Introduction to Geology: Earth’s Structure, Rocks, and Plate Tectonics

Posted on Aug 25, 2024 in Geology

Chapter 1: Introduction to Geology

The Development of Geology

  • Mid-1600s – James Ussher
    • Catastrophism
    • Earth’s landscapes shaped primarily by catastrophes
  • 1795 – James Hutton
    • Uniformitarianism
    • The physical, chemical, and biological laws that operate today have operated throughout geologic past
    • The present is the key to the past

The Earth system is powered by the Sun, which drives external processes in the:

  • Atmosphere
  • Hydrosphere
  • At Earth’s surface

The Earth system is also powered by Earth’s interior.

  • Heat remaining from the formation and heat that is continuously generated by radioactive decay powers the internal processes that produce volcanoes, earthquakes, and mountains.

Earth’s Internal Structure

  • Earth is divided into three major layers by composition:
    • Crust – Earth’s thin, rocky outer skin, divided into the continental and oceanic crust
      • Oceanic crust is approximately 7 kilometers thick and composed of basalt
      • Continental crust is 35–70 kilometers thick and composed primarily of granodiorite
    • Mantle – approximately 2900 kilometers thick and composed of peridotite
    • Core – composed of an iron-nickel alloy
  • Earth’s interior is divided into different zones based on physical properties:
    • Lithosphere – the rigid outer layer of Earth that consists of the crust and the upper mantle.
    • Asthenosphere – the soft, weak layer below the lithosphere
    • Transition zone – a zone marked by a sharp increase in density below the asthenosphere
    • Lower Mantle (mesosphere) – a zone of strong, very hot rocks subjected to gradual flow below the transition zone
    • Outer core – liquid outer layer of the core
    • Inner core – solid inner layer of the core

Rocks and the Rock Cycle

Rocks are divided into three major groups:

  • Igneous
  • Sedimentary
  • Metamorphic

Features of the ocean floor include continental margins, deep-ocean basins, and oceanic ridges. Oceanic ridges are the most prominent feature on the ocean floor and are composed of igneous rock that has been fractured and uplifted.

Features of continents include mountain belts, cratons, shields, and stable platforms

  • Mountain belts are the most prominent features of continents.

Plate Tectonics

Rigid lithosphere overlies weak asthenosphere

  • The lithosphere is Earth’s strong, outer layer
  • The asthenosphere is a hotter, weaker region of the mantle under the lithosphere
  • Because of the differences in physical properties, the lithosphere is effectively detached from the asthenosphere, allowing layers to move separately.

Types of Plate Boundaries:

  • Divergent plate boundaries (constructive margins) – plates move apart
  • Convergent plate boundaries (destructive margins) – plates move together
  • Transform plate boundaries (conservative margins) – plates grind past each other without the production or destruction of lithosphere

Divergent Plate Boundaries

  • Also called spreading centers
  • New ocean floor is generated as two plates move apart

Convergent Plate Boundaries & Subduction

  • Destructive margins
  • Two plates move toward each other and the leading edge of one slides beneath the other
Oceanic–continental convergence
  • The denser oceanic slab sinks into the mantle beneath the buoyant continental block
  • At a depth of ~100 kilometers, melting is triggered when water from the subducting plate mixes with the hot rocks of the asthenosphere
  • This generates magma, resulting in a volcanic mountain chain called a continental volcanic arc
  • Examples include:
    • The Andes
    • The Cascade Range
Oceanic–oceanic convergence
  • When two oceanic slabs converge, one descends beneath the other. As with oceanic–continental convergence, partial melting initiates volcanic activity
  • If the volcanoes emerge as islands, a volcanic island arc is formed
  • Examples include:
    • The Aleutian Islands
    • The Mariana Islands
Continental–continental convergence
  • Continued subduction can bring two continents together
  • Less dense, buoyant continental lithosphere does not subduct
  • This results in continental collision and produces mountain belts of deformed rocks
  • Examples include:
    • The Himalayas
    • The Alps / The Appalachians

Transform Fault Boundaries

  • Also called a transform fault
  • Plates slide horizontally past one another, without production or destruction of lithosphere
  • Most occur on the seafloor joining two spreading centers
    • Known as fracture zones
  • Can move oceanic ridges toward subduction zones
  • A few transform faults cut through continental crust
    • Examples include:
      • The San Andreas Fault
      • The Alpine Fault of New Zealand

Evidence from Paleomagnetism

  • Basaltic rocks contain magnetite, an iron-rich mineral influenced by Earth’s magnetic field
  • When the basalt cools below the Curie point, the iron-rich minerals become magnetized and align with the existing magnetic field
  • The magnetite is then “frozen” in position and, like a compass needle, indicates the position of the north pole at the time of rock solidification

What Drives Plate Motions?

