The origin of the oldest rocks on Earth

The oldest rocks found so far on Planet Earth are at the Acasta River in Canada’s North-West Territories. Discovered about 30 years ago, these rocks are felsic granite, which means that they are rich in quartz and feldspar. 

How old are they? The estimate is that they were formed when Earth was only about 600 million years old, which puts their age at around 3.9 billion years. 

This type of rock, with its particular mix of elements, could only have formed at low pressure and at a temperature of 800 to 900 degrees Celsius during a period of temporary melting of Earth’s surface. But how could such a melting have occurred? 

The answer appears to be that the period of planetary formation was extremely violent for millions of years. For some 600-700 million years after the formation of planet Earth it was subject to almost constant bombardment by asteroids and meteorites, as was the Moon. The heating that this caused would have been sufficient to create the rocks found at the Acasta River. 

If that is the case, then why are similar rocks not found all over the world? This is because Earth – unlike the Moon – is subject to plate tectonics, which means that portions of the Earth’s crust are constantly being renewed and rocks as old as those in question are therefore extremely rare. 

It is thought that rocks of a similar age and type might be found in Siberia, although the discovery has yet to be made.

© John Welford

The classification of sedimentary rocks

There are basically two ways of classifying sedimentary rocks, either by how they originated or their composition. The latter takes into account such matters as whether they contain primarily coarse-textured sands or fine clays, or whether they have a high carbon content due to being composed largely or entirely of plant or animal matter. However, it is generally more convenient to combine the two methods into a single classification, as below:

Mechanical Formation

This group comprises rocks that have been formed after material has been moved in fragments from one or several places to another (by the action of wind, water, ice or gravity), where it has become consolidated either by pressure from later deposits, or by cementation, or both. The original material may have been very fine in nature, such as river-borne silt, or much coarser, such as rounded or angular pebbles or rock fragments.

The material that enables fragments to cement together may be a solution containing minerals of various kinds, such that sandstones may contain quartz, calcium carbonate or iron, the proportions of these determining its colour.

Very fine material will form clays or mudstones, less fine deposits lead to grits forming, and much coarser material results in a conglomerate or brecchia (in the former the pebbles are rounded, whereas they are angular in the latter).

Terms used to distinguish rocks by the size of their particles are Argillaceous (e.g. clay, mudstone, shale); Arenaceous (e.g. sandstone, grit); and Rudaceous (e.g. brecchia, conglomerate, boulder clay).

Organic Formation

These rocks were created from the remains of once living organisms which built up over very long periods of time. These can be further classified according to the nature of the plants or animals that comprised the deposits.

Calcareous rocks (chalks and limestones) consist mainly of calcium carbonate, formed from the skeletons of marine organisms, and are distinguished by the size and nature of the particles that comprise them. The finest particles are seen in pure white chalk. Limestone is more varied, including crinoidal, coral, oolitic and shelly, the terms denoting the type of primitive organism that is mainly represented in its formation. Fossils of much larger organisms are often found embedded in limestone.

Ferruginous is a term that denotes the presence of iron, usually from the precipitation of hydrated iron oxide in the water of ancient lakes and marshes. Decomposing vegetable matter formed the basis of ironstone and “bog iron-ore”.

Siliceous rocks can be formed from the remains of sponges and minute organisms such as diatoms (single-celled plants rich in silica). These include nodules of chert and flint found in other rocks, and beds of diatomite.

Carbonaceous rocks are formed from plant accumulations and are high in carbon content. Depending on the age of the deposits and the pressure they have been put under, they can take the form of peat, lignite or coal.

Chemically Formed

These come about from the precipitation or evaporation of solutions of salts. All water that falls as rain will acquire salt in some form as it runs across the surface or finds its way underground, and these salts are often partially or totally released before the water cycle is completed. Rock formation can occur when sufficient salts accumulate in the same place. Five types of chemical formation of rock types can be distinguished.

Carbonates.  Stalactites and stalagmites in limestone caves, or travertine around hot springs, are examples of carbonate deposition. Dolomite is a chemically formed compound of calcium and magnesium carbonate.

Sulphates. Hydrated calcium sulphate, in the form of gypsum or alabaster, is formed by evaporation in inland drainage basins.

Chlorides. These produce rock-salt, either on the surface or at depth.

Silicates. As well as flint and chert (mentioned above), sinter is a silicate rock, formed around the vents of hot springs.

Ironstones. Most iron ores have accumulated from chemical precipitation within sediments, although some are the result of igneous activity.

Sedimentary rocks are typically laid down in strata of varying thicknesses, and the process can continue at the same place for extremely long periods of time (millions of years in some cases). It is sometimes possible, for example, to detect annual depositions made by ancient rivers, and use these to determine the age of a particular formation.

