JA

8.2 Earth's Crust

Discovery of the Magnetosphere

  • The magnetosphere was discovered in 1958 by instruments on the first US Earth satellite, Explorer 1.

  • It recorded ions (charged particles) trapped in its inner part.

  • The regions of high-energy ions in the magnetosphere are often referred to as the Van Allen belts, named after the University of Iowa professor who built the scientific instrumentation for Explorer 1.

  • Since 1958, hundreds of spacecraft have explored various regions of the magnetosphere.

  • Further insights into the interaction between the magnetosphere and the Sun are discussed in a later chapter.

Earth

Learning Objectives

  • Understand the primary types of rock that constitute Earth

  • Explain the theory of plate tectonics.

  • Describe the difference between rift and subduction zones.

  • Describe the relationship between fault zones and mountain building.

  • Explain the various types of volcanic activity occurring on Earth.

Overview of Earth

  • Earth

  • It is noted to be the most active geological surface compared to other planets and moons.- Example: Io, one of Jupiter

Composition of the Crust

  • The Earth's crust primarily consists of:- Oceanic basalt

    • Continental granite

Types of Rock

  1. Igneous Rock: Formed from molten material.- All volcanically produced rock is classified as igneous.

    • Example: Basaltic lava, which flows quickly and can travel distances greater than 20 kilometers.

    • Reference: Figure 8.6 illustrates the formation of igneous rock from liquid lava cooling and solidifying.

  2. Sedimentary Rock: Formed from fragments of igneous rock or shells of organisms deposited and cemented without melting.- Includes sandstones, shales, and limestones.

  3. Metamorphic Rock: Altered through high temperature or pressure affecting igneous or sedimentary rock.- The term metamorphic means "changed in form."

    • These rocks are created due to geological activity where surface rocks are pushed down and then returned to the surface.

  4. Primitive Rock: Represents early, unmodified materials from which the planetary system was formed.- There are no primitive rocks left on Earth; they can be found in comets, asteroids, and small moons.

    • Example: A block of quartzite illustrating the transformation through different rock types.

Plate Tectonics

Definition and Mechanism

  • Geology involves studying Earth

  • The theory of plate tectonics explains the movement of large crustal segments driven by slow motions in the mantle, leading to continental drift and geological formations.

  • Plate tectonics functions as a heat transfer system for Earth, crucial for cooling processes.

Structure of the Crust

  • Earth

  • Movement details:- Plates can move apart (e.g., Atlantic Ocean), converge or collide (e.g., west coast of South America).

  • The movement of plates is due to convection currents in the mantle, with heat escaping from Earth leading to upward flows of warm material.

Plate Movement

  • Average movement rate: 4 to 5 centimeters per year.

  • The US Geological Survey provides information on earthquake activities relative to tectonic plate boundaries.

  • Four types of interactions at plate boundaries:1. Pull Apart

    1. Subduction (one plate is forced beneath another)

    2. Sliding Alongside Each Other

    3. Jamming Together

Historical Context: Alfred Wegener

  • Alfred Wegener (1880-1930) proposed the theory of continental drift in 1915, suggesting continents were once joined.

  • Evidence included:- Similarities in fossils on different continents (e.g., South America and Africa).

    • zoological resemblances that could be explained by former continental connections (Pangaea).

  • Faced skepticism due to lack of a mechanism; research continued long after his death in 1930.

  • Wegener

Rift and Subduction Zones

Rift Zones

  • Locations where plates pull apart, mainly under oceans.

  • Example: Mid-Atlantic Ridge and the central African rift on land.

  • New crust is formed from basaltic lava.

Action of Rift Zones

  • About 60,000 kilometers of active rifts identified, with average separation of 5 centimeters per year.

  • New crust area added annually is approximately 2 square kilometers, allowing for the renewal of the oceanic crust over ~100 million years.

Subduction Zones

  • Formed when two plates converge, typically where one plate subducts beneath the other.

  • Ocean trenches mark subduction zones; example: Japan Trench.

  • Significant geological processes and destructive events (earthquakes) emerge from these zones.

  • Major historical earthquakes have occurred here, such as:- 1923 Yokohama Earthquake (death toll: 100,000)

    • 2004 Sumatra Earthquake (death toll: 200,000)

    • 2011 Tohoku Earthquake (nuclear meltdown risks).

Fault Zones and Mountain Building

Fault Zones

  • Faults are cracks at plate boundaries synonymous with motion of the plates.

  • Example: San Andreas Fault in California demonstrates significant slippage due to plate movement.

  • Motion rates and earthquake predictions:- Regular build-up of stress leads to eventual violent releases (earthquakes).

  • Example calculation: Average motion of 5 cm/year, significant earthquakes in various regions show varied slippage and stress accumulation.

Mountain Formation

  • Continental collision results in the folding and buckling of Earth's crust, leading to mountain formation.

  • Example: Alps formed from the African plate colliding with the Eurasian plate.

  • Mountains shaped by erosion processes post formation (influence of ice and water).

Volcanism

  • Volcanoes signify locations where lava reaches the surface and are concentrated at:- Mid-ocean ridges (e.g., oceanic crust formation)

    • Subduction zones

    • Hot spots (e.g., Hawaiian islands, which have produced a chain of volcanic activity over 100 million years).

  • Volcano shapes: Some form large mountains while others create lava plains from rapid flows (e.g., Snake River Basalts, Deccan Plateau).