Plate Tectonics: Boundaries, Magmatism, and Earthquakes

Class logistics and planning

• Students were reminded they can ask questions via text or social media; the key is to ask four people a question on Wednesday.
• Quiz 2 will be posted today and is due on Thursday, the eighteenth. It covers material from Tuesday and today.
• Friday lab section: you may take the quiz whenever it suits you.
• Tuesday section: it may be beneficial to take the quiz after completing the lab (second part after the lab) to solidify concepts; you may choose to take it before if schedule dictates.
• If there are technical glitches with Canvas or a link, please report them before starting or when you encounter an issue; the instructor will fix issues promptly.
• The instructor emphasizes not to “muscle through” a broken process and to report issues so grading isn’t unfairly affected.
• Plan for today: focus on plate tectonics — three plate boundary types and what happens to the crust at each boundary; in-class sketch activity; follow-up with a key posted on campus.
• In-class activity: draw and label the divergent, convergent, and transform boundaries; later compare with a posted key.
• The broader aim is to connect earthquake and volcano activity to plate boundaries, and to revisit these processes at the end of the unit.

Learning goals for today

• Describe the three types of plate boundaries and what happens to the crust in each example.
• Be able to sketch each type of boundary (in-class activity with a sketch sheet; a key will be provided after class).
• Identify features such as deep-sea trenches, underwater mountain belts, rift valleys, and mid-ocean ridges.
• List processes that occur at plate boundaries and the relationship between earthquakes and volcanoes; locate these processes relative to plate boundaries.
• Understand how plate tectonics can be a unifying theory for geoscience concepts.

Global map context and boundary terminology

• Three main plate boundary options when plates meet:
  • Divergent: plates move apart.
  • Convergent: plates come together (collision/subduction).
  • Transform: plates slide past one another.
• Sub-types by crust type:
  • Divergent: oceanic crust vs continental crust (two pathways for opening space).
  • Convergent: ocean-ocean, ocean-continent, continent-continent (three possibilities).
  • Transform: can involve ocean crust or continental crust.
• Global view (color-coded map conventions):
  • Red boundaries = convergent boundaries.
  • Pale yellow boundaries = divergent boundaries.
  • Orange boundaries = transform boundaries.
• Classic example of transform boundary: San Andreas Fault (Pacific Plate vs North America Plate).
• Transform boundaries often link divergent boundaries or convergent margins.
• Earthquakes and volcanoes tend to be located at or near plate boundaries, but the exact distribution depends on the boundary type.

How plate tectonics explains earthquakes and volcanoes

• At plate boundaries, edges are stuck due to friction; the rest of the plate continues moving, causing deformation right at the boundary.
• Deformation stores up potential energy in rocks; when friction is overcome, rapid movement occurs, releasing energy as seismic waves (P-waves and S-waves).
• The energy release results in earthquakes and surface shaking.
• The velocity and scale of movement can be dramatic: rock movement during an earthquake can reach about ext{order of } 50 ext{ m} in a few seconds.
• The pattern and depth of earthquakes reflect the boundary type and subduction geometry; note the relation between earthquakes and the distribution of melting and volcanism.

Divergent plate boundaries

• Basic idea: plates move apart; a gap opens and magma upwells to fill the space, creating new crust.
• Crustal outcomes:
  • Oceanic divergence creates new ocean crust and mid-ocean ridges.
  • Continental divergence forms rift valleys that can become new oceans if the rifting continues.
• Seismic activity:
  • Earthquakes are shallow and occur right at the boundary (crust is thin here).
  • Depth range for these earthquakes is typically 0 \, \le \; d \; \le \; 30 \text{ km}.
• Magmatic activity:
  • Upwelling magma fills cracks; occasional magma intrusions and rift volcanism can occur, more common underwater but also on land (continental rifts).
• Key features and examples:
  • Rift valleys form due to tensional forces pulling crust apart (block drops and downthrown blocks).
  • Melting near the surface can produce volcanoes as crust stretches; rift-related volcanism is more common underwater.
  • Classic terrestrial example: East Africa Rift; thinning crust leads to low-lying topography and potential for sea opening over geological time.
  • Continental breakup example: Red Sea (progression from a continental rift to an ocean basin).
  • Oceanic divergence example: Mid-Atlantic Ridge running through the Atlantic Ocean; oceanic crust is created at the ridge.
  • Iceland as an active example of a divergent boundary with a hotspot influence along the Mid-Atlantic Ridge.
• Quantitative note: eastern Africa rift rate is on the order of v \approx 30 \ \text{mm/yr} (varies by location).
• Relationship to volcanoes: rifting can produce “drift volcanoes” where crust is being stretched; volcanoes are commonly seen near divergent boundaries, especially where crust is thinner.

