So, banded iron formations only occur when oxygen levels are low enough for all free oxygen to be periodically depleted. The oceans must alternate between aerobic and anoxic conditions. This characteristic makes banded iron formations direct indicators of oceanic oxygen levels, providing us with a record of Earth’s oxygenation.
Why are there banded iron formations in the ocean?
How do banded iron formations relate to oxygen levels? The conventional hypothesis is that the banded iron layers were formed in sea water as the result of oxygen released by photosynthetic cyanobacteria. As photosynthetic organisms generated oxygen, the available iron in the Earth's oceans precipitated out as iron oxides. Click to see full answer.
What is banded iron formation (BIF)?
Jan 10, 2022 · Iron oxide is exactly what the name implies; a compound formed when iron reacts with free oxygen. Think of it as ancient rust. Banded …
Why did the banded iron deposits end?
Jan 10, 2022 · So, banded iron formations only occur when oxygen levels are low enough for all free oxygen to be periodically depleted. The oceans must alternate between aerobic and anoxic conditions. This characteristic makes banded iron formations direct indicators of oceanic oxygen levels, providing us with a record of Earth’s oxygenation.
What type of rock is a banded iron formation?
Nov 14, 2016 · From this exercise the students conclude that: 1. free oxygen was rare or absent on the pre-BIF Earth. 2. oxygen was combining with abundant iron in oceans of the BIF Earth to form BIFs and. 3. formation of abundant BIFs stopped once the majority of iron from oceans was used up which resulted in buildup of oxygen in the atmosphere as also ...
What do banded iron formations tell us?
Almost all of these formations are of Precambrian age and are thought to record the oxygenation of the Earth's oceans. Some of the Earth's oldest rock formations, which formed about 3,700 million years ago (Ma), are associated with banded iron formations.
Why did oxygen in the ancient rock depleted?
As the cyanobacteria prospered, they made more and more oxygen. When oxygen concentrations reached a certain level, it poisoned the cyanobacteria, killing the cells. The oxygen concentration then decreased until it attained a level compatible with cyanobacteria growth.Apr 27, 2005
Why are banded iron formations important quizlet?
Banded iron formations are important as a source of iron and as an indicator of atmospheric evolution.
How does oxygen in the atmosphere differ from oxygen in rocks and minerals?
How does oxygen in the atmosphere differ from oxygen in rocks and minerals? Oxygen in the atmosphere typically is in the gas from and is only bound to itself and not other atoms. Oxygen in minerals forms to solid substances.
How did oxygen become abundant in the atmosphere?
The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen.Aug 19, 2009
Why did banded iron formations form between 2.4 and 1.8 Ga quizlet?
photosynthesis. Why did banded iron formations form between 2.4 and 1.8 Ga? a. The oceans became oversaturated in iron.
What two elements make up the banded iron formations?
Most of the major iron deposits worldwide occur in rocks called banded iron formations (or BIFs for short), which are finely layered sedimentary rocks composed of alternating chert (a form of quartz) and iron oxide bands.
Where do banded iron formations form?
banded-iron formation (BIF), chemically precipitated sediment, typically thin bedded or laminated, consisting of 15 percent or more iron of sedimentary origin and layers of chert, chalcedony, jasper, or quartz. Such formations occur on all the continents and usually are older than 1.7 billion years.
What is a banded iron formation?
Banded iron formations (also known as banded ironstone formations or BIF s) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness and extend laterally for several hundred kilometers.
What is banded iron?
Banded iron formation is more precisely defined as chemically precipitated sedimentary rock containing greater than 15% iron. However, most BIFs have a higher content of iron, typically around 30% by mass, so that roughly half the rock is iron oxides and the other half is silica.
How much iron is in Australia?
The banded iron formations here are the thickest and most extensive in the world, originally covering an area of 150,000 square kilometers (58,000 square miles) and containing about 300 trillion metric tons of iron. The range contains 80 percent of all identified iron ore reserves in Australia.
Where was the first iron ore discovered?
They were first discovered in northern Michigan in 1844. Banded iron formations account for more than 60% of global iron reserves and provide most of the iron ore presently mined.
What is the thickness of banded iron?
A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe 3 O 4) or hematite (Fe 2 O 3 ), alternating with bands of iron-poor chert, often red in color, of similar thickness. A single banded iron formation can be up to several hundred meters in thickness and extend laterally for several hundred kilometers.
Where are BIFs found?
Similar BIFs are found in the Carajás Formation of the Amazon craton, the Cauê Itabirite of the São Francisco craton, the Kuruman Iron Formation and Penge Iron Formation of South Africa, and the Mulaingiri Formation of India. Paleoproterozoic banded iron formations are found in the Iron Range and other parts of the Canadian Shield.
When was banded iron discovered?
Banded iron formation was first discovered in northern Michigan in 1844, and mining of these deposits prompted the earliest studies of BIFs, such as those of Charles R. Van Hise and Charles Kenneth Leith.
What are banded iron formations?
Banded iron-formations are sedimentary rock formations with alternating silica-rich layers and iron-rich layers that are typically composed of iron oxides (hematite and magnetite), iron-rich carbonates (siderite and ankerite), and/or iron-rich silicates (e.g., minnesotaite and greenalite).
