved in life processes. Says Russell: ‘What does a waterfall do? It drains water from a greater to a lesser height, giving the water a lower gravitational energy. What does a warm spring do? It is a plumbing system draining thermal energy from underground and dissipating it on the surface. So, what does life do? Life is a chemical system that drains and dissipates chemical energy. For example,
animals and plants gain chemical energy from sugar in food by burning it in inhaled oxygen, a process we call respiration.’
On Earth today, nearly all life ultimately derives its energy from the Sun, which drives the whole process. Green plants catch the rays and use them to ensnare carbon from the air and water from the soil to produce big organic molecules, with which they build their tissues. The waste product of this process (called photosynthesis) is oxygen, which animals then use to break down the carbon compounds in the food they eat, thus releasing their energy and generating the raw materials they need to grow. Plants are the origin of nearly all life as we know it because only they can use pure energy to build their bodies; bodies that animals at the bottom of the food chain must eat to build theirs. All flesh, as the Bible says, is grass. This has been the way the living world has worked for billions of years. But not always. Certainly not from the beginning.
Mike Russell and Allan Hall think that the first living cells formed on the floor of the Earth’s first newly condensed oceans, where warm, alkaline submarine springs focused chemical energy, and the mixing of the hot spring water and seawater caused simple chemicals to precipitate out as thin, inorganic films.
Under today’s oceans, at regions where the heat-flow is high, such as the mid-ocean ridges, seawater leaking into the hot rocks of the seabed is superheated (sometimes as high as 700°C), becomes charged with minerals and is then extruded into the cold sea at submarine hydrothermal vents known as ‘black smokers’, because the dissolved minerals immediately come out of solution with the sudden fall in temperature and create the impression of chimney smoke. But these waters can be hot enough to melt lead, and are only kept from boiling by the intense pressure. They are far too hot for organic life to have originated in or near them, even though they are often densely colonized today by specially adapted organisms.
However, a similar hydrothermal process can also happen far away from the hot ridge, on much older (and cooler) ocean floor. At these vents, percolating seawater itself is responsible for creating heat, by hydrating the mineral olivine (the basic mineral component of the mantle) to create another mineral, serpentine. These springs reach only relatively moderate temperatures (the hottest being about 170°C); but these off-ridge alkaline vents (first discovered in 2001, though predicted by Professor Russell some years before) do grow much larger than their hotter equivalents on ridges. They are also a distinctive ghostly white (since they are mainly made of calcium carbonate) and the largest of them are known to rear up over thirty metres from the ocean floor.
Ocean-floor springs also contain many essential minerals which all organisms require, such as phosphorus, nickel and zinc. At a time when the Earth’s surface was inimical to the existence of organic molecules, here was somewhere safe and protected, where life could form uninterrupted. The gradients of temperature and acidity/alkalinity could provide the energy while the minerals brought chemical food within a solid structure built of a mix of carbonates, silica, clays and sulphides of iron and nickel. Mike Russell, who began his career apprenticed as a chemical engineer studying nickel catalysts, recognizes in this system what an industrial chemist would call a ‘continuously regulated flow reactor’.
A further hint that life truly originated in these dark, submarine places soon after the oceans first condensed perhaps 4.4 billion years ago, is that the microbes still living today among modern hot springs include some of the most genetically simple life-forms on Earth. Vent communities may form a closed ecosystem, but Russell believes life eventually escaped from them to colonize the world. And that great escape was probably brought about by the processes of plate tectonics: familiar to us as elements in the Supercontinent Cycle.
Russell’s insight into the way life originated on the Hadean ocean floor began while he was entertaining his son Andy by making ‘chemical gardens’ with the sort of chemical kit you can buy from science museum shops. These crystal gardens seem to ‘grow’ in a plant-like way; but the snaking, knobbly tubes rising from the beaker bottom are purely inorganic, forming at the interface of the crystals that the experimenter drops into the solution. However, Russell remembers that on the night after starting the chemical garden, Andy started to break it up. ‘Deaf to my pleas to join us at supper,’Russell remembers, ‘he announced, from behind the locked bathroom door, “Hey Dad, these things are hollow!”’
