An Introduction to Environmental Chemistry

source warms the water, causing it to expand and become less dense, forcing it
upward again through the crust in a huge convection cell (Fig. 6.15). We can view
these convection cells as having a recharge zone and low-temperature ‘limb’ of
subsiding seawater, a hot reaction zone closest to the magma chamber, and a high-
temperature rising ‘limb’ of chemically modified seawater discharging at the
seabed (Fig. 6.15). The overall process is called ‘hydrothermal’ (hot water) con-
vection. It is currently estimated that about 3¥ 1013 kg yr-^1 of seawater is cycled
through the mid-ocean ridges of the Earth’s crust. It therefore takes about 3.3¥
107 years to cycle the entire volume of seawater through the axial part of the mid-
ocean ridges (Table 6.5).
It is not possible to measure directly the maximum temperature to which water
becomes heated in the basaltic crust. However, hot springs of hydrothermal water
discharge from the seabed at the apex of the convection cell. Temperature mea-
surements taken from ridge axis hot springs range from 200 to 400°C (average
around 350°C). This implies that temperatures in the hot reaction zone above
the magma chamber (Fig. 6.15) are typically not less than 350–400°C. Owing to
chemical reactions between this convecting hot water and the basaltic crust
(shaded region on Fig. 6.15), these waters are acidified (typical pH 5–7) and rich
in dissolved transition metals leached from the crust. Iron (Fe), manganese (Mn),
lead (Pb), zinc (Zn), copper (Cu) and hydrogen sulphides rapidly precipitate a

The Oceans 209

Active

axial

~ 400°C

Heat source

Passive

off-axis

< 200°C

Sea water

Ocean crust

Magma

chamber ++

+++

Fig. 6.14Convection systems at mid-ocean ridges. Active circulation at the ridge axis is
driven by magmatic heat. Off-axis circulation is driven by passive cooling of the crust and
lithosphere. Modifed from Lister (1982).