Download Scotland`s Time Lords

Scotland’s Time Lords
Prof Stuart K Monro, Scientific Director, Our Dynamic Earth and
Honorary Professor, University of Edinburgh
Prologue
This article is an opportunity to reflect on how Earth science was born in
the City of Edinburgh and how ideas about Earth processes have
changed up to the present day. There have also been some dramatic
changes in our attitudes to public engagement with science in the 21st
century. As a geologist, I frequently quote the phrase “geologists are good
company, especially for other geologists” which reflects the unfortunate
attitude of some specialists in a variety of professions to engage more
with each other than with the wider community. I contrast that with the
nature of Society during the Edinburgh Enlightenment when individuals
with widely different interests came together in discussion, often over a
jug of claret, thus fostering the development of the polymath. It was in this
climate that the modern understanding of geology, the science of the
Earth, was born, here in Edinburgh over 200 years ago. Here is where our
story, represented in the first gallery of Dynamic Earth, begins…
James Hutton, medic, chemist, farmer and the Father of Modern Geology
James Hutton was born in Edinburgh on the 3rd June
1726, the son of the City Treasurer. At the age of 17, he
was apprenticed to a lawyer; but this was not a successful
move. According to his biographer John Playfair,“…
happily the force of genius cannot always be controlled by
the plans of a narrow and short-sighted prudence. The
young man…was often found amusing himself and his
fellow apprentices with chemical experiments instead of
copying papers”. With an interest in chemistry, it was
medicine that he studied when he attended the University
of Edinburgh. In those days, chemistry was an integral
part of a medical education though few students actually
matriculated for a degree programme, preferring instead to attend individual lectures.
Three years later he continued his studies in Paris pursuing “…with great ardour the
studies of chemistry and anatomy” and took the degree of doctor of medicine in 1749
at Leyden. His thesis was on the circulation of blood. His departure to the continent
to complete his studies was a common enough occurrence then, however it may
have been precipitated by having had a son during his early twenties by a Miss
Edington!
In 1750 he returned to Edinburgh and resumed chemical experiments with his friend
James Davie. Their work on the production of sal ammoniac – ammonium chloride, a
salt used in dyeing and working with brass and tin – led to a profitable partnership.
Hutton also had a keen interest in agriculture and farmed at Slighhouses and Nether
Monynut in Berwickshire, farms he had inherited from his father. Hutton wished to
apply his scientific understanding to agricultural practice and travelled extensively to
gain new ideas, introducing the Suffolk plough to Scotland.
It was undoubtedly these travels that fostered his interest in the processes that
shape the natural world. He visited Arran and Jedburgh and discovered the
geological relationship, the uncomformity, in both places where horizontal or gently
dipping rocks rest on top of more steeply dipping rocks. More importantly, he
recognised the story that these exposures were telling. In Berwickshire he had seen
the steeply dipping rocks that make up the Southern Uplands and also the more
gently dipping rocks around Dunbar. He surmised that somewhere on the
Berwickshire coast, these two rock formations would come together. Thus, with two
friends, Sir James Hall and John Playfair, he sailed from Dunglass, southwards
along the coast to discover Siccar Point and its dramatic unconformity. Here Hutton
revealed to Hall and Playfair the true significance of what they were seeing.
Hutton recognized that the vertical rocks would have been initially laid down
horizontally then subsequently squeezed into a vertical position forming large-scale
folds and building mountains. Before the second, gently dipping rocks are laid down;
there would need to be a long period of erosion, removing the mountain. Finally the
second set of rocks would be laid down. The unconformity between the two sets
represents, therefore, a period of time when mountains were raised and then eroded
away. These observations should be seen in the context of the establish wisdom of
the time, that of Archbishop Ussher of Armagh who pronounced that the world was
formed on the evening prior to the 23rd of October 4004BC. Hutton demonstrated to
his friends at Siccar Point that the Earth had to be very much older and thus started
a quest for the age of the Earth which continued through to the 1940s when Arthur
Holmes produced a revised age akin to that accepted today.