© John Welford

The classification of igneous rocks

Igneous rocks are solid forms of magma that has been extruded by volcanic processes. The word “igneous” derives from “ignis”, the Latin for “fire”. Magma is molten rock that rises from deep within the Earth’s crust and which cools and solidifies as it approaches the surface. Igneous rocks can therefore be classified according to the manner in which the cooling took place, and also according to the chemical composition of the magma from which they were formed.

Chemical Composition

Nine elements make up about 99% of all igneous rocks, with the most common compound being silica (silicon dioxide, SiO₂). The rocks can therefore be classified according to the proportion of silica they contain. If this is greater than 65% the rock is said to be “acid”, and if lower than 55% it is “basic”. Rocks between these points are “intermediate”, whereas those with a silica percentage lower than 45% are “ultrabasic”. Where the silica percentage is low, that of other (basic) oxides is high, and vice versa.  Acid rocks are generally lighter in colour and weight than basic rocks.

Examples of rocks of the various types are:

Acid: granite, obsidian

Intermediate: diorite, andesite

Basic: gabbro, basalt

Ultrabasic: peridotite

Cooling of the Magma

Where the magma cools has much to do with the rate at which it cools. Not all magma reaches the surface, and it may therefore cool slowly at some point below the surface. The magma from a single event can cool at different rates depending on how close it gets to the surface, and may therefore produce a wide range of igneous rocks.

Rocks formed from magma that has reached the surface are termed “extrusive” whereas those formed below the surface, and exposed by later erosion or earth movements, are termed “intrusive”.

Cooling magma will produce crystals of nine silicate minerals, each being produced at different temperatures, from olivine to quartz.

The rate of cooling will determine the size of the crystals, such that the longer the process takes, the larger will be the crystals, with some having been found at 40 feet of length in the Black Hills of South Dakota.

Rocks formed from very slow cooling of intrusive magma are termed “plutonic” and are compact, coarse-textured and large-crystalled, examples being granite, diorite, gabbro and peridotite.

Rocks formed from rapid cooling at the surface are termed “volcanic”. These contain very small crystals or are glassy in appearance (e.g. obsidian). Non-glassy volcanic rocks include rhyolite, andesite and basalt.

Sometimes magma will penetrate weaknesses in the original rock and cool at a rate that is intermediate between plutonic and volcanic rocks. The magma may cool at different rates as it progresses, thus producing crystals of varying sizes. These are termed “hypabyssal”, of which porphyry (in its various forms) is an example.

The Igneous Rocks Matrix

The two classifications mentioned above, namely according to chemical composition and rate of cooling, cross each other and thus produce a matrix.

We can therefore distinguish the following groups of igneous rocks (with examples; but note that not every logically possible combination is apparent in terms of actual rocks):

Acid plutonic (granite)

Intermediate plutonic (diorite)

Basic plutonic (gabbro)

Ultrabasic plutonic (peridotite)

Acid hypabyssal (granophyre)

Intermediate hypabyssal (porphyries)

Basic hypabyssal (dolerite)

Acid volcanic (rhyolite, obsidian)

Intermediate volcanic (andesite)

Basic volcanic (basalt)

Mention should also be made of “pyroclasts”, which are rocks formed during volcanic eruptions from rough balls of material that are spat out and comprise a mixture of lava, cinders, ash and dust.

© John Welford

Beware the magma blobs!

Use of the word “beware” implies that there is some action one can take to avoid the bad consequences of a possible or probable event. However, in the case of the magma blobs the only action that might save one’s skin would be to leave Planet Earth altogether!

Most people are aware of the concept of the “supervolcano”, which could erupt with devastating force and cause millions of deaths. The Lake Toba eruption in Indonesia some 75,000 years ago was one such event, as have been the eruptions, over millions of years, that created the Yellowstone Caldera in the United States.

However, these are as nothing compared to what might happen were one of two “magma blobs” that are thought to be deep inside the Earth to become active and send a plume of super-hot material to the surface.

When “ordinary” volcanoes erupt they emit lava that is chemically young. This is because it comes from the upper mantle (beneath the Earth’s crust) where there is constant churning of material – usually at plate margins. However, analysis of lava in Greenland, that erupted 60 million years ago, has shown that it was already very much older than that. This suggests that magma from a much deeper region of the mantle, below the “churn zone”, can sometimes reach the surface.

The fossil record shows that life on Earth periodically goes into reverse, with mass extinctions wiping out the majority of living organisms and evolution having to start again after a long period of time. These events often coincide with dramatic geological events that involve the outpouring of vast amounts of magma over what can be hundreds of thousands or even millions of years. These events often mark the end of geological periods and usher in new ones.