How old is banded iron?
Banded iron formations are worldwide layered sedimentary deposits rich in iron that range in age from 1.5 to 3.8 billion years old, the age of the earliest known rocks. They contain 30–60% Fe3+. No molecular oxygen was present in the earth's atmosphere until 1.5–2.0 billion years ago.
What is a BIF?
Banded iron formation (BIF) consists of finely interstratified chemical sediments rich in iron oxides, carbonates, and/or silicates that are precursor deposits to the world's largest iron ore bodies (see Chapter 13.13 ). The second class of deposits, granular iron formations (GIF), comprises iron oxide- and silicate-rich granular material deposited in high-energy environments on broad continental shelves. Enrichment of BIF and GIF to create higher-grade ore deposits is variously attributed to supergene and hydrothermal processes. Deposition of iron formation was most prevalent between 2.65 and 1.85 Ga and was closely tied to the evolution of atmospheric oxygen, but causes and effects of ocean oxidation are still widely debated ( Bekker et al., 2010; Clout and Simonson, 2005 ).
What are BIFs in greenstone?
BIFs are a distinct component of Neo- to Mesoarchean greenstone belts. These BIFs are either intercalated with submarine volcanic rocks or associated with sedimentary strata of continental-shelf environments. Good examples of continental shelf BIF deposits are found on several cratons that experienced transient crustal stability prior to the global magmatic event at 2.7 Ga. The 2.83–2.70 Ga Manjeri Formation of the Zimbabwe craton is a fluvial to shallow-marine succession that was deposited unconformably on older greenstones and granitoids ( Hofmann and Kusky, 2004; Hunter et al., 1998 ). The IF of this unit is intercalated with quartz arenite, shale, and carbonate strata, and lacks obvious links to coeval volcanic rocks. Lithologically similar sequences include the ~ 2.8 Ga Central Slave Cover Group ( Bleeker et al., 1999) of the Slave craton, the ~ 2.7 Ga Steep Rock Group in the Wabigoon greenstone belt and correlative sequences in Canada ( Stone, 2010; Tomlinson et al., 2003; Wilks and Nisbet, 1988 ), and the ~ 2.7 Ga Bababudan Group of the Dharwar craton in India ( Srinivasan and Ojakangas, 1986; Trendall et al., 1997 ).
Where is BIF located?
Fifty percent of the iron ore deposits hosted in BIF are located in China and to some extent in North America. China represents around 80% of the BIF mined globally with a yearly production of 1200 Mt.
What are the layers of banded iron?
Banded iron formations typically have alternating layers: 1 FIRST LAYER: The first layer is either magnetite (Fe 3 O 4) or hematite (Fe 2 O 3) a few millimeters to a few centimeters in thickness. 2 SECOND LAYER: The next layer usually is iron-poor shales often red in color.
Where can I find banded iron?
We can find banded iron formation all over the world such as: Australia. Canada. South Africa. For example, South Africa ’s largest iron mines at Sishen and Thabazimbi both extract BIF. Hematite-rich ore can be mined which are found in banded iron formations.
Overview
Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Almost all of these formations are of Precambrian age and are thought to record the oxygena…
Description
A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor chert, often red in color, of similar thickness. A single banded iron formation can be up to several hundred meters in thickness and extend laterally for several h…
Occurrence
Banded iron formations are almost exclusively Precambrian in age, with most deposits dating to the late Archean (2800-2500 Ma) with a secondary peak of deposition in the Orosirian period of the Paleoproterozoic (1850 Ma). Minor amounts were deposited in the early Archean and in the Neoproterozoic (750 Ma). The youngest known banded iron formation is an Early Cambrianformatio…
Origins
Banded iron formation provided some of the first evidence for the timing of the Great Oxygenation Event, 2,400 Ma. With his 1968 paper on the early atmosphere and oceans of the earth, Preston Cloud established the general framework that has been widely, if not universally, accepted for understanding the deposition of BIFs.
Economic geology
Banded iron formations provide most of the iron ore presently mined. More than 60% of global iron reserves are in the form of banded iron formation, most of which can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States.
Different mining districts coined their own names for BIFs. The term "banded ir…
See also
• Iron-rich sedimentary rocks – Sedimentary rocks containing 15 wt.% or more iron
• Stromatolite – Layered sedimentary structure formed by the growth of bacteria or algae
• Taconite – Iron-bearing sedimentary rock, in which the iron minerals are interlayered with quartz, chert, or carbonate
Further reading
• Harnmeijer, J.P. (2003). "Banded Iron Formation: A Continuing Enigma of Geology". University of Washington. Archived from the original on 8 September 2006.
• Klein, C. (October 2005). "Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins". American Mineralogist. 90 (10): 1473–99. Bibcode:2005AmMin..90.1473K. doi:10.2138/am.2005.1871.
External links
• Media related to Banded iron formation at Wikimedia Commons
• Banded-iron formation at the Encyclopædia Britannica
• "Jaspilite" . Encyclopedia Americana. 1920.