288 Resources from the Ocean Floor
Redondo Beach, California; beaches in western Oregon yield chromite and other
heavy minerals as well as gold and platinum; titanium minerals are taken from beach
sands along the eastern Florida coast.
How do placers originate? The process of concentrating heavy particles on the
beach has much in common with panning for gold - water motion works on the
different settling velocities of the particles and on their different sizes, to separate
heavy from light, large from small.
Terrigenous beach sands commonly consist of more than 95 % quartz, tropical
ones of calcareous grains. Volcanic rocks and other igneous sources supply minerals
which are considerably heavier than quartz or calcite (density of 2.65 and 2.70).
These minerals have densities of greater than 2.85 g/cm 3 . The "heavy minerals" are
ubiquitous, usually making up a few percent of the sand. The back-and-forth movement of the waves washing over the beach face can concentrate them greatly (Figs.
10.8 and 10.9). Under the microscope, one recognizes minerals such as ilmenite
(iron-titaniumoxide), rutile (titaniumoxide), zircon (zirconiumsilicate), and monazite
(phosphate, containing cerium and thorium).
Thick placers of heavy particles form only in the beach zone. Offshore placers,
therefore, were formed when the sea level stood lower. The gold deposits off Nome,
Alaska, are a case in point. Glaciers of the last ice age brought gold-containing debris
from the hinterland and dropped it on the shelf. This morainal material was then
worked up by the surf, as the sea level rose.
River deposits may contain heavy mineral concentrations, too, such as cassiterite
(Sn02) in Thailand, Malaysia, or Indonesia. In these areas, former river beds on the
shelf, which are now in down to about 100 m water depth, are of potential economic
interest and have been exploited offshore in shallower water for nearly a century. At
present, the offshore revenue reaches some 14 % of that of worldwide tin exploitation
on land.
10.3.4 Sand and Gravel. Considerable amounts of sand are taken locally for building
roads and houses, as well as for coastal protection (dams) and for fill. However, the
main use of beach sand is for building sand castles and for plain enjoyment. Recreation is one of the most important uses of beaches - by far exceeding mineral
extraction in business value. Beaches are commonly eroded by winter storms (Sect.
4.2.2); in some areas sand is brought in from offshore to replace the eroded material,
usually at considerable expense.
Gravel only rarely reaches the sea, unless high mountains are near, or unless
glaciers bring morainal debris. Such debris, when washed by waves or rivers on the
bare ice age shelves can then yield gravel. In the Baltic and in the North Sea, for
example, gravel is mined for a filler for concrete.
In summary, the economic importance of raw materials from shelves is very
modest: they contribute less than 1 % of the more conventional onshore production,
with the exception of tin (not to mention sunken treasures from antiquity, and from
Spanish pirates, licensed and otherwise). For economic impact of marine resources
on a global scale, one must look to hydrocarbons, fisheries, recreation, and waste
diposal.
Redondo Beach, California; beaches in western Oregon yield chromite and other
heavy minerals as well as gold and platinum; titanium minerals are taken from beach
sands along the eastern Florida coast.
How do placers originate? The process of concentrating heavy particles on the
beach has much in common with panning for gold - water motion works on the
different settling velocities of the particles and on their different sizes, to separate
heavy from light, large from small.
Terrigenous beach sands commonly consist of more than 95 % quartz, tropical
ones of calcareous grains. Volcanic rocks and other igneous sources supply minerals
which are considerably heavier than quartz or calcite (density of 2.65 and 2.70).
These minerals have densities of greater than 2.85 g/cm 3 . The "heavy minerals" are
ubiquitous, usually making up a few percent of the sand. The back-and-forth movement of the waves washing over the beach face can concentrate them greatly (Figs.
10.8 and 10.9). Under the microscope, one recognizes minerals such as ilmenite
(iron-titaniumoxide), rutile (titaniumoxide), zircon (zirconiumsilicate), and monazite
(phosphate, containing cerium and thorium).
Thick placers of heavy particles form only in the beach zone. Offshore placers,
therefore, were formed when the sea level stood lower. The gold deposits off Nome,
Alaska, are a case in point. Glaciers of the last ice age brought gold-containing debris
from the hinterland and dropped it on the shelf. This morainal material was then
worked up by the surf, as the sea level rose.
River deposits may contain heavy mineral concentrations, too, such as cassiterite
(Sn02) in Thailand, Malaysia, or Indonesia. In these areas, former river beds on the
shelf, which are now in down to about 100 m water depth, are of potential economic
interest and have been exploited offshore in shallower water for nearly a century. At
present, the offshore revenue reaches some 14 % of that of worldwide tin exploitation
on land.
10.3.4 Sand and Gravel. Considerable amounts of sand are taken locally for building
roads and houses, as well as for coastal protection (dams) and for fill. However, the
main use of beach sand is for building sand castles and for plain enjoyment. Recreation is one of the most important uses of beaches - by far exceeding mineral
extraction in business value. Beaches are commonly eroded by winter storms (Sect.
4.2.2); in some areas sand is brought in from offshore to replace the eroded material,
usually at considerable expense.
Gravel only rarely reaches the sea, unless high mountains are near, or unless
glaciers bring morainal debris. Such debris, when washed by waves or rivers on the
bare ice age shelves can then yield gravel. In the Baltic and in the North Sea, for
example, gravel is mined for a filler for concrete.
In summary, the economic importance of raw materials from shelves is very
modest: they contribute less than 1 % of the more conventional onshore production,
with the exception of tin (not to mention sunken treasures from antiquity, and from
Spanish pirates, licensed and otherwise). For economic impact of marine resources
on a global scale, one must look to hydrocarbons, fisheries, recreation, and waste
diposal.