CHALK
Chalk
is very distinctive variety of limestone that is soft, whitish, and
powdery. Chalk is composed of calcite (CaCO3), and will bubble
in acid. The most spectacular chalk locality on Earth is the White Cliffs
of Dover, along the southern shores of Britain. The rocks there are
Cretaceous in age (“creta” means “chalk”).
Chalk is a biogenic sedimentary rock, but it is not at
all obvious how this white powdery material represents the remains of
once-living organisms. When examined under a scanning electron
microscope, chalk powder is seen to be composed of immense numbers of exceedingly
small microfossils, principally coccoliths.
Coccoliths are calcitic plates that once covered a living cell (see example photo).
The cell was an entire organism called a coccolithophorid (Kingdom Protista,
Phylum Chrysophyta, Class Coccolithophorida). Coccolithophorids are
unicellular, photosynthetic organisms.
They are often called "algae", but they’re better called
photosynthetic protists. When they die, the cell degrades, and the
numerous hard calcitic plates covering the cell fall to the seafloor.
Chalk generally forms in moderately deep marine
environments (but not in the deepest ocean depths), where high numbers of
coccolith plates can accumulate as sediments, without calcite dissolution, and
undiluted by muddy or sandy sediments washed in from the continents.
Chalk
from the "Upper Chalk" Formation (Cenomanian Stage, lower Upper
Creatceous) at Dover Cliffs (White Cliffs of Dover), southeastern coast of
Kent, southeastern-most England.