North Pacific Seafloor: Dominated by Abyssal Plains at 4,500-6,000 Meters Depth

The North Pacific Ocean floor is predominantly abyssal plains — vast, flat, sediment-covered expanses at 4,500-6,000 m depth. They cover ~50% of the basin, making them Earth's most extensive single landform.

Abyssal Plain Formation

Turbidity currents (density-driven sediment flows) race down continental slopes through submarine canyons, depositing graded beds (Bouma sequences) across the plain. Pelagic rain (foraminifera, coccolithophores, diatoms, clay) adds 1-5 mm/kyr. Result: smooth topography (<100 m relief over 100 km).

Sediment Sources

• Terrigenous: Rivers (Columbia, Fraser, Yukon, Amur, Huang He, Yangtze) — but most trapped on shelves.
• Volcanic: Ash from Ring of Fire eruptions (distinct layers = chronostratigraphy).
• Biogenic: Siliceous (diatoms, radiolarians) south of 40°N; calcareous (forams) north — but CCD (Calcite Compensation Depth) ~4,500 m dissolves CaCO₃.
• Hydrogenous: Manganese nodules (potato-sized, 10-20% Mn, Ni, Cu, Co) — grow mm/Myr. Mining interest.

Life on the Plains

Low biomass, high diversity. Meiofauna (nematodes, copepods, kinorhynchs) dominate numbers. Macrofauna: Polychaetes, isopods, amphipods, holothurians (sea cucumbers — "vacuum cleaners"), xenophyophores (giant protists, 10-20 cm). Megafauna: Rattails (grenadiers), cusk-eels, tripod fish, octopuses (Graneledone). Scavengers on whale falls.

Other North Pacific Seafloor Features

• Seamounts: 10,000+ (Emperor, Hawaiian, Marcus-Wake chains). Biodiversity hotspots.
• Trenches: Mariana, Kuril-Kamchatka, Aleutian, Japan, Izu-Bonin — hadal depths.
• Ridges: Juan de Fuca, Gorda, Explorer (spreading centers). Hydrothermal vents.
• Plateaus: Shatsky, Hess, Ontong Java (thickened crust).

The abyssal plain is the default seafloor — the "background" upon which all other features are superimposed.