Earth's Lithosphere and Internal Structure

Earth's Internal Structure

• Earth's internal structure is divided into three main parts: crust, mantle, and core.
• Crust: 3-70 km thick, ranges from 3 km at oceanic ridges to 70 km in collisional orogens/mountains.
• Mohorovicic discontinuity (Moho) defines the base of the crust.
• Lithosphere: 50-300 km thick, includes the crust and uppermost mantle, thinnest in oceanic regions and thicker in continental regions.
• Asthenosphere: extends from the base of the lithosphere to 660 km, characterized by high temperature and pressure, and viscous flow.
• Upper Mantle: extends from the Moho to the 660 km discontinuity, includes the lower part of the lithosphere and the upper part of the asthenosphere.
• Transition Zone: region from the 410 km to the 660 km discontinuity.
• Lower Mantle: extends from the 660 km discontinuity to the 2900 km discontinuity at the core-mantle boundary.
• Outer Core: extends from the 2900 km to the 5200 km discontinuity, composed of mostly iron and nickel.
• Inner Core: extends from the 5200 km discontinuity to the center of the Earth, primarily a solid ball composed of an iron-nickel alloy.

Lithosphere

• Lithosphere is the outermost layer of Earth, composed of the crust and the brittle part of the upper mantle.
• The term "lithosphere" is derived from the Greek words "lithos" (stone) and "sphaira" (globe or ball).
• At the lithosphere-asthenosphere boundary, rocks take on a viscous nature and flow, albeit very slowly, due to high temperature and pressure.

Plate Tectonics

• Plate tectonics is the concept that the lithosphere is divided into a small number of nearly rigid plates that move over the asthenosphere.
• The Earth's crust is divided into 12 major plates that move in various directions, causing them to collide, pull apart, or scrape against each other.
• Each type of interaction causes a characteristic set of Earth structures or "tectonic" features.
• The word "tectonics" refers to the deformation of the crust as a consequence of plate interaction.

Plate Composition and Movement

• Plates are made of rigid lithosphere, which is composed of the crust and the upper part of the mantle.
• Below the lithosphere is the asthenosphere.
• Plates are moved around by the underlying hot mantle convection cells.
• The Earth's magnetic field originates in the outer core, which is a metallic fluid consisting mainly of iron.

Convection and Conduction

• Convection: the movement of fluid due to differences in temperature and density, resulting in the transfer of heat.
• Conduction: the process of direct heat transfer through a material without movement.

Oceanic and Continental Plates

• Oceanic plates are formed at mid-oceanic ridges, composed of heavy rocks (mafic in nature), dense (approximately 3200 kg/m3), and old (less than 200 million years).
• Continental plates are formed at subduction zones, composed of lighter rocks (felsic in nature), less dense (approximately 2600-2750 kg/m3), and older (up to 4 billion years).
• Oceanic plates cover approximately 71% of the Earth's surface, while continental plates cover 29%.

Plate Boundaries

• There are three types of plate boundaries: divergent, convergent, and conservative.
• Divergent plate boundaries: plates move away from each other, resulting in the creation of new plate material.
• Convergent plate boundaries: plates move towards each other, resulting in subduction, collision, or destruction of plate material.
• Conservative plate boundaries: plates slide past each other, resulting in faults and magma outflow.

Convergent Plate Boundaries

• Collision of oceanic plates: one plate subducts beneath the other, forming a subduction zone and trench.
• Collision of continental plates: plates collide and push against each other, resulting in intense folding, faulting, and mountain formation.

Conservative Plate Boundaries

• Along conservative boundaries, lithosphere is neither created nor destroyed, and plates move laterally relative to each other.
• Examples include the San Andreas Fault in California, USA.