Iron is one of the most abundant metals on Earth. It is a major component of the Earth’s core and is found in rocks and minerals across the planet’s crust. But is iron truly an Earth metal? To answer this question, we need to examine the properties of iron and how it relates to the structure and composition of our planet.
What is Iron?
Iron is a metallic chemical element with the symbol Fe and atomic number 26. On the periodic table, iron belongs to the first transition series and Group 8. It has five stable isotopes, with Fe-56 being the most abundant.
Iron is a hard, heavy, grayish metal that rusts in moist air. It has relatively high melting and boiling points (1538°C and 2862°C respectively). Iron has high tensile strength and is malleable when hot. It is also ferromagnetic, meaning it becomes strongly magnetic when placed in a magnetic field.
Iron makes up over 90% of all metals used by humans and is present in hemoglobin, the protein that carries oxygen in blood. It is an essential nutrient for life and integral to many enzymatic reactions in cells. The average human adult has about 5 grams of iron distributed throughout their body.
How is Iron Formed?
Iron is formed via stellar nucleosynthesis in the cores of massive stars. As a star runs out of hydrogen fuel in its core, it starts fusing helium and other heavier elements. Temperatures and pressures reach billions of degrees as the core condenses and contracts. Under these extreme conditions, atomic collisions can force three helium nuclei to fuse into a carbon nucleus.
Carbon nuclei produced this way can then fuse with more helium to generate elements up to iron on the periodic table. Fusing iron consumes more energy than it produces, so iron builds up in the star’s core as one of the end products. After a supernova explosion, the newly synthesized iron is ejected into space within the debris.
Over billions of years, expelled iron from ancient supernovae became incorporated into gas clouds and protostellar systems that gave rise to solar systems like ours. Iron on Earth primarily came from the original nebula that condensed into the solar system. Additional meteoritic iron from stellar debris also made its way to Earth later through collisions.
Abundance of Iron on Earth
Iron is the sixth most abundant element in the universe and makes up over 35% of the mass of Earth’s crust and mantle. The core, however, contains about 85% of Earth’s iron. The following table shows the breakdown of iron content in different layers:
| Earth Layer | Iron Content |
|---|---|
| Inner Core | 85-88% iron |
| Outer Core | 80-85% iron |
| Lower Mantle | 5-10% iron |
| Upper Mantle | 3-5% iron |
| Continental Crust | 3-5% iron |
| Oceanic Crust | 6-10% iron |
The abundance of iron decreases as you move from the inner layers outward. But even the crust still contains huge iron deposits, making it readily accessible compared to elements found only in the core.
Properties of Iron in the Core
Iron adopts different properties and mineral forms depending on location and interactions with other elements. In the hot, pressurized core, iron takes on a liquid state and forms an iron-nickel alloy due to the presence of other metals.
Earth’s core is structured with the inner core being solid iron crystals surrounded by the liquid outer core. The inner core started crystallizing about a billion years ago as the planet cooled down. Slow growth of the inner core releases latent heat that helps maintain the outer core in a liquid state.
The core’s fluid iron churns and flows due to convection and Earth’s rotation. This motion produces the dynamo that maintains our global magnetic field. So in effect, the abundance and conductive properties of iron in the core enable Earth to have a life-protecting magnetosphere.
Iron in the Mantle and Crust
Iron takes on different mineral forms in the cooler mantle and crust compared to the core. Common iron-containing minerals found in igneous and metamorphic mantle rocks include:
- Olivine – (Mg,Fe)2SiO4
- Pyroxene – XY(Si,Al)2O6 (X=Mg,Fe,Ca,Na; Y=Mg,Fe,Ca,Li,Zn)
- Magnetite – Fe3O4
- Hematite – Fe2O3
- Ilmenite – FeTiO3
The crust is chemically differentiated from the mantle with higher silica content. Common iron ores found in sedimentary crustal rocks include hematite, magnetite, limonite, siderite, and taconite. Banded iron formations are a distinctive type of metamorphic rock loaded with thin bands of iron oxides.
Iron oxides and hydroxides produce the wide range of red, yellow, and brown colors characteristic of many soils, clays, and rocks on Earth’s surface. So in this form, iron plays an important role in the geology and appearance of terrestrial landscapes.
How Iron Influences the Atmosphere
Iron interacts with Earth’s atmosphere in a couple key ways. As iron rusts in the presence of oxygen and water, it can strip oxygen molecules (O2) from the air. Over geologic time scales, this oxidation of massive iron deposits may have contributed to the shift in Earth’s atmosphere from anoxic to oxic.
Iron-containing minerals in sediments can also influence the carbon cycle. Certain bacteria can utilize iron and carbon compounds in sediments to process methane. This methanotrophic microbe activity helps reduce greenhouse gas emissions from wetlands and aquifers.
Is Iron Truly an Earth Metal?
Based on its vital presence across Earth’s layers and systems, iron seems qualified as a fundamentally Earthly metal. A strong case can be made that iron is essential to the existence of our planet as we know it.
Iron’s abundance sets Earth apart compositionally from other rocky planets like Mercury and Mars. The size and dynamics of Earth’s core allow generation of a crucial magnetic field powered by the iron dynamo.
Iron minerals constitute major components of terrestrial rocks. Iron facilitates biological processes that shape our atmosphere. In all of these ways, iron’s properties and planetary roles make it inextricably linked to the character of Earth.
Conclusion
Iron’s unique characteristics and distribution within Earth establishes its identity as a primary building block of our planet. From the inner core to the upper crust, iron leaves its mark across each layer. Earth’s composition, atmosphere, magnetic field, geology, biology, and more are all influenced by this single metallic element.
Iron is truly Earth’s metal. Our planet’s physiology depends on iron as much as humans depend on iron in hemoglobin. Earth would not be the dynamic, living world it is without the abundant presence of iron in its body. From its violent stellar origins to its steady activity in the core, iron remains one of the most important Earth metals we have.
