1.2 Earth's structure and composition
2 min read•july 24, 2024
Earth's interior is a complex system of layers, each with unique properties. From the thin we live on to the scorching , these layers interact to shape our planet's geology and habitability.
The structure and composition of Earth's layers drive and other geological processes. Understanding these internal dynamics helps explain everything from earthquakes to the formation of continents and oceans.
Earth's Internal Structure
Structure of Earth's interior
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- Crust forms outermost layer ranging 5-70 km thick composed mainly of silicate rocks (granite, basalt)
- extends from base of crust to -mantle boundary divided into upper and lower sections primarily composed of iron and magnesium-rich (, )
- consists of liquid iron and nickel generating Earth's magnetic field with temperatures 4,400-6,100℃
- Inner Core forms solid sphere at Earth's center composed of iron and nickel under extreme pressure and heat up to 5,400℃
Lithosphere vs asthenosphere
- forms rigid outer layer 50-280 km thick including crust and uppermost mantle broken into tectonic plates (Pacific, North American)
- comprises weak, ductile upper mantle layer beneath lithosphere allowing plate movement through convective flow
- Plate tectonics driven by lithospheric plate interactions at boundaries (convergent, divergent) and asthenospheric convection currents
Composition of crust and mantle
- Crust major elements: oxygen 46.6%, silicon 27.7%, aluminum 8.1%, iron 5.0%, calcium 3.6%
- Mantle major elements: oxygen 44.8%, magnesium 22.8%, silicon 21.5%, iron 5.8%
- Common crust minerals: , , ,
- Common mantle minerals: olivine, pyroxenes,
Oceanic vs continental crust
- Oceanic crust:
- Basaltic composition
- 5-10 km thick
- Density ~3.0 g/cm³
- Younger than 200 million years
- Continental crust:
- Granitic composition
- 30-50 km average thickness
- Density ~2.7 g/cm³
- Can be billions of years old
- Oceanic crust thinner but denser subducts beneath continental crust at convergent boundaries (Mariana Trench)
- Continental crust more buoyant resists forms stable landmasses (Eurasia, North America)
Key Terms to Review (31)
Amphiboles: Amphiboles are a group of silicate minerals that are characterized by their elongated crystals and double-chain structure. They play a crucial role in the composition of igneous and metamorphic rocks, providing important insights into the conditions of their formation. These minerals are often rich in iron and magnesium, contributing to the overall mineralogy and texture of rocks within Earth's crust.
Asthenosphere: The asthenosphere is a semi-fluid layer of the Earth's upper mantle located beneath the lithosphere, characterized by its ability to flow and deform slowly over time. This layer plays a crucial role in tectonic plate movement and interactions, allowing the rigid plates of the lithosphere to slide over it. The asthenosphere's properties are vital for understanding geological processes such as volcanism, earthquakes, and the overall dynamics of Earth's interior.
Carbonates: Carbonates are a class of minerals that contain the carbonate ion, CO₃²⁻, and are formed through various geological processes. These minerals are important components of sedimentary rocks and play a crucial role in the carbon cycle, contributing to both the structure of Earth's crust and the biochemical processes that regulate climate. Carbonates can also indicate past environmental conditions, making them vital for understanding Earth's history.
Convergent boundary: A convergent boundary is a tectonic plate boundary where two plates move toward each other, leading to geological processes such as subduction or continental collision. This interaction often results in significant geological features, such as mountain ranges, deep ocean trenches, and volcanic activity, highlighting the dynamic nature of Earth’s surface and its internal processes.
Core: The core is the innermost layer of the Earth, composed primarily of iron and nickel. It is divided into two parts: the solid inner core and the liquid outer core. The core plays a crucial role in generating the Earth's magnetic field through the movement of molten metal in the outer core.
Crust: The crust is the outermost layer of the Earth, composed of solid rock, and varies in thickness from about 5 kilometers under the oceans to up to 70 kilometers beneath some mountain ranges. It is divided into two main types: oceanic crust, which is thinner and denser, and continental crust, which is thicker and less dense. The crust plays a crucial role in Earth's internal structure and dynamics, serving as the foundation for landforms and geological processes such as plate tectonics and earthquakes.
