Scottish Agates - The Geology

 

The geology of Scotland for its size is very complex. It represents a varied assemblage of geologically diverse fragments of the earth crust accreted at various times over the almost unimaginable length of time of some 3300 million years.

The geological map of Scotland is reproduced by permission of the British Geological Survey. © NERC. All rights reserved. IPR/95-33CW. (ipr@bgs.ac.uk)

The ancient Scottish Precambrian crust lay approximately 30 degrees South of the equator about 3300 million years ago. Together with Labrador and Greenland the Outer Hebrides and the Northwest Highlands formed part of an old and stable part of the Archaen crust. The oldest rocks in Scotland are therefore those exposed in the far north-west composing the Outer Hebrides and a narrow tract of ground extending from the region of Cape Wrath to the Sleat peninsula of Skye and to the islands of Tiree and Iona. These rocks, generally known as the Lewisian from their widespread occurrence on the Isle of Lewis are between 2,900 to 3,100 million years old. The Lewisian consists largely of coarsely crystalline, striped grey and white or black rocks referred to as gneisses, which are the end products of long and complex histories. The basic ingredients that initially went into the making of these gneisses probably included sands and muds deposited in ancient seas over 3,000 million years ago. As well as this material lava from ancient volcanoes as well as intrusive igneous rocks almost certainly also featured among this starting material. Due to this repeated folding and recrystalisation during mountain building processes these original sediments and igneous rocks were so altered (or Metamorphosed) into the gneisses that only a guess can be made as to the original material they were formed from.

By about 2,400 to 2,000 million years ago however the record comes a bit cleared. Dykes that are formed by the intrusion of magma are seen abundantly in the northwest of Scotland transecting the Lewisian gneisses.  Dykes start off vertical or steeply dipping but can later fold or rotate into other attitudes by earth movements. Before about 1,000 million years ago it is thought that there were episodes involving the growth of continents and the rifting apart of continents once formed with the creation of new oceans. These oceans waxed (through growth along constructional plate boundaries), and then waned (when rates of destruction along subduction zones exceeded those of growth). The resultant closure of the oceans, involving collision of the adjacent continents, caused uplift, folding and metamorphism, i.e. orogenesis. One such transient ocean may have opened and closed in the interval 1,100 to 1,000 million years ago, starting with the break up of the great continent of Paleopangaea, closing with the Grenvillian Orogeny and the formation of a continent called Rodinia.

The Pre-Cambrian Earth. This map illustrates the break-up of the supercontinent, Rodinia, which formed 1100 million years ago. The Late Precambrian was an "Ice House" World, much like the present-day. The Red Arrow shows the approximate position of what would become Scotland. [Copyright C.R. Scotese, Paleomap Project]

From about 1,000 million years ago the geological record starts to be more complete. Great thicknesses of sediments were laid down on the floors of shallow seas and/or from great river deltas flowing into these seas. Much of the landscape of the Grampians and the Northern Highlands has been carved out of rocks composed of these ancient sediments. At about 870 million years ago, still in the Precambrian, it is thought that the crust thinned (lithospheric stretching) in this area causing a major outbreak of basalt volcanism. There is no longer any evidence of extrusive structures but there are a great number of minor intrusions north of the Great Glen Fault that probably represent intense swarms of dykes which would have almost certainly been associated with fissure volcanoes erupting either on dry land or on the sea floor. 

Some time around 600 million ago the “over-stretched” continent of Rodinia ruptured allowing the generation of another ocean, Iapetus. Due to the fact that the severed continents on either side largely corresponds to those now seen in North America on the one side and Europe and North Africa on the other, Iapetus is sometimes referred to as the “proto-Atlantic Ocean”. The remains of lavas and intrusions generated by the volcanism at the “birth” of this ocean can be seen scattered throughout the Grampians but can be seen at their best on the Tayvallich Peninsula, Argyll on the west coast. “Pillow lavas” as evidence of this volcanism can been seen in this area. It had no sooner reached its maximum size when subduction of the ocean floor below the continental crust on both sides led to shrinkage and its ultimate closure about 455 million years ago. With the collision of the bordering continents this led to contorting and uplifting of the rocks involved and the creation of a majestic mountain range on a “Himalayan” scale. These subduction processes that caused the closure of the Iapetus Ocean were also associated with extensive volcanism.

This mountain building episode reached its peak about 430 million years ago and is known as the Caledonian Orogeny. This major folding and faulting of these rocks of this now deeply eroded mountain belt are recognised far beyond Scotland and compose much of the Appalachian Mountains in eastern North America, the Norwegian mountains and those of north-eastern Greenland.