Hutton’s other great controversial idea was that some rocks had been molten and
injected into sediments. He was a Plutonist. In this he was at odds with Robert
Jameson, the Neptunist, who believed that these rocks were formed by precipitation
from some primeval ocean. The evidence for Hutton’s view can be clearly seen at
what is now called “Hutton’s Section” at the foot of Salisbury Craigs. The relationship
between the Craigs rocks and the underlying sediments indicates that the sediments
have been forced up and broken off by forceful emplacement of what must have
been molten rock.
Charles Lyell, the great communicator
Hutton’s ideas were the subject of two lectures given to the
newly-formed Royal Society of Edinburgh on 7 March 1785
and 4 April 1785. However, he did not publish his Theory of
the Earth in book form until 1794. Unfortunately the book
was so badly written that few succeeded in reading it. In
1802, five years after Hutton’s death, John Playfair tried to
explain Hutton’s theories in his Illustrations of the Huttonian
Theory of the Earth. However Hutton’s concepts were not
widely recognised until Sir Charles Lyell included them in
his Principles of Geology in 1833. Charles Lyell was born in Kinnordy, Forfarshire
(now Angus) and was the great communicator of Hutton’s geological ideas and
concepts. His Principles of Geology brought Hutton’s ideas forward. Importantly,
Charles Darwin had a copy of Lyell’s book with him on his voyage on the Beagle. It
was a book which greatly influenced Darwin’s thinking, giving him a framework of
deep time in which evolution by natural selection could work. Lyell went on to be one
of the foremost geologists of his time and held the post of Professor of Geology at
King's College London in the 1830s.
He was a great advocate of uniformitarianism, a concept that related back to
Hutton’s thinking that the earth was shaped entirely by slow-moving forces still in
operation today, acting over a very long period of time. The other side of the coin
was that of catastrophism, where the Earth has been affected in the past by sudden,
short-lived, violent events, possibly worldwide in scope. Now of course, it is
recognized that both processes are operative; slow-moving change in many areas
but violent and catastrophic events like volcanic eruptions, earthquakes and tsunami
in others. The scientific understanding of our Earth continues to evolve.
The “Readers of the Rocks”, Ben Peach and John Horne
Fundamental to Hutton’s ideas was the
concept that rocks would be put into a
vertical disposition by major compressive
forces, building mountains. Though Hutton
surmised this process, he was unaware of
the mechanism by which it could be
achieved. During the 19th century
prominent
geologists
conducted
a
prolonged and bitter debate, the Highland
Controversy,
about
the
geological
relationships exposed in the north-west
Highlands of Scotland. The protagonists
were, on the one hand Roderick Murchison and Archibald Geikie and on the other,
James Nicol and Charles Lapworth. This was finally resolved in 1907 by the work of
mapping geologists from the Geological Survey, Ben Peach and John Horne. The
main issue was that the Moine schists at the top of the crag appeared to be older
than the Cambrian and Ordovician rocks lower down. Murchison and Geikie believed
the sequence was wrong and that the Moine schists must be the younger rocks. The
conundrum was explained by Peach and Horne to be due to the action of a lowangle thrust fault bringing older rocks from the east over the top of the younger
rocks. This was evidence of the major compressive forces envisaged by Hutton.
Subsequently investigations in areas where there are much younger mountain
chains such as the Alps and the Himalayas have confirmed that major compressive
forces can build mountains by generating large-scale overfolds called nappes and
the stacked, thrust slices which Peach and Horne recognized in the north-west
Highlands of Scotland. However, what was yet unknown, was the mechanism by
which these major compressive forces could be generated. That was all to change in
this evolving story.
The Second Time Lord, Arthur Holmes
Working in Durham and then in Edinburgh, Arthur Holmes
was able to use the emerging science of radioactive decay to
date rocks and establish a numerical timescale for the events
that shaped the Earth. His interest in geochronology dates
back to his undergraduate experience where he dated a
Devonian rock from Norway as 370my. For the first time,
rocks and the events associated with their formation could be
accurately dated and a chronological story of the evolution of
the Earth could be started to be written. Holmes was
undoubtedly the other historical character who could stand
alongside Hutton as a “Time Lord”.
Global impact of these innovators
There is an international dimension to this story which starts with a German polar
explorer and meteorologist, Alfred Wegener who, in 1912, considered that the
geometrical fit between South America and Africa suggested that they had been at
one time joined and through time had drifted apart; the theory of continental drift.
