03. Earth Structure - Earth Structure A17DE

Heriot-Watt University Overview

Course: BSc in Physical Geography A17DE - Dynamic Earth 2

Session: 3

Topic: Earth Structure

Learning Objectives and Outcomes

  • Understand how the Earth is subdivided into layers, focusing on both compositional and mechanical layering.

  • Appreciate the various physical and chemical properties of each layer, including temperature, pressure, and material composition.

  • Show how surface processes are influenced by dynamic actions occurring within different layers of the Earth’s structure, including volcanic activity, earthquakes, and plate tectonics.

  • Understand the historical context that has shaped the Earth’s structure and its ongoing evolution, including the role of meteorite impacts and tectonic activity.

Geological Time

  • The Earth formed approximately 4.5 billion years ago (Ga) through the accretion of particles within the solar nebula, subsequently leading to a differentiated planet.

  • Early conditions were characterized by extreme volatility, with frequent meteor impacts leading to a mostly molten surface that gradually cooled.

  • Over geological time, differentiation occurred, leading to the formation of the Earth's core, mantle, and crust, alongside the emergence of liquid water and the conditions facilitating the development of life.

  • Layering of the Earth occurred rapidly during the molten phase and continues to be influential in geological processes observed today.

Zoned Earth

  • The formation of the Moon is believed to have occurred after the Earth had undergone initial zonation, affecting its rotational dynamics and geological evolution.

  • Zonation refers to the segregation of materials into layers based on density and composition, with lighter materials forming the crust and denser materials sinking towards the core, a process impacted by gravitational differentiation.

Current Earth Structure

  • Understanding the contemporary layout of the Earth's layers involves analyzing the crust, mantle, and core, each uniquely contributing to geophysical phenomena.

Layer Composition

  • Bulk Earth Composition:

    • Crust: Solid and divided into oceanic and continental types. Primarily composed of Oxygen (O), Silicon (Si), Aluminum (Al), as well as other elements.

    • Mantle: Exists in a plastic state, rich in silicates containing Magnesium (Mg), Iron (Fe), and Aluminum (Al), characterized by slow convection currents that drive plate tectonics.

    • Core:

      • Outer Core: Liquid and primarily composed of Iron (Fe) and Nickel (Ni), generating the Earth’s magnetic field through convection currents.

      • Inner Core: Solid and temperatures reaching up to 5,700 °C, primarily composed of Fe and Ni.

Layer Processes

  • Mantle: Exhibits slow convection; the material becomes denser and harder toward the core, influencing tectonic movement and surface processes.

  • Core: Processes involve convection of the liquid outer core, responsible for generating the Earth’s magnetic field through dynamo action.

Seismicity and Its Importance

  • Seismic waves, particularly Primary (P) and Secondary (S) waves, are vital for understanding the Earth’s internal structure through the detection of shadow zones and variations in wave speed, providing insights into physical states within the layers.

  • P-wave arrival times aid in estimating the density of the outer core, while S-wave behavior confirms the liquid state of the outer core due to their inability to travel through liquids.

Seismic Imaging

  • Utilizes seismic reflection surveys to create detailed images of geological structures from the crust to the mantle.

  • Notable examples include studies in the Himalayas which specifically refer to the Mohorovicic Discontinuity (Moho), marking significant differences in material composition and temperature distribution within the Earth’s layers.

Compositional Evidence

  • Mid-Ocean Ridge Basalts (MORB) formed through mantle melting during seafloor spreading events.

  • Differences in volcanic eruption types are observed: basaltic eruptions dominate oceanic settings, whereas kindesitic and rhyolitic eruptions are more common on continents, often due to interaction with continental crust material.

  • Ophiolite Complexes: These geological structures expose both the mantle and oceanic crust, playing a critical role in understanding mountain-building processes and orogeny.

The Uppermost Layers

  • Definition of the Lithosphere: Comprising the crust and the uppermost mantle, the lithosphere exhibits both plastic and rigid properties, influencing surface interactions and tectonic behavior.

  • Crust Types:

    • Oceanic Crust: Typically basaltic, thinner and denser, formed at Mid-Ocean Ridges, characterized by rapid geological activity.

    • Continental Crust: Significantly older and more complex, composed of various rock types, including sedimentary, igneous, and metamorphic rocks, and can be substantially thicker compared to oceanic crust.

Earth’s Interior Processes

Effects on Surface Processes

Processes occurring within the Earth’s layers have notable effects on surface conditions, including:

  • Surface faulting and folding, influencing topography.

  • Sedimentation processes leading to the formation of sedimentary rocks.

  • Lower crust deformation influencing mountain-building and erosion patterns.

  • Igneous intrusions/extrusions affecting landforms and geological diversity.

  • Transitions from solid rock to plastic mantle impacting tectonic activity.

  • Mantle convection currents driving plate tectonics and resulting geological phenomena.

Plate Tectonics

  • The movement of tectonic plates creates dynamic geological processes including:

    • Areas of collision leading to mountain building and volcanic arcs.

    • Divergent boundaries facilitating seafloor spreading and ocean basin formation.

    • Shear zones resulting in lateral displacements and earthquake occurrences.

  • Understanding these movements is critical for comprehending the formation of metamorphic regions and the geodynamics of the Earth.

Earth’s Molten Core

  • Core Composition includes a solid inner core surrounded by a molten outer core, generating convection currents vital for Earth's magnetic field generation.

  • The convection of iron-nickel alloys contributes to the Earth's electrical and magnetic currents under a relatively weak and complex magnetic field.

Magnetic-Reversal Stratigraphy

  • The study of geomagnetic episodes through the analysis of MORBs provides insights into the Earth's magnetic field history, revealing magnetic reversals and helping to establish a timeline for geological and biological events over the last 80 million years (Ma), alongside greater uncertainties for earlier periods.

Summary

In summary, the Earth is structured into multiple layers with distinct compositions and processes that are dynamic and interconnected. This overview discussed the early evolutionary history and the current layered structure of the Earth while illustrating the implications of internal processes on surface conditions, contributing to our understanding of geology and plate tectonics.