Since that time the only other major tectonic event would occur was not until about 60 million years ago when the North Atlantic Ocean started its existence. This was the Tertiary Volcanic period. 

Scotland is really a collection of continental fragments (terranes) that were assembled over this enormous period of time by plate tectonics. It is not certain where all this component parts were originally positioned on the ancient earths surface. The study of rock magnetism can provide a clue due to the fact that the Earths magnetic field becomes locked or fossilised into the rocks as they form. Thus measurements of the rock’s magnetic properties allows geologists to determine not only in which direction the Earth’s magnetic poles lay at the time of the rock’s formation but also the angle of the dip (declination) of the magnetic field. Since the declination changes with latitude, palaeomagnetic research can identify the latitude in which rocks form. Such studies have shown that arpund about 1,200 million years ago the terranes that ultimately formed Scotland lay deep in the southern hemisphere. In the last 500 million years there has been a generalised northward migration that brought all the rocks across the equator some 200 million years ago and on to their present day latitude between 54 and 61 degrees N.

 

Of the different terranes that eventually become Scotland the principal 5 are:

  1. Southern Upland Terrane, bounded to the south by the so-called Iapetus suture through northern England and to the north by the Southern Upland Fault
  1. Midland Valley Terrane, lying between the Southern Upland Fault and the Great Glen Fault
  1. Grampian Highland Terrane between the Highland Boundary Fault and the Greta Glen Fault
  1. Northern Highland Terrane, bounded on the SE by the Great Glen Fault and to the NW by a fault with a shallow easterly dip, called the Moine thrust
  1. Hebridean Terrane to the west of the Moine thrust, embracing the Outer Hebrides and a narrow strip of land along the NW margin of the Scottish mainland.

 

Distinct fossils first made their appearance around 540 million years ago when marine organisms developed shells or carapaces that had a reasonable chance of preservation. Major changes in the evolution of life, particularly as indicated by animal fossils provide the basis for useful time breaks. There are three principal time divisions, or Eras, primarily based on such palaeontological evidence:

The Palaeozoic – extending from 540 to 250 million years ago

The Mezozoic – from about 250 to 60 million years ago

The Cainozoic – from the end of the Mesozoic to the present day.

Collectively these three Eras are known as the Phanerozoic.

Each of these “Eras” is then sub-divided into a number of “Periods” [See diagram] In Scotland there is no evidence of volcanic activity in the Mesozoic Era, which consists of the Triassic, Jurassic and the Cretaceous Periods. In the Cainozoic Era volcanic activity in Scotland is almost totally confined to the early part, the Tertiary Volcanic period. It is mainly in the Palaeozoic that  we find a long and complex history of volcanic eruptions in Scotland. The Palaeozoic is thus divided into six periods. They are, in decreasing order of age, Cambrian, Ordovician, Silurian (composing the “Lower Palaeozoic”) and the Devonian, Carboniferous and Permian periods making up the “Upper Palaeozoic”

This timescale is used by geologists to describe the timing and relationship between events that have occured during Earth's history. The table below is a very much simplified version that agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy. Different spans of time scale are usually determined by major geological and paleontological events such as mass extinctions. In this table I have not subdivide into Series/Epochs.

The time between the end of the Neogene and to the modern day is known as the Quaternary. This spans a time of 1.8 million years and due to the fact that most of mans evolution took place in that time it is also known as the "Age of Man" It is interesting to note that with all this massive, almost unimaginable, length of time we have only been on the planet this short time. It should also be noted that in many scientific circles the combined period of the Paleogene and the Neogene is also known as the Tertiary Period. I have referred to this time period particularly in relation to the last great volcanic period of Scotland.

Click on the appropriate Period to take you to its page. You are probably better starting with the earliest and working forward. As well as a general discussion of the Period I have also tried to put a Scottish perspective in the text.

EON
ERA
PERIOD

PHANEROZOIC

543 mya to present

CENOZOIC ERA

65 mya to today

NEOGENE

24 to 1.8 mya

PALAEOGENE

65 to 24 mys

MESOZOIC ERA

248 to 65 mya

CRETACEOUS

144 to 65 mya

JURASSIC

206 to 144 mya

TRIASSIC

248 to 206 mya

PALAEOZOIC ERA

543 to 248 mya

 

PERMIAN

290 to 248 mya

CARBONIFEROUS

354 to 290 mya

DEVONIAN

417 to 354 mya

SILURIAN

443 to 417 mya

ORDOVICIAN

490 to 443 mya

CAMBRIAN

543 to 490 mya

PROTEROZOIC

2500 to 543 mya

PRECAMBRIAN

4500 to 543 mya

ARCHAEN

3800 to 2500 mya

HADEAN

4500 TO 3899 MYA