### Models of Hawaiian Hotspot Volcanism Hawaiian volcanoes are located in the middle of t… — ACT

### Models of Hawaiian Hotspot Volcanism

Hawaiian volcanoes are located in the middle of the Pacific Plate, far from plate boundaries. Two models explain the source of magma and the age progression of the Hawaiian-Emperor seamount chain, where volcanoes get older further northwest.

Model 1 (Deep Mantle Plume Model)
A narrow plume of hot mantle material rises from the core-mantle boundary (about $2{}900\text{ km}$ deep) to the crust. This plume is stationary relative to the deep mantle. As the Pacific Plate moves northwestward over this fixed 'hotspot,' decompression melting of the plume creates a chain of volcanoes. The source of magma is the deep mantle, which is rich in primordial helium ( $^3\text{He}$ ) and contains higher concentrations of primitive trace elements compared to the upper mantle.

Model 2 (Shallow Plate-Tectonic Extension Model)
Magma rises from the shallow upper mantle (asthenosphere, less than $200\text{ km}$ deep) due to localized crustal extension (cracking) of the Pacific Plate. Tectonic stresses bend the plate, causing propagating fractures. Magma is not fed by a deep plume but is passive melting of the upper mantle drawn upward into the fractures. The northwestward age progression occurs because the stresses that cause fracturing propagate along the plate over time. The magma source is the recycled oceanic crust in the shallow mantle, characterized by normal ratios of helium ( $^3\text{He}/^4\text{He}$ ) and depleted trace elements typical of the upper mantle.

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A geologist is comparing the mechanisms and geochemical predictions of Model 1 and Model 2 for the origin of Hawaiian volcanism. Match each model-specific claim or prediction on the left with its corresponding underlying assumption or explanation on the right.

Geochemical signature of primordial $^3\text{He}$ in lava samplesUnderlying mantle source is tapped at the core-mantle boundary (Model 1)
Volcanic chain alignment reflecting lithospheric plate motion relative to the lower mantleMagma generation occurs above a plume that is stationary relative to the deep mantle (Model 1)
Passive extraction of magma from a depleted upper-mantle sourceDecompression melting occurs in the shallow asthenosphere beneath lithospheric cracks (Model 2)
Volcanic age progression driven by a moving zone of structural failureStresses bend the plate, causing fractures to propagate along the plate over time (Model 2)

Answer

The geochemical signature of primordial

^3\text{He}

matches the core-mantle boundary source (Model 1). The volcanic chain alignment reflecting plate motion relative to the lower mantle matches the stationary deep-mantle plume (Model 1). The passive extraction of magma matches shallow asthenospheric melting beneath lithospheric cracks (Model 2). The volcanic age progression driven by structural failure matches propagating stresses and fractures over time (Model 2).

The correct pairings align the deep mantle origins (

2{}900\text{ km}

source, primordial

^3\text{He}

signature, and stationary hotspot reference) with Model 1, and the shallow plate extension origins (

< 200\text{ km}

source, passive cracking, and stress propagation) with Model 2.

Step-by-Step Solution

Analyze the claims and characteristics associated with Model 1.

Model 1 features a deep core-mantle boundary origin (explaining primordial

^3\text{He}

) and a plume stationary relative to the deep mantle over which the plate moves.

This establishes the deep-mantle plume chemical and structural premises.

Analyze the claims and characteristics associated with Model 2.

Model 2 features a shallow upper-mantle/asthenosphere origin (explaining depleted trace elements and passive melting under cracks) and age progression driven by propagating lithospheric fractures over time.

This establishes the shallow plate-tectonic extension chemical and structural premises.

Match each left item to the right item representing its correct physical or chemical mechanism.

Primordial

^3\text{He}

matches the core-mantle boundary source; plate motion over a stationary plume matches the lower mantle reference; passive extraction from a depleted source matches shallow asthenospheric melting; and age progression via structural failure matches propagating fractures.

Completes the matching alignment according to each model's distinct physical mechanism.

Key Concept

Comparing and Contrasting Models

Estimated Time:2m 0s

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