Banded Iron Formations

الأحد مايو 17, 2015 5:23 pm

Banded Iron Formations

[size=15]In the Investigate, you looked at pictures of banded iron formations. You
saw that they were made up of alternating dark and light layers of rock.
Some of the layers contain iron-rich minerals. The minerals are hematite
and magnetite which are common. Some of the layers are a fine-grained
silica rock called chert. Chert is a chemical sedimentary rock that forms
when tiny quartz crystals grow from a silica-rich solution.
Today, banded iron formations can be found all over the world, including
North America, South America, Africa, Australia, Greenland, India, and
Asia. They are commonly associated with greenstone belts. They were
moved onto the continents by accretion. This is the process by which the
cores of ancestral continents grew by the subduction of adjacent oceanic
crust and the welding of crust to the continental margin.
Scientists think that during the late Hadean Eon the oceans contained vast
amounts of iron and silica. One source of the iron seems to have been
underwater volcanoes. The iron was released in fluids from hydrothermal
vents. There, the material from the upper mantle mixed with seawater.
Another source for the iron and silica in the oceans was Earth’s crust.
Earth’s young basaltic crust was rich in minerals that contain iron and
silica. Earth’s new crust was exposed to the warm primitive atmosphere.
This caused rocks to weather. The weathering process released iron and
silica from the crust. They found their way into rivers in solution and in
mineral fragments. The rivers eventually deposited the iron and silica into
the oceans.
Iron was being added to seawater for several hundred million years.
However, it was not being removed. Also, at that time, there were no
organisms that required silica to form shells. As a result, the ocean water
became saturated with silica as well. Iron and silica are the chemicals
found in the iron-rich and chert layers that form banded iron formations.
Then a change began to occur. Banded iron formations began to appear.
Scientists have determined that this occurred in the following way.
Between about 3.5 and 2 billion years ago, the iron and silica began to
react with oxygen dissolved in Earth’s ancient ocean. Iron reacted to form
iron-oxide minerals. They built up in layers on the ocean bottom. Silica
in the water also reacted with the oxygen. As a result, layers of silica-rich
chert were formed.
Banded iron formations developed in two major settings. Some formed
in deep marine environments where volcanism released iron and silica.
Others formed in shallow basins along the margins of continents. Some
scientists think that the layering of banded iron formations reflects the
competing influences of hydrothermal processes and continental fluxes
of material. However, there is still much debate about their layering.
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Banded Iron Formations

Banded Iron Formations

رد: Banded Iron Formations