Atlantis Cruise 3-31

Southern EPR 17°-19°S:
The STOWA Expedition


Ken Macdonald's Relative Age Scale for lavas on the EPR axis
Note: this working definition of lava ages was employed by observers in Alvin and is used in many of the dive transcripts and maps.

AGE 0

Very glassy and black
NO sediment
Populated only by mobile fauna and white bacterial mats/snow
NO sessile macrofauna
Either NO mineral deposits at vents, or else incipient chimneys (<1 m
high, simple "organ pipes" unpopulated by organisms, white =
anhydrite or bacterial mat)

AGE 1

Very glassy
Black-to-dark grey
On pillow lavas; lots of glassy ornaments and buds.
Only a dusting of sediment (no pockets), or a thin veneer coating flat sheet flows
Sessile macrofauna and mineral chimneys present at hydrothermal vents;
Chimneys are more mature (at least a few meters in height, have complex morphology, often
populated by attached organisms, white = Alvinelline tubes)

AGE 2

Some glass
Medium grey
Sediment accumulating in pockets and cracks
Mineral deposits are large and complex in morphology
Chimneys/edifices often have well-developed flanges
Few if any sessile organisms found on the lower parts of the chimneys
Extinct vents are common

AGE 3

No glass
Heavy sediment (pockets between pillows interconnected, sheets and lobates
blanketed)
Active hydrothemal vents very rare
Inactive deposits very weathered (lots of red, white and yellow material
on and around the deposits)

AGE 4

Almost complete sediment cover. Very few outcrops except for scarps and volcanic mounds.


[ATL331 home page | Atlantis Home Page]

This page last modified on 4 Mar. 1999