Lecture 5 for test 1
Transform boundaries
The transform boundary is highlighted as the best-studied, most well-exposed example when asked about plate boundaries.
The classic example given: the San Andreas Fault, a transform boundary that connects ridges.
Location context in Southern California region: east of the Southern Sea, through the Inland Empire (Riverside, San Bernardino), through the Mojave Desert, north of the San Gabriel Mountains, and extending toward Los Angeles, San Diego, and up toward San Francisco.
The boundary “transforms” connect ridges and accommodate plate motion without creating or destroying lithosphere.
Directionality in plate labels: we typically refer to plate directions using geographic directions (east, north) rather than left/right on a map to avoid misinterpretation when students look at maps.
In-class prompt examples: identify boundaries by watching motion on maps; the discussed boundary is
Divergent: Mid-Ocean Ridge (new lithosphere formed as asthenosphere rises and lithosphere is created at ridges).
Convergent: Subduction zones (oceanic crust being pushed down into the mantle).
Continental collision: Convergent boundary involving continental crust.
Transform: Connects ridges and slides past one another (San Andreas is the prototypical example).
Hotspots are introduced as separate from plate boundaries and are discussed in a later section; the hotspot concept is used to explain island chains that form as plates move over relatively stationary mantle plumes.
Some plate motion questions were posed to students to illustrate concepts like the direction of plate motion and the relative motion across boundaries.
Hotspots and mantle plumes
Hotspots are mantle plumes that originate deep in the mantle and rise by convection due to their high temperature and lower density.
The mantle plume itself is largely stationary; the tectonic plate moves over it.
The result is a chain of volcanoes/islands that get progressively older away from the current hotspot position.
Hawaii is the best-studied hotspot example; Pele the goddess of volcanoes is used as a narrative to personify the hotspot under Hawaii.
Key Hawaii chain observations:
The Big Island (Hawaii) is the youngest island, currently over the hotspot, and is the most volcanically active.
Islands farther along the chain toward the northwest get progressively older (e.g., Kauai is among the oldest islands mentioned, with ages in the range of millions of years).
The age progression along the chain reflects plate motion over the stationary hotspot: the plate moves, and the hotspot creates a new island where it sits, aging islands as the plate moves away.
Lōʻihi is a submarine volcano just southeast of the Big Island; it is expected to become the next above-sea island in the chain in roughly 5 imes 10^5\ ext{years} (about five hundred thousand years).
Direction of plate motion relative to the hotspot in Hawaii is toward the northwest; thus the island chain trends northwest with older islands farther from the current hotspot.
The underlying mechanism for island growth: a mantle plume feeds magma to surface volcanism, creating a volcanic island over the hotspot; the lithosphere above the plume is buoyant due to the heat and low density, contributing to surface uplift.
Plate tectonics context:
Oceanic lithosphere melts more easily than continental lithosphere; mantle rocks melt in the melting region and erupt as basaltic lava at surface.
Subduction zones and mid-ocean ridges represent complementary processes that regulate the Earth’s mass balance (new lithosphere at ridges, consumption of lithosphere at trenches).
Hawaii-specific notes:
Mauna Loa and Kilauea are two of the most active volcanoes in Hawaii; Mauna Loa is the world’s most massive volcano when measured from the ocean floor to the summit.
Kilauea has exhibited nearly continuous lava flows since 1983, contributing hundreds of acres to the island (e.g., ~550 acres).
Lōʻihi remains submerged and is the next stage in the hotspot track; as the plate continues to move, Lōʻihi is expected to become an above-sea island in the future.
The Hawaii volcanic system produces basalt lava; eruptions near the summit and flows to the ocean are common.
Formation of volcanic islands over hotspots is a key example of how volcanic activity can be decoupled from plate boundaries.
An animation illustrating hotspot stability shows a plate moving over a stationary plume; as the plate moves, the plume appears stationary while the surface volcanoes form and then become inactive as the plate moves away.
Observational takeaway: the progressive aging of islands along the Hawaiian chain supports the hotspot theory and plate motion hypothesis.
Anecdotal note: there is a caution about potentially dangerous lava flows and “blowouts” during eruptions, highlighting the ongoing hazard around active volcanic islands.
Hawaii Volcanoes and the volcanic cycle
Hawaii Volcanoes National Park includes two supervolcanoes (Mauna Loa and Kilauea) and features Luihi (Lōʻihi) as part of the hotspot track.
Mauna Loa is described as the world’s most massive volcano when measured from the ocean floor to the summit.
Kilauea is one of the world’s most active volcanoes with long-running lava flows since 1983, significantly contributing to the growth of the island.