Convergent plate boundaries

• Basic idea: plates collide; outcomes depend on crust types involved and subduction dynamics.
• Subduction (one plate dives beneath another): occurs when oceanic crust is involved (ocean-ocean or ocean-continent).
  • Oceanic crust is denser and sinks beneath the overriding plate; the overriding plate is typically land or less dense ocean crust.
  • In ocean-ocean convergence: the older, colder, denser ocean crust subducts; results in a deep-sea trench and volcanic island arcs on the overriding plate.
  • In ocean-continent convergence: the oceanic plate subducts under the continental plate; results in a continental volcanic arc and a trench on the ocean side.
  • Water released from subducting slab (at about d \approx 150 \text{ km} depth) lowers the melting point of the mantle above, generating magma that rises to form volcanoes on the overriding plate.
  • Volcanoes at subduction zones are typically offset from the trench and form a volcanic arc parallel to the trench; distance from trench depends on slab dip angle.
  • Deep earthquakes are common along the subducting slab; the deepest earthquakes occur at subduction zones.
• Continental-continental collision (no subduction): when two continental plates collide, their buoyant crust resists subduction; the result is crustal thickening and mountain building, with no associated volcanism because there is little to no mantle melting (no subducted oceanic crust to release volatiles).
  • Example: Himalayas (India-Asia collision).
  • Features: high mountains, suture zones, deep and shallow earthquakes along the collision zone; no volcanoes.
• Notable examples:
  • Ocean-ocean convergence: Mariana Trench and the Mariana Islands form a volcanic island arc; deep trenches and a chain of volcanic islands parallel to the trench.
  • Ocean-continent convergence: Andes Mountains (western South America) with a continental volcanic arc and a trench offshore.
  • Cascadia region (Pacific Northwest in North America): subduction of the Juan de Fuca plate beneath North America; associated with Cascade Range volcanoes (e.g., Mount Saint Helens).
  • India-Asia collision: extensive mountain-building and crustal thickening; ongoing, but volcanism largely ceases after oceanic crust is consumed.
  • Example questions used in class: Tohoku earthquake (2011, Japan) – convergent boundary with oceanic-continent collision; Kathmandu earthquake (2015, magnitude M \approx 7.8) – continental-continental collision; Haiti earthquake (2010, magnitude M \approx 7.0) – transform boundary between the Caribbean and North America plates; 2023 Turkey-Syria earthquake – transform boundary event involving relative plate motion.
• Key takeaways:
  • Subduction zones produce deep earthquakes, volcanic arcs, and trenches; the arc location depends on slab dip and angle of subduction.
  • Oceanic crust is recycled at subduction zones; continents do not subduct beneath other continents.
  • Continental collisions produce high mountain belts without active volcanism; the process can persist for millions of years.

Transform boundaries

• Definition: plates slide horizontally past one another; motion is parallel to the boundary, creating shear stress.
• Fault type: transform faults; often connect other boundary types (e.g., linking segments of mid-ocean ridges or connecting a ridge to a subduction zone margin).
• Earthquakes: typically shallow because there is no significant rendering of subducting material; no volcanism is expected at transform boundaries.
• Movement pattern: offsets between plate segments; the total length of the transformed segment remains roughly constant while surrounding crust is added on either side at divergent boundaries.
• Classic example: San Andreas Fault (Pacific Plate vs North American Plate).
• Consequences of transform motion: directional offset of landscapes (e.g., roads, fences, coastline features) alongside the boundary; long-term potential for major city-scale displacement over geological timescales.
• Visual intuition: if two blocks slide past each other with a boundary in between, the material experiences shear, not compression or extension.

Earthquakes, volcanoes, and depth patterns by boundary type

• Divergent boundaries:
  • Earthquakes: shallow and right at the boundary (thin crust).
  • Volcanoes: possible where upwelling magma causes crustal melting; more common underwater; on land, associated with continental rifting.
  • Depth pattern: generally shallow only.
• Transform boundaries:
  • Earthquakes: shallow; no melting; no volcanoes.
  • Depth pattern: shallow only.
• Convergent boundaries with subduction:
  • Earthquakes: shallow to deep as distance from the trench increases along the downgoing slab (shallower near the plate interface; deepest earthquakes trace the downgoing slab geometry).
  • Volcanoes: present on the overriding plate (volcanic arcs); location offset from the trench and dependent on slab angle.
  • Depth pattern: a continuum from shallow near the boundary to deep within the subducting slab; a characteristic rainbow-drawn pattern on maps highlights increasing depth with distance from the trench.
• Continental-continental convergent boundaries:
  • Earthquakes: can be shallow to deep within the collision zone (suture zone).
  • Volcanoes: generally absent because there is no subduction and thus little mantle melting to generate magma.
  • Depth pattern: not associated with a volcanic arc.

