Question

What are the two most abundant elements in the Earth? Describe the major categories of materials constituting the Earth. Does the crust have the same composition as the whole Earth?

Short answer

The two most abundant elements in the Earth are iron (Fe), making up about $35\mathrm{\%}$ of Earth's mass, and oxygen (O), making up about $30\mathrm{\%}$. Earth can be divided into three main compositional layers: the crust, the mantle, and the core. The materials constituting these layers are categorized into two major types: silicates and metals. Silicates, composed of silicon, oxygen, and other elements, form the majority of Earth's crust and mantle, while metals like iron and nickel are predominantly found in Earth's core. The crust's composition differs from that of the whole Earth, as it is primarily composed of silicate minerals with abundant oxygen, silicon, and lighter elements rather than an equal distribution of iron and oxygen throughout the different layers.

Step-by-step solution

Identify the two most abundant elements in the Earth

The two most abundant elements in the Earth are iron (Fe) and oxygen (O). Iron makes up about $35\mathrm{\%}$ of Earth's mass, while oxygen makes up about $30\mathrm{\%}$. Together, these two elements constitute approximately $65\mathrm{\%}$ of Earth's total mass.

Describe the major categories of materials constituting the Earth

The Earth can be divided into three main compositional layers: the crust, the mantle, and the core. The materials constituting these layers can further be categorized into two major types: silicates and metals. 1. Silicates: Silicates are minerals composed of silicon, oxygen, and other elements, such as aluminum, calcium, and magnesium. These minerals make up the majority of Earth's crust and mantle. Some common examples of silicate minerals are quartz and feldspar. 2. Metals: Metals are mostly found in Earth's core, particularly the outer core, which is composed primarily of iron and nickel. These metals exist in solid form in the inner core and in liquid form in the outer core.

Compare the composition of the crust with the whole Earth

The composition of the crust is different from that of the whole Earth. Although iron and oxygen are the most abundant elements in the entire Earth, they are not equally distributed across the different layers (crust, mantle, and core). The Earth's crust is primarily composed of silicate minerals, which include abundant amounts of oxygen, silicon, aluminum, potassium, and lighter elements. The mantle, which lies beneath the crust, is also composed mostly of silicate minerals but contains significantly more iron and magnesium compared to the crust. The core, on the other hand, is mainly comprised of metals such as iron and nickel. In summary, the crust has a distinct composition compared to the entire Earth, being primarily composed of silicate minerals instead of an equal distribution of iron and oxygen throughout the different layers.

Key concepts

Silicate Minerals

Silicate minerals are the most common group of minerals found on Earth. They are compounds made primarily of silicon (Si) and oxygen (O), which are the fundamental building blocks known as silicate tetrahedra. Each tetrahedron consists of one silicon atom surrounded by four oxygen atoms. This simple structure can connect to form a wide variety of complex minerals.

These minerals play a crucial role in Earth's crust and mantle, making up around 90% of these regions. Silicates are divided into several classes based on how their tetrahedra are arranged and linked together:

• Nesosilicates: Consist of isolated tetrahedra linked by metal ions.
• Sorosilicates: Have paired tetrahedra.
• Cyclosilicates: Contain rings of linked tetrahedra.
• Inosilicates: Chain structures, either single or double chains.
• Phyllosilicates: Form sheets of tetrahedra.
• Tectosilicates: Form a three-dimensional network.

Common silicate minerals include quartz, feldspar, and mica. These minerals not only define much of Earth's geology but also significantly influence the landscape and life on the planet.

Earth's Layers

The Earth is structured in distinct layers, differentiated by their composition and physical state.

• Crust: The outermost layer of Earth, the crust is relatively thin compared to other layers. It ranges from only a few kilometers thick under the oceans to about 70 kilometers thick under continental masses. The crust is mainly composed of silicate minerals.
  
  
• Mantle: Beneath the crust lies the mantle, which extends to a depth of around 2,900 kilometers. The mantle accounts for most of Earth's volume and is rich in silicates that are denser than those in the crust. It is composed mostly of elements like magnesium and iron in addition to silicon and oxygen.
  
  
• Core: The innermost layer of Earth, the core is divided into the outer and inner core. The outer core is in a liquid state and consists mainly of iron and nickel, while the inner core is solid. The core is responsible for generating Earth's magnetic field.
  
  

Each layer has its own unique characteristics and plays a significant role in the geological activities of our planet, from plate tectonics in the crust to convection currents in the mantle.

Iron and Oxygen Abundance

Iron and oxygen are the most abundant elements in Earth's composition, making up approximately 65% of the planet's total mass.

• Oxygen (O): As a key component of many common minerals, oxygen is abundant in Earth's outer layers, especially in the crust and mantle where it pairs with silicon to form silicates. Oxygen's prevalence in these minerals contributes to much of Earth's geological features and processes.
  
  
• Iron (Fe): Iron is predominantly found in Earth's core. Due to its density, iron sinks toward the center during the planet's formation, leading to a massive accumulation in the core. Iron also appears in the mantle but in smaller quantities compared to its concentration in the core.
  
  

Though these two elements are prevalent throughout Earth, their distribution is uneven. This is crucial to understanding the different properties and activities between the Earth's layers, such as the generation of the magnetic field due to the movement of iron in the liquid outer core or the formation of rocks and soils in the crust and mantle.