Luihi (Lōʻihi) is the underwater volcano currently rising toward the surface; it is located near the Big Island and represents the next stage in the island chain’s growth.
The Big Island is the only currently active island in Hawaii; the activity shifts to other islands as the plate moves over the hotspot.
The island chain’s age progression is explained by the fixed hotspot and the northwestward motion of the Pacific Plate.
Basalt lava is produced by melting of oceanic lithosphere; the basalt lava flows down to the coast and into the ocean; Hawai‘i’s volcanism produces fluid lava flows characteristic of shield volcanoes.
The volcanic activity is intense and dynamic; rapid changes can occur on daily timescales in Hawaii, unlike most other geological settings.
Plate tectonics, convection, and mantle dynamics
The Earth’s interior drives plate tectonics via convection in the mantle: hot mantle rocks rise due to lower density and buoyancy, then cool and sink, creating convective cells.
The mantle is mostly solid rock (the asthenosphere behaves plastically under pressure, enabling plate motion).
Heat sources include residual heat from Earth’s formation and ongoing radiogenic heating from elements such as uranium (U), potassium (K), and thorium (Th). The decay of these radioactive elements releases heat, slowing the Earth’s cooling.
Convection in the mantle provides the driving force for plate tectonics, with forces including slab pull (gravity-driven sinking of dense oceanic lithosphere at subduction zones) and ridge push (new lithosphere at mid-ocean ridges pushing the plates apart).
The cycle of mantle convection, ridge creation, and subduction explains the global distribution of earthquakes, mountain belts, and volcanic arcs.
The creation of new lithosphere at ridges and its destruction at trenches balances the Earth’s mass and drives plate motions.
Earthquakes: basics and real-world examples
Earthquakes are caused by the rapid release of energy as rocks fracture and slip along faults.
The site of an earthquake is described by two terms:
Focus (hypocenter): the location within the Earth where the earthquake originates; depth is typically within the crust and often shallow (on the order of the first 15 km, i.e., the crustal lithosphere).
Epicenter: the point on the Earth’s surface directly above the focus.
Ground shaking is caused by seismic waves radiating from the focus outward in all directions.
Plate boundaries are the most seismically active regions because rocks are under high stress and prone to slipping along faults.
The San Andreas Fault is a classic example of a transform boundary with predominantly horizontal (strike-slip) motion.
Earthquakes around the Pacific Rim are concentrated along subduction zones (the Pacific Ring of Fire) where oceanic plates dive beneath adjacent plates.
Megathrust earthquakes are the largest earthquakes and occur at subduction zones; they have extremely long recurrence intervals and very high magnitudes (typically in the 8–9+ range).
Recurrence intervals are qualitatively long for megathrust events; higher magnitude commonly corresponds to lower frequency.
Pacific Rim earthquakes and epicenters are often visualized as red dots along subduction zones and transform boundaries; mid-ocean ridges show seismicity as well, but subduction zones host the largest events.
Notable case studies referenced:
02/2010 Easter Sunday earthquake near Baja California (magnitude ~7.2): occurred south of the border; caused by a major fault crossing the region; led to damage in a sparsely populated area; a notable incident involved a person killed by a car after escaping a building.
2023 Turkey earthquakes: devastating collapse with pancake-style building failures; Turkey suffered extensive destruction; Wasatch Fault in Utah shown as an example of local faulting with vertical motion and hazard.
Loma Prieta earthquake (1989): South of San Francisco; a student visited the epicenter area and shared an observational photo; the event is used as a case study for ruptures and hazard understanding.
California earthquake context:
California has many faults with varying strike-slip and dip-slip movement; San Andreas is a major right-lateral strike-slip boundary.
Faults connect to form a network around the plate boundary rather than existing in isolation.
Earthquakes can occur along faults that cut across landscapes, creating fault scarps and offset features.
A common teaching tool is to compare across faults by looking at cross-sections (footwall vs hanging wall) and map views (strike-slip faults along which rocks slide horizontally).
Faults: types, terminology, and geometry
A fault is a fracture along which rocks move; the movement can be horizontal, vertical, or oblique.
Fault plane: the surface along which sliding occurs; the two blocks on either side are the hanging wall and the footwall.
Hanging wall: the block above the fault plane.
Footwall: the block below the fault plane.
Vertical motion (dip-slip faults) and horizontal motion (strike-slip faults) are distinguished:
Normal fault (tensional regime): hanging wall moves down relative to the footwall; the fault forms due to extension; common at divergent boundaries; leads to valley formation and uplift of blocks elsewhere.
Reverse fault (compression): hanging wall moves up relative to the footwall; footwall moves down; associated with crustal shortening and mountain building; a subset is the thrust fault (reverse fault with dips of 45° or less from the surface).