There was strong scientific evidence for this with identical, terrestrial fossils present
on both continents however the scientific world would have none of it there being no
mechanisms by which continents could move. Holmes championed Wegener’s
theory at a time when it was deeply unfashionable with his more conservative peers.
Holmes proposed that Earth's mantle contained convection cells that dissipated heat
produced by radioactive decay and moved the crust at the surface.
The next real advances were stimulated by the Cold War; the Americans wanted to
know where Russian submarines were and vice versa. As a result research was
funded into the nature of the ocean floor topography. Marie Tharp was an American
geologist and oceanographic cartographer working at the Lamont-Doherty Earth
Observatory of Columbia University, who, along with her colleague Bruce Heezen,
mapped the ocean floor including the Mid-Ocean Ridges, a line of undersea
mountains. Their first physiographic map of the North Atlantic was published in 1957
and was part of the evidence that led Harry Hess from Princeton University in the
1960s to the concept of seafloor spreading from a mid-ocean ridge, opening up
oceans and moving continents. Further evidence to support this concept came from
work on determining the age of the ocean floor and the geomagnetic signature
contained within the magnetic minerals. Sampling of the basaltic rocks of the ocean
floor revealed a symmetrical pattern either side of the mid-ocean ridge. The ages of
the rocks were shown to be progressively older away from the ridge. This pattern
was reinforced by evidence from the magnetic minerals which recorded the periodic
reversals that have taken place in the Earth’s magnetic field through time. This
symmetrical pattern revealed by Fred Vine and Drum Matthews working in
Cambridge University and Lawrence Morley of the Canadian Geological Survey
demonstrated conclusively that magma was rising up along the Mid-Ocean Ridge
and moving away from the ridge giving the mechanism that Wegener was looking for
to move continents.
The corollary of this process is that if the Earth were not to be seen as continually
expanding then there must be zones where crust is destroyed, compensating for the
generation of new crust in the Mid-Ocean Ridges. A processes known as subduction
emerged from studies of location and depth of the epicenters of earthquake activity
together with the information from the ocean floor topography. Oceanic trenches
were identified with the epicenter of earthquakes becoming progressively deeper
away from the trench. From this emerged the concept of a dense oceanic plate
moving below a lighter continental plate. As the oceanic plates moves into the hotter
areas of the crust and mantle it begins to melt, helped by the addition of water which
reduces the melting temperature. In this way the oceanic crust is destroyed
producing magma which rises to the surface, producing volcanoes which increase
the volume of continental crust.
Hutton’s ideas revisited
This story moves us a long way from the original ideas of Hutton, but taking the time
to reflect might well help make meaningful connections between our modern
understanding of how the Earth works and the concepts that emerged from Hutton’s
work. These ideas fall into three categories; deep time, the Earth’s heat engine and
the concept of cycles.
Hutton’s idea of Deep Time is fundamental to much of modern thinking on Earth
science processes. These generally work slowly through time though the occasional
catastrophic event is recorded in the geological time scale. Without this concept our
ideas of the nature of the Universe and of evolutionary processes would not have
emerged in the way they have.
The concept of Cycles that Hutton recognized in his agricultural activities also fed
through to his geological understanding. Primarily, he recognized a continuing
process of erosion leading on to deposition of sediments which eventually become
lithified to form rock. Looking at modern processes of Plate Tectonics, that too
operates in cycles with oceans opening, oceans closing building mountains which
are then eroded and deposited in the oceans forming new rock.
The Heat Engine that Hutton conceived of is the driving force for plate movement
with convection in the mantle, though it is now widely recognized that the “pull” of a
subducting cold plate is probably more important than the “push” from upwelling
along the Mid-Ocean Ridge.
Epilogue
The following letter, purported to come from James Hutton and bearing
marks of scorching was, according to “A Geological Miscellany”, received
recently by the editor of Séance de la terre:
“… We all look forward to meeting some of your young colleagues who
have made such spectacular advances in plate tectonics, and I am
especially glad to hear that they are using my heat to drive their plates…
The science has moved on since Hutton’s day but his legacy remains as
the foundation for our modern understanding of how the Earth works.