Rock Composition of Mount Fuji: Basaltic Andesite Layers

Mount Fuji is a **classic stratovolcano (composite volcano)** built from **alternating layers of lava flows and pyroclastic deposits (tephra)**. Its bulk composition is **basaltic andesite** (intermediate silica: **53–57% SiO₂**), characteristic of **subduction zone volcanism** (Pacific Plate subducting under Philippine Sea/Okhotsk Plates).

Lava Flows (Effusive)

• Primary Rock Type: **Basaltic Andesite** (also **Two-Pyroxene Andesite**).
• Mineralogy: **Phenocrysts:** Plagioclase feldspar (dominant), **Orthopyroxene (hypersthene), Clinopyroxene (augite), Olivine (forsteritic)**. **Groundmass:** Microlites of same + magnetite, glass.
• Compositional Range: **Basalt (48–52% SiO₂, high Mg, olivine-rich) → Basaltic Andesite → Andesite (57–63% SiO₂, hornblende appears).** **No dacite/rhyolite.** Reflects **fractional crystallization + magma mixing** in a **mid-crustal reservoir**.
• Flow Morphology: **Aa (blocky, rough) & Pahoehoe (ropy) near vent; thick, blocky 'a'a flows** dominate lower flanks. **Lengths up to 10–15 km.** Volume per flow: **0.01–0.1 km³.**

Pyroclastic Deposits (Explosive)

• Tephra Layers: **Scoria (vesicular basaltic andesite), Ash (vitric/crystal), Lapilli, Bombs.**
• Key Deposits: **Hoei Tephra (1707 – last eruption, 0.7 km³, Plinian),** **Older Fuji Tephras (Ko-Fuji, Pre-Ko-Fuji).** **Widespread across Kanto Plain (Tokyo).**
• Composition: **Same basaltic andesite spectrum.** **Pumice** in Plinian phases (more evolved, andesitic).

Eruptive History & Magma System

• Four Stages: **Sen-komitake (pre-Fuji) → Komitake → Old Fuji (Ko-Fuji, 100–17 ka) → New Fuji (Shin-Fuji, 17 ka–present).**
• New Fuji: **High eruption rate (~1.5 km³/kyr).** **Basaltic andesite dominant.** **Frequent flank eruptions (parasitic cones: ~70).** **Summit crater (500m x 200m) active.**
• Magma Reservoir: **Mid-crustal (10–20 km depth), zoned (basaltic base → andesitic top).** **Mafic recharge triggers eruptions.**

Why Not "Basalt" or "Andesite" Simply?

The **consistent intermediate composition (basaltic andesite)** across lava and tephra, across stages, defines Fuji. It sits at the **basalt-andesite boundary**—**primitive enough for high eruption rates/fluid flows, evolved enough for explosive Plinian potential (Hoei 1707).** This **dual personality (effusive + explosive)** makes it hazardous.

Mount Fuji = Basaltic Andesite Stratovolcano. Its layered lavas and tephras of this single intermediate composition build the iconic cone.