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Tertiary Igneous Activity on Mull |
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Ardmeanach Peninsula. Lava Flows - Trapp Topography. |
60 MY ago, a lengthy period of volcanicity started in North West
Scotland associated with the opening of the North Atlantic.
During this time of crustal tension the Eurasian plate split
apart from the North American plate and new oceanic crust formed
the widening Atlantic Ocean. Lavas, flow upon flow, were
extruded on to Mull’s basement rocks to a total thickness of
6000 feet. As the lava extruded was basalt, low in silica and
therefore viscosity, the extrusions were not explosive. The lava
flows, made of ‘runny’ lava, often in vast volumes, spread over
extensive areas. They were fed by dykes along NW-SE trending
fissures. Later, igneous intrusions created Mull’s Central
Igneous Complex. Volcanicity continues today along the length of
the Mid-Atlantic Ridge, in Iceland and Jan Mayen. |
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Mull’s
volcanic activity can be divided into three stages:-
1. The Lavas:
There are two groups:-Plateau lavas (3000 feet), were
the first to be erupted and today form Mull’s
characteristic ‘trap’ scenery best seen on Ardmeanach,
Ulva, and northern Mull. The flows cover an area of 325
square miles and are of two different types, simple, or
compound. The Plateau group are mainly tholeiites. The
Central lavas (3000 feet), mainly alkali basalts, are
less extensive. They formed after the Plateau group and
have largely been removed by denudation, but some are
preserved within Mull’s caldera, including pillow lavas
which show that they were extruded into a crater lake.
The caldera was created when a ring
fracture formed over the by then depleted magma chamber.
The weight of the lavas above the part empty magma
chamber resulted in the collapse of the overlying lavas.
The collapse caused the once horizontal lava flows to
dip gently towards the centre.
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Lava Tube, Ardtun
Columnar Basalts, Ardtun |
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This can be seen in the flows on the south
side of Loch na Keal. Pyroclastic layers (volcanic bombs
showing explosive activity) within the lava pile are rare and it
is thought that the lava was extruded quietly on to the land
surface. Individual flows are less than 50 feet thick and some
may be traced over wide areas, as much as 13 square miles.
Columnar jointing, a cooling phenomenon, is characteristic of
flows in S.W. Mull. Here the basalt has cooled and contracted at
right angles to the grounds cooling surface into mainly
pentagonal and hexagonal sided columns with horizontal cross
joints. Red bole may often be seen between flows. This
represents the weathered tops of lava flows and shows that there
were lengthy times when lava was not poured out. Although most
flows erupted over land, where there are pillow lavas (lave
extruded under water) it suggests there was a caldera lake.
Sediments between the lavas may be found, particularly near the
base of the lava pile, eg. the Ardtun leaf beds of S.W. Mull
where sediments accumulated in lakes and braided streams. Mull’s
lava pile was built up from fissure eruptions fed by dykes. Some
of the dykes are exposed today, though none has as yet been
discovered feeding a lava flow.
Mull’s lavas
vary from black to light grey in colour and show a variety of
textures and structures. Textures:- Vesicles, Amygdales, Vugs,
Geodes, Zeolites. Structures:- Columns, Giant Columns,
Interbedded sediments, Red Bole, Ropey tops, Slaggy bottoms. |
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Ben More 3169 feet and A' Chioch |
Mull’s 6000 foot lava sequence consists of:-
Central
Group 3000 feet. 6. Interior zone. Non-porphyritic tholeiitic
basalts. 5. Middle zone. Feldspar phyric basalts. 4. Outer zone.
Sparsely feldspar phyric and aphyric basalt. Plateau Group 3000
feet. 3. Pale suite of Ben More. Alkali-olivine, mugearite,
feldspar phyric basalts. 2. Main suite. Alkali-olivine, feldspar
phyric and picrite basalts. 1. Staffa suite. (S.W.Mull).
Pyroxene rich alkali-olivine basalt. Basal red mudstone 3-10
feet thick, - a lateritic, weathered mudstone. |
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2. Dyke swarms:
Dykes
can be seen cutting lava flows of older ages. Dyke activity
continued late into the volcanic activity of Mull as dykes can
be seen cutting some late stage structures such as the Loch Ba
ring dyke. The structural trend of the dykes is NW-SE and is
associated with the tensional spreading of the diverging
European and North American plates. Crustal dilation due to dyke
intrusion may be as much as 9% at Croggan in SE Mull. |
Loch Ba Ring Dyke
Eroded Tertiary Dyke in
Torridonian and Lewisian rocks, Iona |
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Last modified
Friday December 07, 2007
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