Episode 9: Tertiary volcanism
About 60 million years ago, in the earliest Tertiary, a dramatic episode of igneous activity took place along the western seaboard of mainland Britain. The resulting bedrock has played a major role in forming features of the landscape of the western Hebridean, Northern Highland and Midland Valley terranes. Successions of lava flows formed volcanic lava fields tens to hundreds of metres thick in many areas of the Inner Hebrides and northern Ireland. Distinct fields have been dated around Eigg and Muck at 60.5 million years old, around Skye and Canna at 58 million years old and around Mull and Morvern at between 58.5 and 55 million years old. The layered (‘stepped’) landscapes eroded in the bedrock of these lava fields are striking, and are due primarily to differences in erosional resistance between the lower and upper parts of each lava flow.
FIG 31. General pattern of processes thought to underlie a typical igneous centre.
Even more striking are the centres of volcanic activity and igneous intrusion that developed in a scatter of localities shortly after the lava fields formed (Fig. 31). The coarsely crystalline intrusive rocks of these centres dominate the landscapes of their surroundings, because of the resistance of this material to erosion. The eroded remains of these ancient igneous centres now form the remarkable Cuillin and the Red Hills of Skye, the mountains of Rum, the hills of the Ardnamurchan peninsula and the main mountains of Mull and Arran, not to mention the islands of St Kilda and Ailsa Craig.
In wider geographical terms, these Tertiary igneous activities, along with the associated uplift and erosion, were responses to the tectonic plate divergence movements that created the Atlantic Ocean, with additional igneous input related to ‘hot-spot’ activity in east Greenland, Iceland, the Faroes, western Scotland and northern Ireland.
CHAPTER 5
Later Surface Modifications
THE PREVIOUS CHAPTER dealt with nine episodes recorded in the bedrock of Scotland. This chapter deals with three more recent episodes (Episodes 10–12; Fig. 21) which have modified the surface, removing bedrock and adding soft material to the surface blanket.
SURFACE-MODIFICATION EPISODES
Episode 10: Tertiary landscape erosion
Dating of the lavas extruded in Episode 9 suggests that Tertiary igneous activity in Scotland lasted for only about 5 million years and finished about 55 million years ago. This was followed by more than 50 million years of Tertiary and Quaternary landscape erosion (Fig. 21), during which time the main valleys of present-day Scotland increasingly approached their present shape and size.
Sedimentary bedrock of Tertiary age (Palaeogene and Neogene) is very largely absent on land in Scotland, even where volcanic and other igneous bedrock is present. This suggests that the crust below the present land area of Scotland was moving upwards and was subjected to net erosion during most of the Tertiary. Part of the evidence for this is the large thickness of Tertiary sandstones and mudstones that are found offshore to the east, north and west of Scotland, as shown by extensive oil exploration.
The valleys and mountains of Scotland, along with the lochs, sea lochs and offshore rock basins, have all been shaped by this erosion, principally by Tertiary rivers but also by more recent glacial ice (Episode 11). The present-day drainage pattern in Scotland (see Chapter 2) represents the latest phase in the evolution of this erosional system, and provides clues to the way it may have developed over the past 55 million years.
Episode 11: the Ice Age
During the nineteenth century, it became generally accepted that much of Britain had been subjected to glaciation by ice sheets and valley glaciers. Since then, this distinctive episode in the history of the British landscape has been referred to as the Ice Age, broadly equivalent to the Quaternary period of the internationally accepted series of time divisions (Fig. 21).
Over the last few years of geological research, one of the most far-reaching developments has been the establishment of the detailed record of fluctuating climate changes that have occurred during the Ice Age. A key step in this advance was the realisation that various indicators (often called proxies) of climate change can be measured at very high time resolution in successions of sediment or ice. The first of these successions to be tackled covered only the last few thousand years, but further work has now provided estimates of global temperature extending back several million years.
One of the best climate indicators has turned out to be variations in the ratios of oxygen isotopes (oxygen-16 versus oxygen-18), as recorded by microfossils that have been deposited over time on deep ocean floors. When alive, these organisms floated in the surface waters, where their skeletons incorporated the chemistry of the ocean water – including the relative amounts of oxygen-16 and oxygen-18. During cold climatic periods (glacials) water evaporating from the oceans may fall as snow on land and may be incorporated within ice sheets. Because oxygen-16 is lighter than oxygen-18 it evaporates more easily, so during cold periods the newly formed ice sheets tend to be rich in oxygen-16, relative to the oceans. The ratio of oxygen isotopes in the world’s oceans, as recorded by microfossils, can therefore be used to distinguish glacial and interglacial periods. Other useful indicators of ancient climate have come from measuring the chemical properties of ice cores, which preserve a record of the atmospheric oxygen composition, to complement the oceanic data from sediment cores.
Ratios of the isotopes of oxygen have turned out to provide one of the most important indicators of climate change, because they depend principally on ocean temperature and the amount of water locked up in the world’s ice sheets. There are, however, numerous other factors that can affect the ratios in ice and sediment cores, so interpretation of the data is rarely straightforward.
Figure 32 shows corrected oxygen isotope ratios as an indicator of temperature over the last 3.3 million years. The numbers on the vertical axis are expressed as δ18O values (pronounced ‘delta 18 O’), which compare the oxygen-18/oxygen-16 ratios in a given sample to those in an internationally accepted standard. The greater the proportion of heavy oxygen-18 in a sample the larger the δ18O value and, as described above, the lower the corresponding ocean temperature. For this reason, the vertical axis on Figure 32 is plotted with the numbers decreasing upwards, so that warmer temperatures are at the top of the figure and cooler ones at the bottom. The pattern shown in Figure 32 is of an overall cooling trend with, in detail, a remarkable series of over 100 warm and cool periods or oscillations. These alternations have been numbered, for ease of communication by the scientific community, with even numbers for the cold periods and odd numbers for the warm periods.
FIG 32.Oxygen isotope ratios track the more than 100 climate fluctuations over the last 3.3 million years. Warm episodes (red lines above the curve) alternate with cold episodes (blue lines below the curve). These have been used as the basis for numbering the global oxygen isotope stages, as shown.
Our next step involves looking in greater detail over roughly the last 400,000 years (Fig. 33). Over this period, there has been a distinctive pattern of increasingly highly