2.3 Plate Boundaries and Associated Landforms

Earth's crust is like a jigsaw puzzle, with pieces called tectonic plates. These plates move, creating boundaries where they meet. At these boundaries, amazing things happen: mountains rise, volcanoes erupt, and earthquakes shake the ground.

Plate boundaries come in three flavors: divergent (plates move apart), convergent (plates collide), and transform (plates slide past each other). Each type creates unique landforms and geological features, shaping our planet's surface in dramatic ways.

Plate Boundaries: Divergent, Convergent, Transform

Types and Characteristics of Plate Boundaries

• Divergent boundaries form where two plates move away from each other, creating new oceanic crust through seafloor spreading (mid-ocean ridges, rift valleys)
• Convergent boundaries occur where two plates collide, resulting in subduction of one plate beneath another or collision of two continental plates (subduction zones, deep-sea trenches, mountain ranges)
• Transform boundaries exist where two plates slide past each other horizontally, causing friction and stress buildup (San Andreas Fault)

Plate Motions and Interactions

• Plates move apart at divergent boundaries, allowing magma to upwell and create new oceanic crust
• At convergent boundaries, plates collide, causing subduction of the denser plate or collision and uplift of continental crust
• Transform boundaries involve plates sliding past each other, generating friction and stress along the fault line

Landforms and Geological Features of Plate Boundaries

Divergent Boundary Landforms

• Mid-ocean ridges form long, underwater mountain ranges where new oceanic crust is created (East Pacific Rise, Mid-Atlantic Ridge)
• Rift valleys develop as the crust stretches and thins, creating deep, linear depressions (East African Rift Valley)
• Submarine volcanoes and hydrothermal vents are common features associated with seafloor spreading and magmatic activity

Convergent Boundary Landforms

• Deep-sea trenches form where the subducting plate bends and descends into the mantle (Mariana Trench, Peru-Chile Trench)
• Volcanic island arcs develop above subduction zones as magma generated by the melting of the subducting plate rises to the surface (Aleutian Islands, Caribbean Islands)
• Mountain ranges form through the collision and uplift of continental crust (Himalayas, Andes)

Transform Boundary Landforms

• Linear fault lines mark the boundary where plates slide past each other (San Andreas Fault, Alpine Fault)
• Offset landforms, such as displaced stream channels or ridges, are characteristic of transform motion
• Shallow earthquakes are common along transform faults due to the buildup and release of stress

Processes at Plate Boundaries

Seafloor Spreading and Subduction

• At divergent boundaries, upwelling magma from the mantle creates new oceanic crust, which is pushed away from the ridge as it cools and solidifies
• Subduction occurs at convergent boundaries where the denser oceanic plate sinks beneath the less dense continental plate, leading to the formation of deep-sea trenches, volcanic arcs, and earthquakes
• The subducting plate melts as it descends, releasing fluids that trigger partial melting in the overlying mantle and generate magma

Mountain Building and Isostatic Adjustment

• Mountain building, or orogeny, occurs at convergent boundaries where continental plates collide, causing the crust to thicken and uplift (Himalayas formed by the collision of the Indian and Eurasian plates)
• Isostatic adjustment is the process by which the Earth's crust and mantle equilibrate to maintain balance, causing vertical movement of landmasses in response to the removal of overlying rock, ice, or the addition of sediments (post-glacial rebound in Scandinavia)

Plate Boundaries and Earthquakes vs Volcanoes

Earthquake Distribution and Characteristics

• Earthquakes are most common along plate boundaries, where the interaction between plates causes stress buildup and sudden release
• Shallow earthquakes occur along divergent and transform boundaries, where plates are moving apart or sliding past each other (East African Rift Valley, San Andreas Fault)
• Deep earthquakes occur along convergent boundaries, particularly in subduction zones, as the subducting plate descends into the mantle (Wadati-Benioff zones)

Volcano Distribution and Characteristics

• Volcanoes are concentrated along convergent and divergent boundaries, where magma is generated and able to reach the surface
• At divergent boundaries, volcanoes are typically submarine and produce basaltic lava due to decompression melting of the mantle (Iceland, Mid-Atlantic Ridge)
• At convergent boundaries, volcanoes are often subaerial and produce more viscous, explosive eruptions due to the melting of the subducting plate and the mixing of magmas (Mount St. Helens, Mount Fuji)

Intraplate Seismicity and Volcanism

• Intraplate earthquakes and volcanoes can occur away from plate boundaries, often due to hot spot activity or local tectonic stresses within plates
• Hot spots are fixed regions of magmatic activity that can create chains of volcanoes as the plate moves over the mantle plume (Hawaiian Islands, Yellowstone)
• Intraplate earthquakes may be caused by the reactivation of ancient fault lines or the buildup of stress within the plate interior (New Madrid Seismic Zone)