NB0332 : Gneiss at Uig Sands, Lewis

taken 21 years ago, near to Eadar Dha Fhadhail, Na h-Eileanan an Iar, Great Britain

Gneiss at Uig Sands, Lewis
Gneiss at Uig Sands, Lewis
The grey striped rock is Lewisian gneiss, and the pink is late
Laxfordian pegmatitic granite. The gneiss is very old (c 3,000
million years), metamorphic (changed by heat and pressure deep
in the Earth) and polycyclic (having been buried and resurfaced
several times. It underlay a range of Himalayan-height mountains
that once lay where the Outer Hebrides now are, but were then
eroded to sea level. The granite was intruded into the gneiss
about 1,700 million years ago. Many beaches in Lewis are covered
with smooth rounded boulders of Lewissian gneiss, showing pink
and black stripes most attractively.
The gneiss has been dated by various methods, partly by analogy
with similar rocks in Greenland, to 3,000 Ma plus or minus a
couple of hundred million years. It was formed by compression
during a period of continental collision.
This was followed around 2900 - 2700 Ma by the main Scourian
event, an extensional regime which resulted in the intrusion of
the Older Basics, which are more mafic (richer in iron and
magnesium) and less silica-rich than the gneiss, and then in the
late Scourian, around 2500 to 2400 Ma, by the intrusion of the
Younger Basics, which are also more mafic. The Older Basics can
be shown to cut the fabric of the gneiss, therefore they are
later, and dating the OBs by geophysical methods has provided a
minimum age for the gneiss.
Around 1900 to 1700 Ma was the main Laxfordian event, another
episode of compression and deformation. In the late Laxfordian,
1700 - 1600 Ma, more extension allowed the intrusion of the
granites, mainly in the Uig Hills and north Harris.
Uplift commenced after the Laxfordian, around 1400 Ma.
[Thanks to Anne Burgess for this description]
Creative Commons Licence [Some Rights Reserved]   © Copyright John Palmer and licensed for reuse under this Creative Commons Licence.
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NB0332, 26 images   (more nearby )
Photographer
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Date Taken
Wednesday, 22 May, 1996   (more nearby)
Submitted
Monday, 16 March, 2009
Category
Rock strata   (more nearby)
Subject Location
OSGB36: geotagged! NB 036 324 [100m precision]
WGS84: 58:10.8760N 7:2.6219W
Camera Location
OSGB36: geotagged! NB 036 326
View Direction
SOUTH (about 180 degrees)
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