  • Researchers agree that convection in the mantle is the ultimate driver of plate tectonics
  • Forces that drive plate motion:
    • The subduction of cold, dense oceanic lithosphere is a slab-pull force
    • Elevated lithosphere at oceanic ridges will slide down due to gravity, causing the ridge-push force

Minerals

Atomic Structure

  • Atomic number
    • The number of protons in the nucleus of an atom
    • Determines the atom’s chemical nature
  • Atomic weight (mass number)
    • The number of protons and neutrons in the nucleus of an atom
  • Isotopes
    • Varieties of the same element that have different mass numbers; their nuclei contain the same number of protons but different numbers of neutrons

Chemical Bonding

  • Ionic bonding
    • Atoms gain or lose outermost (valence) electrons to form ions (positively and negatively charged atoms)
    • Ionic compounds consist of an orderly arrangement of oppositely charged ions
    • Ionic bond: the attraction of oppositely charged ions to one another
    • Example:
      • Halite (table salt)—NaCl
  • Covalent bonding
    • Atoms share one or more valence electrons
    • Attraction between oppositely charged particles:
      • Positively charged protons
      • Negatively charged electrons
    • Covalent compounds are generally stronger than ionic bonds
  • Metallic bonding
    • Valence electrons are free to migrate among atoms
    • Accounts for the high electrical conductivity of metals
    • Weaker and less common than ionic or covalent bonds

Mineral Properties

  • Color
    • Generally unreliable for mineral identification
    • Often highly variable due to impurities or slight changes in mineral chemistry
  • Streak
    • Color of a mineral in its powdered form
    • Obtained by rubbing mineral across a porcelain streak plate
    • Not every mineral produces a streak when rubbed across a streak plate
    • Although a mineral’s color may vary, its streak is usually consistent in color
  • Hardness
    • Resistance of a mineral to abrasion or scratching
    • All minerals are compared to a standard scale called the Mohs scale of hardness

Mineral Groups

  • Silicates
    • Most important mineral group
    • Comprise most rock-forming minerals
    • Very abundant due to large percentage of silicon and oxygen in Earth’s crust

Polymorphs

  • Minerals with identical composition but different crystalline structures
  • Examples include diamond and graphite, both made entirely of carbon atoms
  • Transforming one polymorph into another is called a phase change

The eight elements that make up the vast majority of rock-forming minerals represent more than 98% (by weight) of the continental crust!

  • Oxygen 46.6%
  • Silicon 27.7%
  • Aluminum 8.1%
  • Iron 5.0%
  • Calcium 3.6%
  • Sodium 2.8%
  • Potassium 2.6%
  • Magnesium 2.1%

Silicate Structures

Single tetrahedra are linked together to form various structures including:

  • Isolated tetrahedra
  • Ring structures
  • Single and double chain structures
  • Sheet or layered structures
  • Complex 3-dimensional structures
Silicate minerals with independent tetrahedra
  • One of the simplest silicate structures
  • Oxygen ions are bonded with positive ions (such as Mg2+, Fe2+, Ca2+)
    • Olivine (Mg, Fe)2SiO4
    • Garnet
Silicate minerals with three-dimensional framework
  • All oxygen ions are “shared” between tetrahedra
  • The most common silicate structure
  • Examples include:
    • Quartz
    • The feldspars
Other Silicate Structures
  • Pyroxene Group
    • Single chain structures involving iron and magnesium
  • Amphibole Group
    • Double chain structures involving a variety of ions
  • Mica Group
    • Sheet structures that result in one direction of perfect cleavage
    • Biotite is the common dark-colored mica mineral
    • Muscovite is the common light-colored mica mineral
  • Feldspar Group

Igneous Processes

  • Magma that crystallizes at depth forms plutonic or intrusive igneous rocks
    • These rocks are observed at the surface following periods of uplifting and erosion of overlying rocks
  • The solidification of lava or volcanic debris forms volcanic or extrusive igneous rocks

Igneous Rock Composition

Igneous rocks are divided into two broad groups:

  • Granitic (Felsic) versus Basaltic (Mafic) Compositions

Granitic or felsic composition

  • Light-colored silicates
  • Composed almost entirely of quartz and potassium feldspar
  • Termed felsic (feldspar and silica) in composition
  • High silica (SiO2) content
  • Contain about 10% dark silicate minerals
  • Major constituent of continental crust

Basaltic or mafic composition

  • Contain at least 45% dark silicates and calcium-rich feldspar
  • Contain no quartz!
  • Termed mafic (magnesium and ferrum, for iron) in composition
  • Higher density than granitic rocks
  • Comprise the ocean floor and many volcanic islands
  • Also forms extensive lava flows on the continents

Origin of Magma

  • Earth’s crust and mantle are primarily composed of solid rock
  • Magma is generated in the uppermost mantle
    • Greatest amounts are produced at divergent plate boundaries
    • Lesser amounts are produced at subduction zones
    • Can also be generated when crustal rocks are heated sufficiently to melt

Generating Magma from Solid Rock

  • Geothermal gradient: temperatures in the upper crust increase about 25°C per kilometer
  • Rocks in the lower crust and upper mantle are near their melting points
    • Tectonic processes trigger melting by reducing the melting point
      • Decrease in pressure
      • Addition of water
      • Increase in temperature of crustal rocks

Decompression Melting

  • Melting occurs at higher temperatures with increasing depth (and increasing confining pressure)
  • Reducing confining pressure lowers the melting temperature = decompression melting
  • Solid, hot mantle rocks will ascend to regions of lower pressure, inducing melting
    • Divergent plate boundaries
    • Mantle plumes at hot spots

Addition of Water

  • Water and other volatiles act as salt does to melt ice
    • Causes rock to melt at lower temperatures
  • Occurs mainly at subduction zones