3. Mull’s Central Igneous Complex:

The plateau lavas of Mull and all the Inner Hebrides, were later intruded by plutonic complexes - intrusions arising from the magma chamber.

Besides Mull’s Central Igneous Complex there is a similar one on Ardnamurchan and other well developed centres on Skye and Rum. Additional centres without basalt lavas exist on St Kilda, Arran and the submarine Blackstones Bank. These complexes of intruded igneous rocks are called CENTRAL INTRUSION COMPLEXES because many of the individual intrusions are arcuate in outcrop pattern and arranged about specific sites, or centres. Each complex consists of many separate intrusions, several hundred in Mull and Ardnamurchan, though less in Rum and central Skye. Younger intrusions invade and in some instances partly destroy older ones, resulting in very complex patterns and creating difficulty in interpreting the geological history of each plutonic centre. Mull’s intrusive igneous phase which followed its long extrusive history showed both a change in chemistry of the magma chamber and a shift in its position during its history. This rising cylinder of basic material which underlay Mull gradually moved NNW with time, giving rise to three centres of intrusive activity. Centre 1. was centred on Glen More, Centre 2. on Beinn Chaisgidle and Centre 3. Loch Ba. The first intrusions were both inside and outside the caldera that had formed as a result of the lava collapse. Some of these lavas were pushed aside and folded due to the rise of the basic cylinder underlying Mull. This folding can be seen from Kinloch on Ben More’s lower slopes, and in the anticlines and synclines of Loch Don. Later intrusions include cone-sheets, which can be observed dipping towards their respective centres, later ones cutting earlier ones.

Dolerite Cone Sheet in Layered Gabbro, Craig Quarry, Glen More.


Sill in Columnar Basalts, Ardtun

Sills intruded into the lavas are both concordant and discordant. Ring dykes, formed around several ring fractures above the centres, have steep outward dipping contacts, the best preserved being above centre three, the Loch Ba ring dyke. Viewed on the solid geology map they can be seen to have a circular (annular) outcrop pattern around a centre. The earlier outcrops are harder to discern because they have been cross-cut by later annular outcrops.  Dykes, both simple, basic or acid, or composite, nearly all have a NW-SE trend. Many of the phases of activity are marked by volcanic vents which explosively brought up crustal material from deep below. Some of Mull’s vents contain fragments of Moine schists from the lower basement rocks thousands of feet below. Much of the reason for the extreme complexity of Mull’s Central Igneous Complex is that the magmatic centre under Mull shifted over time from Glen More, to Beinn Chaisgidle, and then finally to Loch Ba. Mull’s igneous activity is thought to have spanned five million years.

Types of igneous intrusion:

1. Dykes -  a) Simple, b) Multiple, c) Composite, 2. Sills a) Concordant, b) Discordant, 3. Cone Sheets, 4. Ring Dykes, 5. Irregular intrusions (large and small), 6. Volcanic vents.