Suddenly Russell thought about the puzzling patterns he had seen in the rocks of an ancient ore body he had studied in Ireland, long after he had left the chemical industry and qualified as a geologist. He had seen columns, chimneys and bubbles in the rock, all made of iron sulphide. These had once been natural ‘chemical gardens’, a garden, he soon realized, in which the seeds of life could have grown. As he discussed the idea the next day with his colleague Allan Hall, he saw how organic molecules could have become trapped and concentrated along those flimsy chemical membranes precipitated as the hot alkaline spring water, rich in minerals, met the cold, acidic sea. These small pockets of proto-cells could have encouraged more complex and unlikely reactions to take place, just as every life-form today uses a membrane to protect and concentrate its contents. Russell says: ‘The precipitation of chemicals on mixing of solutions forms a barrier, preventing further mixing and precipitation. At the warm spring we envisage the formation of a special precipitate that not only formed a barrier, but also provided a template for the assembly of chains of organic molecules, and acted as a catalyst for electrochemical reactions.’
He thinks that along such thin chemical boundaries organic molecules like amino acids, the building blocks of proteins, first became concentrated. These organic molecules would have formed a little deeper in the columnar ‘flow reactor’, where water and its dissolved chemicals were reacting with iron and nickel-rich minerals. The precipitate membrane would then capture and concentrate other chemicals that could participate in reactions, he thinks; but eventually this system would evolve, by a process of ‘organic take-over’, into a cell membrane consisting entirely of organic molecules. Russell and Hall also believe that by acting as a template, the iron-sulphide precipitate could bond chemically to, and assemble a sequence of, the molecular components of RNA, a chain molecule very similar to DNA, which plays a supporting role in genetic evolution.
‘Once organized on the iron sulphide, this RNA could influence the assembly of amino acids into proteins, as well as the assembly of further chains of RNA, and, finally, of DNA. Eventually, these new large organic molecules could reproduce themselves through the interaction of DNA, RNA and proteins, without any need for the original iron-sulphide template.’
But how could life, assuming that it was indeed born on such ocean-floor vents, have escaped from its abyssal refuge? Russell thinks that during life’s first few hundred million years the only safe escape route would have been down. Early organisms could have migrated into the ocean floor, with its warm underlying sediments, and subsisted there on a diet of trapped hydrogen and carbon dioxide. This was the beginning of the so-called ‘deep biosphere’, the mass of microbes that sits silently and invisibly in the pore spaces of the Earth’s lithosphere, but whose total mass even today outweighs all the living things around us on the planet’s surface.
There, safe from lethal solar rays, early life played its long waiting game; until the rocks, now teeming with endolithic microbes, completed their plate-tectonic journey and became involved in a subduction zone. While most of the sea floor slid down into the mantle, some would inevitably become scraped off on the overriding plate to form an ophiolite suite; and, thrust up into the shallows, a few bacteria would have found themselves deep enough to be protected from harmful rays but shallow enough to use the Sun’s longer-wavelength light to build organic molecules from carbon dioxide. It was the beginning of photosynthesis.
Rust world
Earth’s early acid-bath oceans contained huge amounts of dissolved iron, which by mid-Archean time had been spewing into it from ocean-floor hydrothermal springs for a
billion years or more. This iron was held in solution in both its positively charged ionic forms: ferrous iron (an atom lacking two electrons and thus having a positive charge of two) and ferric iron (lacking three electrons, and with a positive charge of three). The ferrous iron mostly came into the water from very hot springs, whereas the ferric form would have been created by oxidation of this ferrous iron by the weak sunlight, or by the waste product of the earliest forms of photosynthesis.
The novel condition of matter we call life probably originated some time around 4.4 billion years ago, probably around the time that the Australian rocks containing those robust little zircons, with their telltale hafnium isotope ratios, were forming. As the mantle roiled beneath, small blocks of light continental crust were forming at the Earth’s surface and sticking together, growing like plaques of scum on a lake: a process that geologists have called cratonization. By three billion years ago these had coalesced into the first known recognizable supercontinent: Ur.
It is interesting that cratonization did not occur at the same pace all over the Earth’s surface. At the time that the supercontinent of Ur was forming (it connected areas of South Africa, Madagascar, Sri Lanka, India, Western Australia and parts of East Antarctica), elsewhere on the Earth small convection cells were still producing ‘greenstone belts’, the typical product of the Earth’s earliest tectonics: small kernels of continental rocks surrounded by highly deformed, ocean-floor-type rocks.