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.