Case studies and real-world examples

• East Africa Rift: example of a continental divergent boundary; active rifting with thin crust, shallow earthquakes, and localized volcanism; tectonic stretching visible in recent aerial/ground observations.
• Red Sea: intermediate stage of continental breakup with an incipient ocean basin.
• Iceland: active divergence along the Mid-Atlantic Ridge with hotspot influence; prominent rifting and volcanic activity.
• Andes Mountains (Chile, Peru): classic example of oceanic crust subducting beneath a continental plate; deep trench offshore; a long volcanic arc on the continental margin.
• Cascade Range (Pacific Northwest): subduction of the Juan de Fuca plate beneath North America; active volcanoes such as Mount Saint Helens; illustrates ocean-continent subduction.
• Himalayan Mountains: India-Asia continental collision; high mountains; extensive crustal deformation; no active volcanism due to the lack of subduction.
• San Andreas Fault: transform boundary linking the Pacific and North American plates; features offset geography and shallow earthquakes.
• Caribbean and North American plates: transform boundary examples (e.g., 2010 Haiti earthquake) illustrate the transform boundary earthquake context.
• Tohoku earthquake (2011, Japan): convergent boundary with oceanic-continental subduction; notable for its large magnitude and tsunami implications (magnitude not specified in transcript).
• Kathmandu earthquake (2015): continental-continental collision; magnitude M \approx 7.8; significant crustal deformation with no volcanism.
• Turkey-Syria earthquake (2023): transform boundary event; notable for strike-slip motion along plate margins.

Sketching and practice tips for today’s activity

• Two main sketch formats:
  • Cross-section view: vertical slice through the Earth to show plate movement and subsurface processes.
  • Map view: top-down view showing surface boundaries and surface features.
• Divergent boundary sketch guidance:
  • Draw two plates moving apart (arrows pointing away from the boundary).
  • Indicate earthquakes with dots along the boundary.
  • Draw melting zones and volcanoes near the boundary (on or near the boundary for shallow magma intrusion).
  • For a cross-section, show an upwelling mantle and thinning crust; for a map view, show a ridge and potential volcanic centers parallel to the boundary.
• Convergent boundary sketch guidance (subduction):
  • Ocean-ocean or ocean-continent: draw one plate subducting beneath the other, with a trench at the boundary.
  • Indicate a volcanic arc on the overriding plate and a trench at the boundary.
  • Show earthquakes that begin shallow near the boundary and deepen with distance from the trench; in a cross-section, illustrate the subducting slab descending with depth.
  • For continent-continent collision: draw two thick, buoyant plates colliding, uplift to form mountains, presence of suture zone, earthquakes along the collision zone, no volcanism.
• Significance of the trench, ridge, and volcanic arcs:
  • Trenches mark subduction zones; ridges mark zones of new crust formation; volcanic arcs indicate magmatic activity due to subduction.
  • Offshore vs on-land differences: rifting and volcanism can be underwater or on land depending on crust type.

Unifying theory and broader implications

• Plate tectonics as a unifying theory:
  • Explains the global distribution of earthquakes and volcanoes.
  • Connects features such as mountain belts, ocean basins, and volcanic arcs with plate interactions.
  • Provides a framework for understanding past, present, and future configurations of Earth’s lithosphere (e.g., supercontinents, continental drift, and ocean basin formation).
• Practical implications:
  • Hazard assessment: understanding boundary types helps explain where earthquakes and volcanoes are likely to occur and how deep they may be.
  • Environmental and societal relevance: major earthquakes and volcanic episodes have long-term impacts on landscapes, infrastructure, and risk planning.

Quick takeaway recap

• Boundaries:
  • Divergent: pull apart; create new crust; shallow earthquakes; potential rift valleys and volcanic activity.
  • Convergent: collide; subduction (oceanic crust sinks) or continental collision (mountains form); volcanic arcs or non-volcanic uplifts; earthquakes across a depth range; volcanism tied to subduction.
  • Transform: slide past; shallow earthquakes; no volcanism; boundaries link other boundary types and create offset landscapes.
• Earthquake depth patterns reflect boundary type and subduction geometry; maps colored by depth reveal the geometry of downgoing slabs.
• Real-world examples (case studies) reinforce how boundary dynamics shape topography and seismic/volcanic activity.

Optional reflection prompts

• How does the angle of subduction influence the distance of volcanoes from the trench?
• Why are some coastlines considered passive margins while others lie near active boundaries with earthquakes and volcanoes?
• In what ways do subduction processes recycle oceanic crust and drive mountain building in adjacent continental crust?
• How do different boundary types explain the distribution of the major earthquake and volcanic belts around the world?