Earth's crust and plate tectonics shape our planet's surface. From igneous, sedimentary, and metamorphic rocks to the movement of massive tectonic plates, these processes create diverse landscapes and geological features we see today.

Plate boundaries, fault zones, and volcanic activity are key players in Earth's dynamic system. Understanding these processes helps us grasp how mountains form, earthquakes occur, and volcanoes erupt, giving us insight into our planet's ever-changing face.

Earth's Crust and Plate Tectonics

Main rock types of Earth's crust

• Igneous rocks form from the cooling and solidification of magma (beneath Earth's surface) or lava (on Earth's surface)
  • Intrusive igneous rocks (granite) form from magma that cools slowly underground
  • Extrusive igneous rocks (basalt, obsidian) form from lava that cools quickly on the surface
• Sedimentary rocks form through the deposition and compression of sediments (sand, silt, clay) over time
  • Clastic sedimentary rocks (sandstone, shale) consist of rock and mineral fragments
  • Chemical sedimentary rocks (limestone) form from the precipitation of minerals from water
• Metamorphic rocks form when existing rocks are subjected to high heat and pressure, causing them to change in texture and mineral composition without melting
  • Foliated metamorphic rocks (slate, schist) have a layered or banded appearance
  • Non-foliated metamorphic rocks (marble, quartzite) have a more uniform texture

Key principles of plate tectonics

• Earth's lithosphere (crust and upper mantle) is divided into several large, rigid plates that move relative to each other
• Plate motion is driven by convection currents in the mantle, which are caused by heat transfer from Earth's hot interior
• Plates interact at three main types of boundaries:
  1. Divergent boundaries (Mid-Atlantic Ridge) where plates move away from each other, creating new oceanic crust
  2. Convergent boundaries (Andes Mountains) where plates collide or subduct, causing mountain building, volcanism, and earthquakes
  3. Transform boundaries (San Andreas Fault) where plates slide past each other horizontally, causing frequent earthquakes
• Plate tectonics explains the formation and distribution of various geological features (mountains, volcanoes, rift valleys, ocean basins) across Earth's surface
• The theory of plate tectonics evolved from the concept of continental drift, proposed by Alfred Wegener in the early 20th century

Rift zones vs subduction zones

• Rift zones occur at divergent plate boundaries where plates move away from each other
  • Characterized by the stretching and thinning of the lithosphere, causing the formation of rift valleys (East African Rift)
  • Accompanied by the upwelling of hot mantle material, leading to volcanism and the formation of new oceanic crust (Mid-Atlantic Ridge)
  • Seafloor spreading occurs at oceanic rift zones, creating new oceanic crust and pushing plates apart
• Subduction zones occur at convergent plate boundaries where one plate sinks beneath another
  • Characterized by the formation of deep-sea trenches (Mariana Trench) and volcanic arcs (Andes Mountains)
  • Associated with intense seismic activity as the subducting plate descends into the mantle, causing earthquakes and tsunamis (Japan, Indonesia)

Fault zones in mountain formation

• Fault zones are areas where rocks have been fractured and displaced due to tectonic forces
• Three main types of faults contribute to mountain formation:
  1. Normal faults (Basin and Range Province) where the hanging wall moves downward relative to the footwall, creating horsts and grabens
  2. Reverse faults (Himalayas) where the hanging wall moves upward relative to the footwall, causing the uplift and thickening of the crust
  3. Strike-slip faults (San Andreas Fault) where blocks move horizontally past each other, causing lateral displacement and local compression or extension
• Compression along convergent plate boundaries leads to the folding and faulting of rock layers, resulting in the uplift and formation of mountain ranges (Rocky Mountains, Alps)
• Orogeny, the process of mountain building, involves complex interactions of tectonic forces, faulting, and rock deformation

Types of volcanic activity

• Shield volcanoes (Mauna Loa, Olympus Mons) have broad, gently sloping flanks and effusive eruptions of fluid, basaltic lava
  • Formed by the accumulation of numerous lava flows over time
  • Characterized by low-viscosity lava that can travel great distances from the vent
• Stratovolcanoes (Mount Fuji, Mount Rainier) have steep, conical shapes and explosive eruptions of viscous, silica-rich magma
  • Formed by alternating layers of lava flows, ash, and pyroclastic material
  • Characterized by high-viscosity lava that tends to pile up near the vent, creating a steep profile
• Cinder cones (Parícutin, Sunset Crater) are small, steep-sided volcanoes built from ejected lava fragments called cinders or scoria
  • Formed during a single eruption or a series of brief, intermittent eruptions
  • Characterized by a circular or oval-shaped crater at the summit
• Lava domes (Mount St. Helens, Soufrière Hills) form when viscous lava accumulates and solidifies near the vent
  • Often associated with explosive eruptions due to the buildup of gas pressure within the magma
  • Characterized by a bulbous or dome-shaped mass of lava that can collapse or explode violently

Earth's crust and mantle dynamics

• Isostasy describes the gravitational equilibrium between Earth's crust and mantle, explaining how the crust "floats" on the denser mantle
• Seismology, the study of seismic waves, provides crucial information about Earth's internal structure and helps identify plate boundaries and fault zones