Setting the Scene
The history of Rum began nearly 3000 million years ago, when some of the oldest rocks in the world were formed - the Lewisian gneisses. These are metamorphic rocks, which were formed when even older granitic rocks were buried and heated deep beneath the Earth's surface. They can be seen in a few scattered outcrops on Rum.
By about 11000 million years ago, the Lewisian gneisses had been uplifted by immense movements within the Earth. Erosion on the surface had worn away these rocks to a bare, hummocky land surface looking rather like that of much of the Outer Hebrides today. The area which today makes up Scotland then lay much closer to the equator and had a warm, arid climate. Torrential rivers flowing across this landscape deposited sand and pebbles that accumulated into a pile of sediment many kilometres in thickness. As the sand and pebbles were compressed under the weight of the overlying sediment they formed a sequence of rocks, known as the Torridonian.
At that time, the land that now makes up the Isle of Rum was part of a huge supercontinent. After deposition of the Torridonian sandstones, this area remained fairly stable for many millions of years, during which time the rocks that make up much of the Highlands today were laid down near the margin of the supercontinent. Scotland was separated from the rest of modern Europe by an ocean that gradually narrowed through the process of continental drift - until England and Scandinavia collided with Scotland around 430 million years ago.
This continental collision led to the formulation of a mountain range that may have been as high as the modern-day Alps. The eroded remnants of this mountain chain form much of the Highlands today.
The next chapter in Rum's story began some 250 million years ago, in the Triassic Period, the mountains had been largely eroded away, and the western side of Scotland was a low-lying, arid area with a tropical climate. Sand and pebbles were eroded from higher land to the east and carried westwards by rivers, to be deposited on lower ground where they eventually formed sedimentary rocks. These rocks look similar to the Torridonian sandstones, but the presence of plant fossils indicates that they are much younger.
For over 100 million years, during the Jurassic and Cretaceous periods, the Hebridean area remained close to sea level, with periodic incursions of the sea. During this time sediments, which later formed rocks such as limestones, sandstones and mudstones, were deposited. These rocks can be seen on the neighbouring island of Eigg, but on Rum they were removed by erosion that occurred when the area was raised up above sea level at the beginning of the Palaeogene Period some 65 million years ago.
The beginning of the Palaeogene Period was a key time in the geological history of Rum. Forces operating deep within the Earth began to pull apart the huge continent that included Scotland. As the continental mass was stretched, the Earth's crust began to crack. Magma, formed by the melting of rock many kilometres below the Earth's surface, welled up through these cracks in the crust and erupted from volcanoes. This magma spread out over much of the area in lava flows similar to those seen on Iceland today. Meanwhile, beneath the volcanoes, huge bodies of magma accumulated in magma chambers deep within the Earth's crust. This magma slowly cooled and crystallised to form igneous rocks such as granite and gabbro. Much of the Isle of Rum represents the eroded remains of one of these volcanoes that formed 60 million years ago. Modern techniques used to date rocks accurately have showed us that the active lifetime of the Rum volcano was very short in geological terms - probably only a few hundred thousand years. The volcanic rocks of Rum have been intensely studied by geologists over the last 100 years, allowing us to understand in detail how the Rum volcano evolved. The volcanic features are described in more detail in the following pages.
By 55 million years ago, the volcanoes of northwest Scotland had ceased to erupt, and volcanic activity had shifted to the west as the North Atlantic began to open. The youngest volcanic rock still preserved in Scotland is the lava flow that now forms the distinctive Sgùrr of Eigg.
During the next 50 million years, the area around Rum underwent periodic erosion in a mostly warm, sub-tropical climate. This continued until about 2.4 million years ago, when the climate cooled dramatically and glaciers formed in Scotland. The landscape of Rum owes its appearance largely to the action of these glaciers; the corries, sharp peaks and deep U-shaped valleys on the island are all products of glacial erosion. During the last 2 million years glaciers have repeatedly scoured away soils and younger sediments, leaving the underlying rocks spectacularly exposed. Since about 750,000 years ago, intensely cold glacial episodes have been interspersed with brief warm interglacial periods. At the peak of one of these glacial episodes the whole island would have been almost completely covered by an ice sheet flowing from the mainland westwards. The most recent of these major glaciations was at its peak on 20,000 years ago. Around 11,500 years ago the climate warmed rapidly to give the temperate, maritime, conditions experienced today. Following the melting of the ice, much of the west of Scotland rose up as it adjusted to the removal of the weight of the glaciers, and hence the relative sea level fell rapidly. The sea level around Rum has continued to fall gradually over the last 6000 years or so as the island steadily rises.