Divergent boundary: A divergent boundary is a tectonic plate boundary where two plates move apart from each other, creating new crust as magma rises to the surface. This process is primarily associated with seafloor spreading and the formation of mid-ocean ridges, highlighting the dynamic nature of Earth's lithosphere and its role in shaping geological features.
Eon: An eon is the largest division of geologic time, spanning hundreds of millions to billions of years, and is a key component in understanding Earth's history. Eons are subdivided into eras, which are then divided into periods, and these time scales help to organize and understand significant events in Earth's geological and biological evolution. The study of eons allows us to piece together the timeline of Earth's structure and composition, revealing how the planet has changed over vast time scales.
Era: An era is a significant period of time characterized by particular events, developments, or conditions that distinguish it from other periods. In the context of Earth's history, eras are subdivisions of the geological time scale that mark major geological or biological changes on the planet, helping to organize and understand the evolution of Earth and life over millions of years.
Erosion: Erosion is the process by which soil and rock are removed from one location and transported to another, typically by natural forces such as water, wind, ice, or gravity. This process plays a crucial role in shaping landscapes and is integral to the rock cycle, as it contributes to the breakdown of rocks and the formation of sedimentary structures.
Feldspars: Feldspars are a group of rock-forming minerals that are abundant in the Earth's crust, primarily composed of silicate minerals. They are crucial to understanding Earth's structure and composition, as they make up a significant portion of igneous rocks, such as granite and basalt, and play a key role in the geological processes that shape our planet.
Felsic: Felsic refers to a category of igneous rocks and minerals that are rich in silica and light-colored materials, primarily feldspar and quartz. This term is crucial for understanding the composition of the Earth's crust, as felsic rocks are prevalent in continental regions and are indicative of the processes that shape our planet's surface.
Garnets: Garnets are a group of silicate minerals that are commonly found in metamorphic rocks, characterized by their isometric crystal structure and diverse colors. These minerals play a significant role in understanding the Earth's composition as they can provide insights into the conditions under which metamorphic rocks form, including temperature and pressure ranges. The presence of garnets can also indicate specific geological processes, making them valuable for interpreting Earth's history.
Inner core: The inner core is the Earth's innermost layer, composed primarily of solid iron and nickel, and it is located beneath the outer core. This extreme environment has temperatures that can reach up to 5,700 degrees Celsius (about 10,300 degrees Fahrenheit), creating a dense and solid state despite the immense pressure. Understanding the inner core helps explain Earth's internal dynamics and contributes to our knowledge of its overall structure and composition.
Isostasy: Isostasy is the equilibrium between the Earth's crust and the denser mantle beneath it, which allows landforms to float at a certain elevation based on their density and thickness. This concept explains how changes in topography, such as mountain formation or erosion, can affect the vertical movement of the crust. Understanding isostasy is crucial for studying the dynamics of Earth's internal structure and how geological processes shape our planet's surface.
Lithosphere: The lithosphere is the outermost layer of the Earth, composed of the crust and the upper part of the mantle. It is rigid and relatively cool compared to the underlying asthenosphere, playing a crucial role in the dynamics of tectonic plates and the geological processes that shape our planet. This layer's interactions with both the atmosphere and hydrosphere contribute to various geological phenomena, including earthquakes and volcanic activity.
Mafic: Mafic refers to a type of igneous rock that is rich in magnesium and iron, characterized by a darker color and higher density compared to felsic rocks. Mafic rocks are primarily found in the Earth's mantle and oceanic crust, and they play a significant role in understanding the planet's composition and internal structure.
Magnetometry: Magnetometry is the measurement of magnetic fields, often used to identify and analyze the properties of geological formations. By studying variations in the Earth's magnetic field, scientists can infer information about subsurface materials and structures, making it an essential tool in geology for understanding Earth's structure and composition as well as mapping archaeological sites or mineral deposits.
Mantle: The mantle is a thick layer of semi-solid rock located between the Earth's crust and the outer core, extending to about 2,900 kilometers below the surface. This layer plays a crucial role in Earth's internal dynamics, influencing tectonic activity and convection processes that drive plate movements. Understanding the mantle's composition and behavior is essential for comprehending the structure of the Earth and how seismic waves propagate during earthquakes.
