8.2 Earth’s Crust
4 min read•june 12, 2024
Earth's and shape our planet's surface. From igneous, sedimentary, and to the movement of massive tectonic plates, these processes create diverse landscapes and geological features we see today.
Plate boundaries, fault zones, and volcanic activity are key players in Earth's dynamic system. Understanding these processes helps us grasp how mountains form, earthquakes occur, and erupt, giving us insight into our planet's ever-changing face.
Earth's Crust and Plate Tectonics
Main rock types of Earth's crust
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- Igneous rocks form from the cooling and solidification of magma (beneath Earth's surface) or lava (on Earth's surface)
- Intrusive igneous rocks () form from magma that cools slowly underground
- Extrusive igneous rocks (, ) form from lava that cools quickly on the surface
- form through the deposition and compression of sediments (sand, silt, clay) over time
- Clastic sedimentary rocks (, ) consist of rock and mineral fragments
- Chemical sedimentary rocks () form from the precipitation of minerals from water
- Metamorphic rocks form when existing rocks are subjected to high heat and pressure, causing them to change in texture and mineral composition without melting
- Foliated metamorphic rocks (, ) have a layered or banded appearance
- Non-foliated metamorphic rocks (, ) have a more uniform texture
Key principles of plate tectonics
- Earth's ( and upper ) is divided into several large, rigid plates that move relative to each other
- Plate motion is driven by currents in the , which are caused by heat transfer from Earth's hot interior
- Plates interact at three main types of boundaries:
- Divergent boundaries () where plates move away from each other, creating new oceanic crust
- Convergent boundaries () where plates collide or subduct, causing mountain building, volcanism, and earthquakes
- Transform boundaries () where plates slide past each other horizontally, causing frequent earthquakes
- Plate tectonics explains the formation and distribution of various geological features (mountains, volcanoes, rift valleys, ocean basins) across Earth's surface
- The theory of plate tectonics evolved from the concept of , proposed by in the early 20th century
Rift zones vs subduction zones
- occur at divergent plate boundaries where plates move away from each other
- Characterized by the stretching and thinning of the lithosphere, causing the formation of rift valleys ()
- Accompanied by the upwelling of hot mantle material, leading to volcanism and the formation of new oceanic crust (Mid-Atlantic Ridge)
- occurs at oceanic rift zones, creating new oceanic crust and pushing plates apart
- zones occur at convergent plate boundaries where one plate sinks beneath another
- Characterized by the formation of deep-sea trenches () and volcanic arcs (Andes Mountains)
- Associated with intense seismic activity as the subducting plate descends into the mantle, causing earthquakes and tsunamis (Japan, Indonesia)
Fault zones in mountain formation
- Fault zones are areas where rocks have been fractured and displaced due to tectonic forces
- Three main types of contribute to mountain formation:
- Normal faults (Basin and Range Province) where the hanging wall moves downward relative to the footwall, creating horsts and grabens
- Reverse faults () where the hanging wall moves upward relative to the footwall, causing the uplift and thickening of the crust
- Strike-slip faults (San Andreas Fault) where blocks move horizontally past each other, causing lateral displacement and local compression or extension
- Compression along convergent plate boundaries leads to the folding and faulting of rock layers, resulting in the uplift and formation of mountain ranges (, )
- , the process of mountain building, involves complex interactions of tectonic forces, faulting, and rock deformation
Types of volcanic activity
- Shield volcanoes (, ) have broad, gently sloping flanks and effusive eruptions of fluid, basaltic lava
- Formed by the accumulation of numerous lava flows over time
- Characterized by low-viscosity lava that can travel great distances from the vent
- Stratovolcanoes (, ) have steep, conical shapes and explosive eruptions of viscous, silica-rich magma
- Formed by alternating layers of lava flows, ash, and pyroclastic material
- Characterized by high-viscosity lava that tends to pile up near the vent, creating a steep profile
- Cinder cones (, ) are small, steep-sided volcanoes built from ejected lava fragments called cinders or scoria
- Formed during a single eruption or a series of brief, intermittent eruptions
- Characterized by a circular or oval-shaped crater at the summit
- Lava domes (, ) form when viscous lava accumulates and solidifies near the vent
- Often associated with explosive eruptions due to the buildup of gas pressure within the magma
- Characterized by a bulbous or dome-shaped mass of lava that can collapse or explode violently
Earth's crust and mantle dynamics
- describes the gravitational equilibrium between Earth's crust and mantle, explaining how the crust "floats" on the denser mantle
- , the study of seismic waves, provides crucial information about Earth's internal structure and helps identify plate boundaries and fault zones
Key Terms to Review (56)
Alfred Wegener: Alfred Wegener was a German scientist who proposed the theory of continental drift in the early 20th century. His groundbreaking work laid the foundation for the modern understanding of plate tectonics and the dynamic nature of the Earth's crust.
