14.3 Earthquakes, Volcanoes & the Rock Cycle

Earthquakes, Volcanoes & the Rock Cycle

Quick Answer: Earthquakes release stored elastic strain along faults; rupture begins at the subterranean focus (hypocenter) directly below the surface epicenter. Seismologists locate epicenters via triangulation using P-S wave lag times from at least three seismograph stations. Volcanoes form through subduction flux melting at convergent margins and decompression melting over stationary mantle hotspots (e.g., Hawaii). Earth materials continuously transform through the rock cycle: igneous rocks crystallize from molten melt (coarse intrusive vs. fine extrusive), sedimentary rocks form via weathering and lithification (the only rock family preserving intact fossils), and metamorphic rocks form through heat and directed pressure without melting (foliated vs. non-foliated).

Chapter 14 concludes by connecting tectonic motion to seismic rupture, volcanism, and the rock cycle, illustrating how Earth continuously recycles energy and mineral matter.

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Earthquake Mechanics & Fault Behavior

An earthquake is the sudden vibration of Earth's crust caused by rapid energy release along a fault. The rupture process follows elastic rebound theory (Harry Fielding Reid):

1. Frictional Locking: Tectonic forces exert continuous stress, but fault roughness (asperities) locks the fault plane.
2. Elastic Strain: Adjacent rock bends elastically, storing potential strain energy.
3. Brittle Rupture: When shear stress overcomes frictional resistance, the fault slips suddenly.
4. Rebound: Rock snaps back to its unstrained geometry, radiating seismic waves.

Focus (Hypocenter) vs. Epicenter

Understanding earthquake geometry is critical for the HiSET exam:

• Focus (Hypocenter): The exact point deep within the crust or mantle where fault slippage begins and seismic waves originate.
• Epicenter: The geographic point on Earth's surface vertically above the focus. The epicenter typically experiences the most severe surface shaking.

Triangulation: Locating Epicenters via P-S Wave Lag Time

Compressional P-waves ($v_P \approx 6.0\text{ km/s}$) travel faster than shear S-waves ($v_S \approx 3.5\text{ km/s}$), arriving at seismograph stations first.

As distance from the epicenter increases, the P-S arrival lag time ($\Delta t = t_S - t_P$) increases systematically. Seismologists convert this lag time into an epicentral distance ($r$):

1. One Station: Yields distance $r_1$. The epicenter could lie anywhere along a circle of radius $r_1$.
2. Two Stations: Two distance circles intersect at two distinct points.
3. Three Stations: A third station's circle intersects at a single unique geographic point, uniquely identifying the epicenter.

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Volcanic Activity: Subduction vs. Hotspots

Volcanism vents molten rock and gases onto Earth's surface across two primary geodynamic settings:

1. Subduction Volcanism (Flux Melting)

At convergent subduction zones, hydrous marine minerals descend into the mantle. Trapped water is released into the overlying mantle wedge, lowering the melting point of mantle peridotite (flux melting). The resulting buoyant magma rises to build volcanic island arcs (e.g., the Aleutians) or continental volcanic arcs (e.g., the Cascades).

2. Hotspot Volcanism (Mantle Plumes)

A hotspot is an intraplate volcanic center fueled by a stationary plume of hot rock rising from near the core-mantle boundary. As a tectonic plate drifts over a stationary mantle plume, a sequential track of volcanoes develops:

• The active volcano sits directly above the plume (e.g., Kilauea on Hawaii's Big Island).
• Extinct, eroded volcanoes trail away in the direction of plate motion (e.g., Maui, Oahu, Kauai, and the submerged Emperor Seamounts).
• Island ages increase linearly with distance, documenting plate motion.

[!NOTE] Magma vs. Lava: Molten rock underground is magma; at the surface, it is lava. Felsic lavas are viscous and explosive (Mt. St. Helens); mafic lavas are fluid and effusive (Hawaiian shield volcanoes).

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The Rock Cycle: Continuous Terrestrial Recycling

The rock cycle illustrates how Earth materials transform among three primary rock families:

Rock Cycle Transformation Pathways

1. Igneous: Magma cooling slowly underground forms coarse intrusive rock (granite); lava cooling rapidly on the surface forms fine extrusive rock (basalt) or glass (obsidian).
2. Sedimentary: Weathering breaks exposed rock into sediment. Overburden weight squeezes grains (compaction), and dissolved minerals precipitate (cementation), lithifying sediment into rock.
3. Metamorphic: Deep burial subjects rocks to heat ($>200^\circ\text{C}$) and pressure without melting, aligning minerals into foliated bands (e.g., slate $\rightarrow$ gneiss). If rock melts completely, it returns to magma.

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HiSET Exam Traps & Strategic Takeaways

• Trap: Focus vs. Epicenter: The focus is subterranean where rupture begins; the epicenter is the surface point directly above it.
• Trap: Triangulation Minimum: Exactly three seismograph stations are required to locate a unique epicenter.
• Trap: Crystal Size: Slow underground cooling yields large crystals (intrusive granite); rapid surface cooling yields microscopic crystals (extrusive basalt).
• Trap: Fossil Preservation: Fossils exist almost exclusively in sedimentary rocks because igneous melting and metamorphic recrystallization destroy organic remains.