Examples of mass extinctions include the end-Permian extinction of 500 million years ago and the end-Cretaceous extinction that saw off the dinosaurs 66 million years ago. The end-Permian event coincided with the outpouring of magma on a truly vast scale in what is now Siberia, and the end-Cretaceous event also marked the creation of the “Deccan traps” in west central India. The amounts of lava involved are mind-boggling – the lava deposits in India, for example, are two kilometres (1.2 miles) thick. These regions – and there are others in different parts of the world – are known as “large igneous provinces”.

The lava outflows would have been accompanied by constant emissions of toxic volcanic gas that would have completely changed the composition of the atmosphere, thus making life extremely difficult for  most advanced species.

One very interesting fact is that analysis of the rocks in large igneous provinces shows them to be very similar in both composition and chemical age, despite the fact that they were emitted hundreds of millions of years apart. This suggests that they originated in specific regions of the mantle that have not been subjected to churning and mixing.

Some geologists are convinced that the magma blobs responsible for producing large igneous provinces still exist. Not only that, but there is no reason why they may not wake up at some future date and produce yet another lava field and a consequent mass extinction. Planet Earth may indeed become uninhabitable again one day, with man-made climate change having absolutely nothing to do with it.

The bad news is that seismic surveys show that two large magma blobs could indeed exist, these being regions of the mantle that do not fit the pattern of the rest. They lie at a depth of around 2,800 kilometres (1,740 miles), one beneath Africa and the other beneath the Pacific Ocean.

Will one of these blobs produce another massive eruption? Almost certainly. In our lifetimes? Almost certainly not!

© John Welford

Jet: a mineral that gives its name to black

Jet is an organic material that is closely related to coal and is often worked as though it were a mineral.

It is mainly found in strata from the Lower Jurassic era, originating from logs of ancient Araucaria trees (ancestors of the modern ‘monkey puzzle’) that fell and were washed out to sea, eventually being buried under other sediment that, over millions of years of heat and pressure, converted them into a relatively soft form of lignite or coal.

Pieces of jet are often found on beaches in such places as Whitby in North Yorkshire, where they have fallen from nearby cliffs as a result of local erosion.

The softness of jet allows it to be carved into many different shapes. It can also be polished to a brilliant sheen, which has long made it a popular material for use in jewellery.

Examples of jet jewellery have been found from Bronze Age and Roman times. It underwent a revival in popularity during the late Victorian era when people copied the fashion set by Queen Victoria for ‘mourning jewellery’ after the death of Prince Albert (she never wore anything that wasn’t black for the rest of her life).

© John Welford

Geological faults

When two sections of the Earth’s crust move relative to each other, the zone in which they do is known as a fault.

The two sections can move towards each other (in relative terms), apart from each other, or laterally (i.e. sideways to each other). Large areas of land can be pushed up or drop down as the result of a fault.

If two faults occur in parallel, the land between the faults can move downwards to form a rift valley, the best example of this being the Great Rift Valley in East Africa.

Fault movements occur suddenly, as pressures that have built up over tens or hundreds of years are released. This is a major cause of earthquakes.

Faults can allow material from deep in the Earth’s crust to come closer to the surface. This can include valuable minerals and deposits such as gold and silver. There is clearly an economic benefit to mankind in exploiting these resources, but there is also a downside in that zones that are mineral rich may also be prone to frequent earthquakes.

A prime example of this is California, where the discovery of gold led to the region attracting huge numbers of people to settle there, but the local geology made it subject to earthquakes. The whole area is riven with faults, the most prominent being the lateral tear fault known as the San Andreas (see picture).

© John Welford

Geysers

Geysers offer spectacular evidence that, in some parts of the world, there are some very hot rocks not far underground.

The word comes the Icelandic for ‘spouter’ or ‘gusher’, and they are a regular feature of the landscape in Iceland, which sits on top of the mid-Atlantic ridge where hot volcanic material is constantly being pushed towards the surface.

A geyser occurs when rainwater percolates down through cracks in the rocks to accumulate in an underground reservoir that is under constant pressure from a heat source such as rising magma. The water is heated to the point where it expands and is forced upwards through a narrow pipe to the surface.

The result is a geyser of hot water and steam that can shoot seventy metres or more into the air.

The reservoir is now ready to receive more water which will also be ejected when it has been heated and forced upwards. As long as the supply of water is constant, the geyser will erupt at predictable intervals.

It is as though one kept a kettle constantly on the boil but with the lid blocked and only a narrow spout. The contents would be forced out violently time after time, provided that water was always being added to the kettle to replace what had been lost.

One of the world’s best-known geysers is Old Faithful in Yellowstone National Park, Wyoming, USA. This often erupts at 90-minute intervals, with each eruption lasting up to five minutes. However, if the supply of water is lessened, it will erupt more frequently but for shorted durations. The heat source is the volcanic ‘hot spot’ that underlies a vast area in this part of the United States.

There is evidence that Old Faithful has been erupting for at least 700 years.

© John Welford