Earth’s Dynamic Processes: Plate Tectonics and Continental Drift

Posted on Nov 23, 2024 in Geology

DYNAMIC EARTH

The Earth is not a static layer of rock but is in continuous transformation.

A) The Earth’s Crust

  • The continental crust has a thickness of 35-40 km, with a varied mineral composition (outer granitic and basaltic layers).
  • Continents extend hundreds of kilometers under the oceans in the so-called continental shelf, ending in a slope called the continental slope.
  • The oceanic crust has a homogeneous thickness of about 7 km, with a basaltic composition and young rocks. It occupies more than 60% of the Earth’s surface and has a greater density than the continental crust.
  • The crust is the layer of solid minerals covering the Earth’s surface. Its study has been slow due to the lack of methods or instruments capable of obtaining direct evidence.
  • Indirect data about the Earth’s inner layers were provided by the study of seismic waves and their transmission mechanisms.

Early Theories

  • Scientists in the late 19th century supported the cooling-contraction theory, which stated that the Earth cools, contracts, and causes surface cracking (faults) and folds (mountains).
  • Another hypothesis attributed the formation of continents to convection currents in the Earth’s mantle (the layer below the crust composed of molten materials at high temperatures). The mantle would rise when heated at depth and sink when cooled in contact with the crust.

Paleontological-Biological Enigma

Currently isolated animals share a common ancestor (e.g., ratites: ostrich and cassowary).

Geographical and Geological Enigmas

B) Hypothesis of Continental Drift

  • In 1915, Alfred Wegener proposed the hypothesis of continental drift to explain the past existence of a large landmass called Pangea, which divided and drifted to form the current continents.
  • He defended his idea until his death (Greenland, 1930). Although initially rejected by many scientists, it is now widely accepted.
  • Supporting evidence includes paleomagnetic and oceanographic tests. After World War II, the oceanic ridge system was discovered, a vast network of underwater volcanic activity with a central depression (Rift Valley) surrounded by submerged mountain chains.

C) Hypothesis of Seafloor Spreading

  • In the 1970s, Harry Hess proposed that the upward convective mantle collided with the oceanic crust, creating vertical thrusts and oceanic ridges.
  • The oceanic crust was displaced to either side of the ridge, while magma broke through at the ridge’s center, cooling and forming new oceanic crust.
  • Conversely, the mantle flow, due to increased density, dragged the oceanic crust down into trenches. This is a continuous cycle of creation at the ridges and destruction in the trenches.
  • Evidence for seafloor spreading came from various sources, most importantly paleomagnetism. It was discovered that the Earth’s magnetic field undergoes periodic reversals, with the polarity of the poles switching. Lava solidifying during these periods shows a pattern of opposite polarity to the current one.
  • The oceanic crust is created at both sides of the oceanic ridge, and as new crust forms, it pushes and moves laterally the older crust.

D) Plate Tectonics

  • The theory of plate tectonics, developed in the 1960s, states that the Earth’s lithosphere is divided into plates of great length and variable thickness (50-150 km) that move horizontally on the asthenosphere.
  • These plates interact at their boundaries, explaining:
    • The existence of mountain ranges on the ocean floor
    • The lack of sediment on the ocean floor
    • Magnetic anomaly bands on the seafloor
    • The distribution of earthquakes and active volcanoes
    • The origin of mountain ranges
    • The increasing depth of earthquake hypocenters as we move away from coasts
  • Isostasy is the adjustment mechanism that explains the vertical movements of the crust.

Causes of Plate Movement

1) Convection Currents

Convection currents exist in the asthenosphere, where warmer, less dense material rises, comes into contact with the crust, is forced to flow horizontally, dragging the plates, and then cools, increases in density, and sinks to deeper layers.

2) Drag of Plates

Newly formed oceanic crust at the ridges has a high temperature and low density. As it moves away from the ridge, it gradually cools, increasing its density until it becomes denser than the asthenosphere, causing it to sink.

3) Push Plate

The oceanic crust is created at the ridges, which are raised areas of the seabed. Gravity and the thrust of the new crust push the older crust towards deeper areas of the seabed.

Types of Plates

  • Oceans: Oceanic crust (Pacific, Philippines)
  • Mixed: Continental and oceanic crust (Eurasian, Indo-Australian)
  • Continental: Continental crust (Iranian)

Plate Size

  • Great (100-150 million km2): American, South American
  • Medium (20-60 million km2): Eurasian, African
  • Small (10 million km2): Juan de Fuca, Arabian

Plate Boundaries

Divergent or Constructive Boundaries
  • Correspond to ridges, characterized by:
    • Rugged submarine ridges (width 1000-4000 km, height 3 km, length 64,000 km)
    • Formed by volcanic rocks
    • Intermittent seismic activity (moderate)
    • A central Rift Valley where plates separate
    • The axis is interrupted by transform faults
Convergent or Destructive Boundaries
  • Occur where one plate subducts beneath another.
  • The Benioff zone is a plane where one plate penetrates beneath another.
Continental-Oceanic
  • An oceanic plate sinks beneath a denser continental plate, creating a subduction zone and a trench (e.g., the Andes).
  • Oceanic trenches are deep, elongated depressions with variable width.
  • Sediments are deformed due to plate pushing, forming an accretionary prism.
  • High volcanic and seismic activity.
  • Magmatism is generated due to increased temperature and pressure at depth, fusing oceanic and continental lithosphere and forming volcanic arcs.
Oceanic-Oceanic
  • One oceanic plate subducts beneath another, creating a trench and an island arc (e.g., Japan).
  • Island arcs are strings of volcanic islands formed on the edge of the subducting plate.
Continental-Continental
  • Two continental plates of similar density collide, with neither subducting.
  • Subduction of oceanic lithosphere can lead to the formation of a volcanic island arc.
  • The collision of two continental blocks forms an intracontinental mountain range (e.g., the Himalayas).
Transform or Conservative Boundaries
  • At these boundaries, lithosphere is neither created nor destroyed; they are called passive margins.
  • Plates move laterally in opposite directions along a transform fault, producing friction and frequent earthquakes.
  • Seismicity is very important at these boundaries.