Much of Africa has outcrops of granite visible. The geology here
in Abuja is reminiscent of a number of similar sites across Africa
where the basement complex rocks are exposed. The basement complex
are large igneous structures that were intruded into the
over-laying rocks. Often referred to as “domes” evidence is seen in
such places in Southern Africa near Cape Town (Paarl Dome) – in
Johannesburg (Lonely Castle Koppie – Earthcache GC1C30Y) and in
Zimbabwe (Matopos – Cecil John Rhodes’ grave).
Interesting is the vegetation clinging to the surface of some of
the cliffs and the dark water marks from the precipitation run
off.
Basement complex rocks are subdivided into migmatite-gneiss
complexes; the older metasediments; the younger metasediments; the
older granites; and the younger granite alkaline ring complex- es
and volcanic rocks. The migmatite gneiss complex is the commonest
rock type in the Nigerian Basement complex. It comprises two main
types of gneisses: the biotite gneiss and the banded gneiss. Very
widespread, the biotitic gneisses are normally fine-grained with
strong foliation caused by the parallel arrangement of alternating
dark and light minerals.
The banded gneisses show alternating light-coloured and dark
bands and exhibit intricate folding of their bands. The migmatite
gneiss complex is the oldest basement rock, and is believed to be
of sedimentary origin but was later profoundly altered into
metamorphic and granite conditions. The older metasediments were
also among the earliest rocks to form on the Nigerian Basement
Complex. Initially of sedimentary origin, with a more extensive
distribution, the older metasediments underwent prolonged, repeated
metamorphism; and now occur as quaitzites (ancient sandstones),
marble (ancient limestones), and other calcareous and relics of
highly altered clayey sediments and igneous rocks.
Most parts of the Basement complex are underlain by belts of
roughly north-south trending, slightly metamorphosed ancient
Pre-Cambrian sedimentary and volcanic rocks known as the younger
metasediments. The major rock types are ancient shaly rocks which
are now referred to as quartz- biotite-muscovite schist. These
change laterally into coarse-grained feldspar-bearing micaceous
schists. Schists with graphite, phyllites and chlorite are common.
Ferruginous quartzites and tale schists also occur. The younger
metasediments contain most of the gold deposits in Nigeria in the
northwest around Maru and Anka, and at Zuru; near Kaduna, and also
at llesha in southwestern Nigeria. Older granites are widespread
throughout the Basement Complex and occur as large circular masses
within the schists and the older migmite-gneiss complexes. The
older granites vary extensively in composition. The younger granite
complexes in Nigeria are found mainly on the Jos Plateau, forming a
distinctive group of intrusive and volcanic rocks that are bounded
by ring dykes or ring faults. Other occurrences approximate a
north-south belt towards the middle Benue in the south where the
ages are younger, and towards Niger Republic in the north where the
younger granites are older. There is enormous variety in the
granite composition of these rocks.
Note: There are plenty of places to see exposed rocks in Abuja,
but please be careful of petty criminals (like most of Africa),
road safety and finally do not take photos of government buildings
or restricted areas, as the police may sometimes confiscate
cameras. In order to substantiate your visit and be able to claim
this Earthcache, a number of questions need to be answered and
submitted to the cache developer, via e-mail.
You should (if possible) also post a photograph on the site with
your log of you or your team and GPSr standing near GZ with one of
the granite domes visible (photos from alternative locations in the
area are also fine).
1. Describe the area in and around Abuja, especially the
prolific granite outcrops seen everywhere. Apart from granitic
rocks – did you see any other geological features to note?
2. Share in your log any local belief/story/folk lore associated
with one (or many) of the mountains and granite outcrops. We came
across many of these stories when talking to taxi drivers and
locals – especially of Mount Zuma or Aso Rock.
3. What is the predominant shape of the granitic rocks in the
area? Why do you think they weathered into this shape?
4. Give a short indication of why you were in Nigeria?
NOTE: You may log your visit prior to approval, but e-mail
submissions that do not meet the above criteria will be
deleted.