Visit with UCAD Geologist Malick Ngom

(Photos top to bottom)  Randi on the southern tip of the Cap Vert Peninsula, with a dramatic exposure of the quaternary volcanics, including basalt columns (a la Devil's Postpiles in CA, or Devil's Tower in WY);  on the west side of Cap Manuel looking toward Ile de Madeleine;  an exposure on the east side of Cap Manuel, showing bedrock limestone with volcanic intrusions.

I'd been looking for a UCAD (Universite Cheikh Anta Diop) geology connection even when still in the States, knowing that I'd need mentorship in learning the geology of the Dakar area.  I found that contact in Professor Papa Malick Ngom, a very warm and engaging geologist with whom Randi and I met on Saturday morning.  Fortunately, his good English more than compensated for our bon jour French.  

This we learned:  The Cap Vert Peninsula was formed as a result of two series of volcanic events, the first during the Tertiary, and the latter during the Quaternary.  Prominent landforms -- the Mamelles, Cap Manuel, the islands of Madeleine and Goree, and the extensive exposures of columnar basalt -- all reflect this turbulent history.

The breakup of Pangaea, and the spreading of the continents, resulted in a series of north-south running faults extending into Senegal, like stretch marks.  In the Tertiary Period, magma squeezed its way through weaknesses in these faults, resulting in simultaneous volcanic events on the Cap Vert Peninsula (Dakar), the Cape Verde Islands, and the Canary Islands.  Several major volcanic events took place during the Miocene (dated to 21 Ma, 15-10 Ma, and 8.5 - 5.3 Ma).  The basalt associated with the Tertiary volcanics in the vicinity of Cap Manual (a hill on the southern end of the Cap Vert Peninsula) is called ankaratrite, or nepheline basalt, a type of basalt characterized by the presence of a particular composition of minerals.  

Ile de Madeleine and Goree were also formed at this time.

The thick ankaratrite layer weathered at its surface forming an red-colored, iron-rich soil called laterite.  As Professor Ngom explained, the laterite is an important reference point with respect to dating:  everything beneath the laterite is Tertiary, and everything above is the result of Quaternary volcanics.

Randi and I walked to Cap Manuel from downtown Dakar, crossing two east-west running faults, which are now evidenced by dips in the road.  We walked the whole way around Cap Manuel, with expansive views of Madeleine and Goree from the high basalt bluffs.

On the east side of the Cap, we happened by several exposures of the bedrock limestone underlying the ankaratrite.  I suppose only the most nerdy of us would find this interesting.  Randi waited for me in the shade of a tree while I went exploring.

When you fly into Dakar, you'll note a lighthouse atop one of two hills toward the northern end of the peninsula.  They are the Mamelles, meaning breasts in French.  (I'll edit out the obvious puns here, so as not to offend the two people who will read this far into a blog entry about rocks.)  It is true that the Mamelles were once volcanos.  The stratigraphy of the area makes this clear.  The coastal bluffs of the Mamelles reveal a number of volcanic events, separated by layers of sediment, very much like the pages of a book.  The basalt here is of another mineral composition, called dolerite, which generally forms in shallow intrusions, pushing between layers of sedimentary rock.

The age of the Mamelles volcanism is around 1 mya.

A little trivia:  On  field trip last week, we walked down to the shore, not too far from school.  Fascinating area.  Complicated.  Lots that I didn't understand at the time.  In the Sierra Nevada of California, there is a kind of rock called a xenolith, or a small rock that becomes absorbed into a larger rock, as when the so-called country rock become integrated into a subsurface bubble of magma.  Driving across the High Sierras from Sacramento to Reno you see loads of examples of xenoliths, as grey patches in the granite.  We found abundant evidence of similar things last week, as if one rock was incorporated into something larger.  Dr. Ngom confirmed that this was the case, and offered several explanations.  By example, the magma ejected from its source may have carried with it some of the basement rock, which became absorbed/incorporated.

I'm sorry . . . I find this stuff interesting, and now you know better than to take me on a drive.  I read in an article by UCAD Archaeology Lab Director Ibrahima Thiaw;  he wrote that many Senegalese equate archaeologists looking for pottery shards and such as a kind of  insanity.  No doubt the spouses of archaeologists agree.  

A fellow sniggered yesterday as I walked by, my eyes to the ground, turning over rocks and pottery, ostrich-like.  I didn't take it personally.  Maybe a bit of Thorazine would help.