Megata

Maar(s) in Japan

Last Eruption: -2050

Key Facts

Elevation

160 m (525 ft)

Type

Maar(s)

Location

39.950°, 139.725°

Region

Northeast Japan Volcanic Arc

Rock Type

Andesite / Basaltic Andesite

Tectonic Setting

Subduction zone

Location

Loading map...

Overview

Megata, a group of three small maars along the tip of the Oga Peninsula in NW Honshu, has been active during the Holocene (Ishikawa et al. , 1957). Lakes 300-600 m wide fill the basaltic-to-dacitic maars, which are a noted locality for mantle-derived xenoliths.

Ichinomegata, the easternmost, largest, and oldest of the three maars, is 600 m in diameter. Ninomegata and Sannomegata are located immediately E and S of Toga Bay, which itself is a maar. The rim of the rhyolitic Toga (Togowan) maar, larger than the Megata maars, is breached by the sea on the western side and was formed about 450,000 years ago.

The Megata maars are mostly late Pleistocene in age, but Murayama (1987) noted that pottery from roughly 4,000 years before present (Jomon Period) has been found within tephra layers from the maars.

Volcanic Hazards & Risk Assessment

Primary Hazards

Risk Level

Population at RiskHigh

Infrastructure RiskModerate

Aviation RiskSignificant

Geological Composition & Structure

Rock Types

Primary

Andesite / Basaltic Andesite

Silica Content

Intermediate (57-63% SiO₂)

Tectonic Setting

Subduction zone

Formed by oceanic plate subduction, typically producing explosive eruptions due to water-rich magmas.

Age & Formation

Epoch

Holocene

Evidence

Eruption Dated

Eruption Statistics & Analysis

Metric	Value	Global Ranking	Significance
Total Recorded Eruptions	Unknown	Low	Moderately active volcano
Maximum VEI	VEI Unknown	Minor	Local impact potential
Recent Activity	4076 years ago	Historical	Historically active

Monitoring & Alert Status

Monitoring Networks

Japan Meteorological Agency (JMA)

Real-time seismic monitoring

Geological Survey of Japan

Geochemical monitoring

Global Volcanism Program

International eruption database

Current Status

Normal

No recent activity. Routine monitoring continues.

Nearby Volcanoes in Northwestern Pacific Volcanic Regions

Regional Volcanic Activity

The Northwestern Pacific Volcanic Regions contains multiple active volcanic systems. Cross-regional magma interactions and tectonic stresses can influence eruption patterns across the entire arc. Monitor regional seismic activity and volcanic alerts.

Quick Info

•Smithsonian ID: 283262
•Evidence: Eruption Dated
•Epoch: Holocene

About the Photo

Sannomegata (center), seen here from the NW with Toga bay in the foregound, is one of three small Megata maars along the tip of the Oga Peninsula. Lakes 300-700 m wide fill the maars, which are noted localities for mantle-derived xenoliths and are located immediately E and S of Toga Bay.

Copyrighted photo by Hiroshi Yagi (Japanese Quaternary Volcanoes database, RIODB, http://riodb02.ibase.aist.go.jp/strata/VOL_JP/EN/index.htm and Geol Surv Japan, AIST, http://www.gsj.jp/).