Alps: The Alps are a major mountain range system that stretch across south-central Europe, forming a natural barrier between the Mediterranean region and the northern European plains. They are known for their dramatic, snow-capped peaks, deep valleys, and diverse ecosystems.
Andes Mountains: The Andes Mountains are a vast mountain range that runs along the western coast of South America. They are the longest continental mountain range in the world, stretching over 4,300 miles from north to south. The Andes are known for their rugged, snow-capped peaks and diverse ecosystems, which play a crucial role in the Earth's crust and tectonic plate movements.
Basalt: Basalt is a type of igneous rock that is dark in color and fine-grained, formed from the rapid cooling of lava. It is a common rock found in the Earth's crust, particularly in volcanic regions and oceanic environments.
Continental Drift: Continental drift is the theory that the continents have slowly drifted apart over geologic time, moving on the Earth's surface and changing position relative to one another. This concept is central to understanding the global perspective, the structure of the Earth's crust, and the evolution of planetary bodies.
Convection: Convection is the process of heat transfer through the movement of fluid (liquid or gas) caused by molecular motion. It plays a critical role in various natural phenomena, including the dynamics of Earth's crust and the energy transport within the Sun.
Convection: Convection is the transfer of heat by the movement of a fluid, such as air or water. It is a fundamental process that drives many important phenomena in Earth's crust, atmosphere, and the atmospheres of the giant planets.
Core: The core is the innermost layer of a planet, primarily composed of metal. It plays a crucial role in generating the planet's magnetic field.
Core: The core refers to the central, innermost region of a planet, star, or other celestial body. It is typically the densest and most massive part of the structure, often composed of highly compressed materials like metals and heavy elements.
Crust: The crust is the outermost solid layer of a planet. It is composed primarily of silicate rocks and varies in thickness.
Crust: The crust is the outermost solid shell of a planet or moon, which is typically composed of relatively light, silicate-rich rock. It is the first and shallowest layer of a terrestrial body, sitting atop the denser mantle and core layers.
Earth’s crust: Earth's crust is the outermost layer of the planet, composed primarily of solid rocks and minerals. It includes both the continental crust, which forms the landmasses, and the oceanic crust beneath the oceans.
East African Rift: The East African Rift is a major geological feature that is slowly splitting the African continent into two separate landmasses. It is a vast, complex system of deep valleys and high mountains that stretches from the Red Sea in the north to Mozambique in the south, and it is closely linked to the composition, structure, and evolution of the Earth's crust and the planets in our solar system.
Faults: Faults are fractures in Earth's crust where significant displacement has occurred. They result from tectonic forces causing blocks of crust to move relative to each other.
Graben: A graben is a depressed block of land bordered by parallel faults or fault systems. It is a type of rift valley formed by the downward displacement of a block of the earth's crust, resulting in a long, narrow trough-like depression with steep walls on either side.
Granite: Granite is a common type of intrusive igneous rock that is composed primarily of quartz, feldspar, and mica. It is known for its hard, durable, and coarse-grained texture, making it a popular choice for various construction and decorative applications.
Granite is an integral part of the Earth's crust, forming the foundation for many landmasses and mountain ranges. Its unique composition and formation process contribute to its widespread use and significance in the study of Earth's geology.
Himalayas: The Himalayas are a vast mountain range in Asia, forming a natural border between the Indian subcontinent and the Tibetan Plateau. They are known for their towering peaks, including some of the highest mountains in the world, and their diverse geological composition and structures, which are relevant to the study of planetary composition and Earth's crust.
Horst: A horst is a raised block of the Earth's crust that is bounded by parallel normal faults. It is a type of tectonic landform that results from the uplift of a section of the Earth's surface relative to the surrounding areas.
Horsts are often found in regions with active tectonic plate movements, where the Earth's crust is being stretched and faulted. This process leads to the formation of a raised block of land, flanked by parallel fault lines on either side.
Igneous rock: Igneous rock forms from the cooling and solidification of molten magma or lava. It is one of the three main types of rocks found on Earth.
Isostasy: Isostasy is the state of gravitational equilibrium between the Earth's crust and the underlying mantle, where the weight of the crust is balanced by the buoyancy of the underlying mantle. This concept is crucial in understanding the formation and dynamics of the Earth's surface features.