Thrust fault: a low-angle reverse fault with dips ≤ 45°; common in compressional regimes and high-strain zones.
Strike-slip fault (lateral or transcurrent): movement is primarily horizontal; movement can be right-lateral or left-lateral depending on the relative motion across the fault when observed from opposite sides.
Right-lateral (dextral): if you stand on one side and observe the other side moving to your right, it’s right-lateral.
Left-lateral (sinistral): the opposite case.
Map-view vs cross-sectional view:
In cross-section, you can see footwall/hanging wall relationships clearly for dip-slip faults.
In map view, you can see the fault trace and the horizontal (strike-slip) motion; the road or landscape appears offset along the fault.
Fault scarps: surface expression of the fault trace after rupture; visible cliffs or step-like topography indicating the location of the fault plane.
California’s fault population is dominated by right-lateral strike-slip faults and various dip-slip faults associated with the transform boundary network around the Pacific Plate; Wasatch Fault (Utah) is an example of normal faulting in a continental setting.
Owens Valley example: a reverse/thrust-like fault cutting across the valley margins; appears as a high-relief boundary between ranges and a fault scarp in the valley floor; the landscape shows tectonic offsets along fault lines.
Practical interpretation: to identify fault type, observe the relative motion across the fault by looking across the fault and noting which block moved up, down, or sideways; for strike-slip faults, the key is the horizontal offset and which side moved to the observer's right or left.
Nuances highlighted: in California, most observed strike-slip faults are right-lateral; left-lateral faults do exist but are less common in the region described.
Key concepts and terminology recap
Focus (hypocenter): the initial point of energy release inside the Earth during an earthquake; depth is often shallow (first 15 km relative to the crust).
Epicenter: the surface projection of the focus; the point on the Earth’s surface directly above the focus.
Magnitude and energy: larger earthquakes release exponentially more energy; megathrust events occur at subduction zones with very high magnitudes (nines and high eights) and very long recurrence intervals.
Plate boundaries and faults are related but distinct: a plate boundary is a broad region where two plates interact; faults are fractures within lithosphere that accommodate movement along or across plate boundaries.
Convective mantle and plate motion: mantle convection drives plate tectonics; hot mantle rises due to lower density, cools, and sinks, creating convection cells; slab pull and ridge push contribute to plate motion dynamics.
Pacific Rim of Fire: the region surrounding the Pacific Ocean with high seismic and volcanic activity, dominated by subduction zones and transform boundaries.
Importance of context: small-scale faults exist within broader boundary systems; earthquakes and volcanoes are interconnected phenomena in plate tectonics.
Quiz and test preparation tips (from lecture cues)
Expect questions about identifying plate boundaries from descriptions or maps (divergent, convergent, transform).
Be prepared to explain the difference between epicenter and focus and how to locate each on a map or cross-section.
Be able to describe the three main fault types (normal, reverse/thrust, strike-slip) and how stress (compression, tension, shear) leads to each.
Know the difference between footwall and hanging wall and how to determine which block moves in various fault types.
Recognize right-lateral vs left-lateral strike-slip faults and how to determine sense of motion by looking across the fault.
Understand hotspot dynamics vs plate motion, and use the Hawaii chain as an example of a fixed hotspot being traversed by a moving plate.
Be able to discuss the hazards associated with earthquakes (pancake collapse, building codes, seismic safety) and reference real-world events (Turkey 2023, Baja 2010, Loma Prieta).
Remember the approximate ages and progression of Hawaii islands relative to the hotspot (oldest to youngest: Kauai, then others toward the Big Island), and the expected future island development (Lōʻihi becoming a new island in the future).
Grasp the concept of convection in the mantle and the heat sources driving it (radioactive decay of U, K, Th) and how this sustains plate tectonics.
d \approx 0-15 \,\text{km}
Important recurrences: megathrust earthquakes have long recurrence intervals and high magnitudes; their frequency is low but impact is high.
Real-world connections and implications
Understanding transform boundaries like the San Andreas helps explain frequent earthquakes in California and the need for building codes and preparedness.
Hotspot theory explains the Hawaii island chain and why islands become progressively older away from the current hotspot; it also demonstrates how plate motion interacts with stationary mantle features to produce a long-lived volcanic arc.
Subduction and megathrust earthquakes remind us of the potential for large, rare, but devastating events along the Pacific Rim and the importance of preparedness and hazard mitigation.
The lecture emphasizes connecting theoretical concepts (boundaries, faults, convection) with real-world examples (Pacific Northwest, California, Turkey, Utah’s Wasatch Fault, Owens Valley) to understand why earthquakes and volcanic activity occur where they do.