Isle of Rum layered intrusion

Layered igneous intrusions have a special place in the study of igneous rocks.

These frozen magma chambers hold the key to many of the processes that lead to the evolution and diversity of lavas. Many are sources of minerals vital to the modern world. The most famous have names as familiar to geologists as the great battles of history: Agincourt, Trafalgar, Waterloo…Skaergaard, Bushveld, Stillwater, Rum…! It is not likely that, in the foreseeable future, we shall be able to look inside one of these mighty, white-hot, pressurized chambers to observe processes directly. We have to make do with the frozen remains, and use our powers of deduction, along with some clever chemistry and physics, to explain what we see.

Rum’s layers

The Rum chamber formed in the Palaeocene, about 61 million years ago, during a major volcanic episode that accompanied the beginning of the opening of the North Atlantic. Layered igneous rocks of the same age occur in centres on Mull and Skye, accompanied by a large thickness of lavas, and also in the Ardnamurchan ring complex. What sets Rum apart, and unquestionably gives it top international status, is the variety of styles and scales of layering. The layered rocks of the Rum Central Complex are divided into three overlapping intrusions, each with its own inventory of igneous structures. The dramatic mountains of Askival and Hallival are part of the Eastern Layered Intrusion, formed mainly of inter-bedded peridotite and troctolite. The two mountains have distinctive steps in their skylines, visible from the Scottish mainland 16 miles to the East, which mark 16 major units averaging 50 m in thickness. These major layers are thought to represent fresh influxes of magma into the chamber. Within the units there are rhythmic and graded layers, on scales of 10s of cms, produced by settling of crystals onto the floor of the magma chamber, sorted by crystal density. Some structures are ascribed to replacement after the crystals had accumulated. The more feldspar-rich layers were given the name ‘allivalite’.

Aladdin’s cave of petrological wonders

The Western Layered Intrusion is relatively small, but is strongly layered and notable for an extraordinary rock texture called ‘harrisite’, named after the locality of Harris in the west of Rum. Harrisite is composed of thin, branching crystals of olivine up to 60 cm long, forming layers up to 1 m in thickness. The fragile olivine grew in place, rather as do crystals in a ‘chemical garden’, indicating extremely quiet conditions in the magma. In contrast the Central Layered Intrusion, which cuts across the Eastern and Western intrusions, is characterised by the development of breccias composed of huge blocks of layered troctolite and peridotite in a feldspathic peridotite matrix. There are avalanches of peridotite ‘pebbles’ and rafts of troctolite tens of metres in extent. The Central Layered Intrusion formed in turbulent times! The Rum intrusion is an Aladdin’s cave of petrological wonders, a unique insight into an unattainable world beneath a great volcano, a Geosite of many mysteries.