Limestone: Limestone is a sedimentary rock composed primarily of the mineral calcite (calcium carbonate). It is a common and widely distributed rock that has been used throughout human history for a variety of purposes, from construction to agriculture.
Lithosphere: The lithosphere is the outermost solid shell of a rocky planet, including Earth. It is composed of the crust and the uppermost portion of the mantle, and it is characterized by its rigidity and strength, which sets it apart from the underlying, more ductile asthenosphere.
Mantle: The mantle is the thick, solid layer of rock between Earth's crust and core, making up about 84% of Earth's volume. It plays a crucial role in plate tectonics and the heat transfer that drives geological activity.
Mantle: The mantle is the thick, rocky layer of the Earth that lies between the crust and the core. It is the largest layer of the Earth, accounting for about 84% of the planet's volume. The mantle is composed of dense, hot, and slowly flowing solid rock.
Marble: Marble is a metamorphic rock composed of recrystallized carbonate minerals, primarily calcite or dolomite. It is known for its beautiful, varied patterns and colors, making it a popular choice for decorative and building purposes.
Mariana Trench: The Mariana Trench is the deepest part of the world's oceans, located in the western Pacific Ocean. It is a crescent-shaped depression in the Earth's crust, formed by the subduction of the Pacific tectonic plate under the smaller Mariana plate.
Mauna Loa: Mauna Loa is a massive shield volcano located on the island of Hawai'i. It is one of the largest volcanoes in the world, both in terms of its sheer size and the volume of its eruptions. Mauna Loa's unique characteristics and geological history make it a significant feature in the study of planetary composition, Earth's crust, and the evolution of planetary bodies.
Metamorphic rocks: Metamorphic rocks are rocks that have undergone transformation due to extreme heat, pressure, or chemically active fluids. They originate from pre-existing igneous, sedimentary, or other metamorphic rocks.
Mid-Atlantic Ridge: The Mid-Atlantic Ridge is a major undersea mountain range that runs the length of the Atlantic Ocean, forming the boundary between the North American and Eurasian tectonic plates. It is a key feature of Earth's crust and plays a crucial role in the process of plate tectonics.
Mount Fuji: Mount Fuji is an iconic, dormant volcano located in central Japan. It is the highest mountain in Japan, standing at an impressive elevation of 12,388 feet (3,776 meters). Mount Fuji's distinctive symmetrical cone shape has made it a revered symbol of Japan, inspiring countless artworks and serving as a popular destination for hikers and sightseers.
Mount Rainier: Mount Rainier is an active stratovolcano located in the Cascade Range of Washington state. It is the highest mountain in the Cascade Range and the highest point in the state of Washington, standing at an impressive elevation of 14,411 feet (4,392 meters) above sea level. As a prominent geological feature, Mount Rainier plays a significant role in the context of Earth's crust and the volcanic processes that shape the landscape.
Mount St. Helens: Mount St. Helens is an active stratovolcano located in the Cascade Range of Washington state, known for its catastrophic eruption in 1980 that dramatically altered the surrounding landscape. As a key feature of Earth's crust, this volcanic mountain provides insights into the dynamic processes that shape the planet's surface and interior.
Obsidian: Obsidian is a naturally occurring volcanic glass formed by the rapid cooling of silica-rich lava. It is a hard, black, or dark-colored igneous rock that is commonly used in various applications, including tools, weapons, and decorative items.
Obsidian is particularly relevant in the context of Earth's crust, as it is a key indicator of volcanic activity and the geological processes that shape the planet's surface.
Olympus Mons: Olympus Mons is a massive shield volcano located on the planet Mars, known for being the largest volcano in the solar system. This remarkable geological feature is closely tied to the composition, structure, and evolution of planets, particularly in the context of the Martian surface and the broader understanding of planetary geology.
Orogeny: Orogeny is the process of mountain building, where tectonic forces within the Earth's crust and upper mantle cause the formation and uplift of mountain ranges. This process is driven by the collision and convergence of Earth's tectonic plates, leading to the deformation and metamorphism of existing rocks.
Parícutin: Parícutin is a cinder cone volcano that erupted in 1943 in the Mexican state of Michoacán. It is a notable example of a young, actively forming volcano that provides insights into the processes that shape the Earth's crust.
Plate Tectonics: Plate tectonics is the scientific theory that describes the large-scale motion of the Earth's lithosphere, which is divided into several rigid plates that move independently over the more fluid asthenosphere. This concept is fundamental to understanding the global perspective, the structure of Earth's crust, the evolution of life and climate, and the geology of other terrestrial planets in our solar system.