Micas: Micas are a group of sheet silicate minerals characterized by their layered structure and perfect basal cleavage, allowing them to be easily split into thin sheets. These minerals are important components in both igneous and metamorphic rocks and play a significant role in understanding Earth's structure and composition, as well as the textures and structures found in metamorphic environments.
Olivine: Olivine is a magnesium iron silicate mineral that is commonly found in igneous rocks and is a significant component of the Earth's upper mantle. Its distinctive green color and granular appearance make it easily identifiable, and it plays a crucial role in understanding both the composition of the Earth and the classification of igneous rocks, particularly basalt and peridotite.
Outer core: The outer core is a fluid layer composed mainly of molten iron and nickel, located between the solid mantle and the inner core of the Earth. This layer is crucial for generating the Earth's magnetic field through the movement of its molten metals, which create electric currents and, consequently, magnetic fields. Understanding the outer core is essential in studying Earth's internal structure and dynamics, as it plays a significant role in geophysical processes and the overall composition of the planet.
Period: In geology, a period is a unit of geologic time that is used to define significant intervals in Earth's history characterized by distinct geological and biological developments. Periods are part of the geologic time scale, which organizes Earth's history into eons, eras, periods, epochs, and ages, allowing scientists to understand the timing and relationships of events in Earth's past.
Plate Tectonics: Plate tectonics is the scientific theory that describes the large-scale movements of Earth's lithosphere, which is broken into tectonic plates that float on the semi-fluid asthenosphere beneath. This theory explains how these plates interact at their boundaries, leading to geological phenomena such as earthquakes, volcanic activity, and the formation of mountains and oceanic trenches.
Pyroxene: Pyroxene is a group of important silicate minerals found primarily in igneous and metamorphic rocks, characterized by their crystalline structure and distinct cleavage. These minerals are generally rich in iron and magnesium and are crucial for understanding the mineral composition of the Earth's crust and mantle. Their presence in various rock types helps geologists interpret the geological processes that shape our planet.
Quartz: Quartz is a common and abundant mineral composed of silicon dioxide (SiO₂), known for its hardness and resistance to weathering. This mineral plays a vital role in Earth's structure, forming the primary component of many rocks and influencing the classification and formation of minerals, as well as contributing to the development of various rock types, including igneous and sedimentary rocks.
Seismology: Seismology is the scientific study of earthquakes and the propagation of seismic waves through the Earth. This field plays a crucial role in understanding Earth's internal structure and composition, as seismic waves provide valuable information about the various layers within the planet, including the crust, mantle, and core. By analyzing how these waves travel and behave, scientists can infer details about materials and conditions deep within the Earth.
Silicates: Silicates are a group of minerals that contain silicon and oxygen, and they make up about 90% of the Earth's crust. This mineral group is essential in understanding Earth's structure and composition because silicates form the basis of many rock types, influencing geological processes such as tectonics, volcanism, and erosion. Silicates can be classified into different categories based on their structures, which impacts their physical properties and occurrences in nature.
Subduction: Subduction is the geological process where one tectonic plate moves under another and sinks into the mantle as the plates converge. This movement is crucial for understanding plate tectonics, as it drives many geological processes, including volcanic activity and earthquake generation, and plays a significant role in shaping Earth's internal structure and dynamics.
Volcanism: Volcanism refers to the processes and phenomena associated with the movement of magma from beneath the Earth's crust to the surface, resulting in volcanic eruptions and the formation of various volcanic features. This term is closely linked to the behavior of magma, the structure of the Earth, and the dynamics occurring at plate boundaries, playing a crucial role in shaping the planet's surface and contributing to geological cycles.
Weathering: Weathering is the process that breaks down rocks and minerals at the Earth's surface through physical, chemical, or biological means. This breakdown is crucial for soil formation and influences the rock cycle by transforming solid rock into sediments that can eventually form sedimentary rocks. Weathering plays an essential role in shaping landscapes and affecting Earth's structure, contributing to the development of clastic, chemical, and organic sedimentary rocks.