Primitive rock: Primitive rock is an original or unaltered rock that forms the building blocks of planetary crusts. These rocks are typically igneous and have not undergone any significant metamorphic or sedimentary changes.
Quartzite: Quartzite is a hard, metamorphic rock that is formed from the metamorphism of sandstone. It is composed almost entirely of the mineral quartz and is known for its high resistance to weathering and erosion.
Rift Valley: A rift valley is a depressed, elongated land feature that forms when the Earth's crust is pulled apart by tectonic forces, resulting in a valley flanked by parallel fault-block mountains or escarpments. This geologic feature is often associated with divergent plate boundaries and active volcanism.
Rift zones: Rift zones are regions where the Earth's crust is being pulled apart, leading to the formation of fractures and faults. These zones often result in volcanic activity as magma rises to fill the gaps created by the diverging crustal plates.
Rocky Mountains: The Rocky Mountains, also known as the Rockies, are a major mountain range in western North America that stretch from northern British Columbia and Alberta in Canada to New Mexico in the southwestern United States. They are known for their rugged, jagged peaks, steep slopes, and diverse ecosystems, and play a crucial role in the geological and geographic features of the Earth's crust.
San Andreas Fault: The San Andreas Fault is a major geological fault that runs through California, USA. It is a transform fault, where the Pacific Plate and the North American Plate meet and move past each other horizontally. This movement causes earthquakes and other seismic activity along the fault line.
Sandstone: Sandstone is a sedimentary rock composed primarily of sand-sized mineral or rock grains cemented together by various natural materials, such as calcium carbonate, silica, iron oxide, or clay. It is a common and widely distributed type of sedimentary rock, formed by the consolidation of sand grains over time through the processes of weathering, erosion, transportation, and deposition.
Schist: Schist is a type of metamorphic rock that is characterized by its distinct foliated or banded appearance, resulting from the alignment of platy or elongated mineral grains. It is formed through the metamorphism of various parent rocks, such as shale, slate, or phyllite, under conditions of moderate to high temperature and pressure.
Seafloor Spreading: Seafloor spreading is a geological process in which new oceanic crust is formed by the upwelling and cooling of molten magma from the Earth's mantle at mid-ocean ridges. This process is a key component of plate tectonics and the continuous renewal of the Earth's surface.
Sedimentary rocks: Sedimentary rocks are types of rocks formed by the accumulation and compression of mineral and organic particles on Earth's surface. They often contain fossils and are typically layered, providing important geological information.
Seismology: Seismology is the scientific study of earthquakes and the propagation of seismic waves through Earth or other planetary bodies. It provides insights into the internal structure and dynamics of the planet, as well as the processes that drive tectonic activity and other geophysical phenomena.
Shale: Shale is a fine-grained sedimentary rock that is formed from the compaction and cementation of clay, silt, or mud. It is characterized by its ability to split or break along thin, parallel layers, making it a common and widely distributed rock type in the Earth's crust.
Slate: Slate is a fine-grained, metamorphic rock that is formed from the alteration of shale or mudstone. It is characterized by its distinct layered or foliated structure and is known for its durability, water-resistance, and ability to split into thin, flat sheets.
Soufrière Hills: Soufrière Hills is an active volcanic complex located on the island of Montserrat in the Caribbean. It is known for its long history of volcanic activity and the devastating eruptions that have impacted the island's landscape and population over the past few decades.
Subduction: Subduction is the process where one tectonic plate moves under another and sinks into the mantle. This typically occurs at convergent boundaries between oceanic and continental plates.
Subduction: Subduction is the process by which one tectonic plate is forced under another, causing the descending plate to be pushed deep into the Earth's mantle. This fundamental plate tectonic process shapes the surface of the planet and drives many geological phenomena.
Sunset Crater: Sunset Crater is a volcanic cinder cone located in northern Arizona, United States. It was formed during a series of eruptions between 1040 and 1100 CE, making it one of the youngest volcanoes in the contiguous United States. The crater's distinctive reddish-orange color, which gives it the appearance of a sunset, is a result of the oxidation of iron-rich minerals in the volcanic ash and cinders that cover the landscape.
Volcanoes: Volcanoes are openings in Earth's crust where magma, gas, and ash can escape from below the surface. They are often found at tectonic plate boundaries and hotspots, and play a crucial role in shaping Earth's landscape and atmosphere.
Wegener: Alfred Wegener was a German meteorologist and geophysicist who proposed the theory of continental drift. He suggested that continents were once a single landmass that gradually drifted apart.