Ogamegata Volcano Group Ichinomegata

Ichinomegata: A Crater Lake on the Oga Peninsula

A near-perfect circle of dark water sits among the forested hills near the tip of Akita's Oga Peninsula. It measures roughly 600 meters across and drops to 44.6 meters at its deepest point. The lake is called Ichinomegata, and despite its calm surface, it marks the site of one of the most violent kinds of eruption a volcano can produce.

Ichinomegata is a maar: a broad, low crater blasted out of the ground by an explosion of steam. It belongs to a trio of such craters known together as the Megata Volcano Group, the others being Ninomegata and Sannomegata. All three now hold freshwater lakes. The Agency for Cultural Affairs designated Ichinomegata a National Natural Monument on 26 July 2007, recognizing it as the clearest example of a maar anywhere in northeastern Japan.

A Crater Born of Steam and Magma

Most volcanoes build themselves up. They erupt again and again from the same vent, piling lava and ash into a cone. A maar does the opposite. It forms in a single burst, and that burst digs a hole rather than raising a hill.

The mechanism is called a phreatomagmatic explosion. Rising magma meets groundwater underground, the water flashes instantly to steam, and the sudden expansion shatters the surrounding rock. The force is extreme. Debris scatters across a wide area, and only a thin layer of material falls back into the crater itself, so a maar lacks the tall encircling rim of an ordinary volcano. Water then collects in the bowl. Megata's craters are deep enough that the lakes never drained away.

The three maars sit almost in a straight line running east-northeast, numbered from the eastern end: Ichinomegata, Ninomegata, Sannomegata. Ichinomegata came first. Wood fragments recovered from an outcrop on its southern shore and dated by radiocarbon analysis, together with the position of a regional ash layer, place its formation roughly 60,000 to 80,000 years ago. Sannomegata, the youngest, formed only about 20,000 to 24,000 years ago. The city of Oga cites a slightly older range for Ichinomegata, a reminder that dating buried eruptions is rarely exact.

Ichinomegata is the largest of the three, with an area of 0.26 square kilometers. Its ejecta, and that of Ninomegata, are calc-alkaline andesite; Sannomegata produced mostly high-alumina basalt.

Why Ichinomegata Is a National Natural Monument

Two things make this lake scientifically rare.

The first is the maar form itself. Northeastern Japan lies under steady east-west compression, squeezed by the Pacific Plate as it slides beneath the islands. That stress regime tends to channel magma through long-lived volcanic systems, so single-eruption volcanoes are unusual here. Megata is the only maar field in the entire Tohoku region. Across the whole of Japan only a handful of comparable sites exist, among them Habu Harbor on Izu Oshima and Yamagawa Harbor near Ibusuki in Kagoshima.

The second is what the eruption carried up. The andesite thrown out by Ichinomegata contains xenoliths, chunks of foreign rock that the magma tore loose and swallowed on its way to the surface. Some of these fragments came from the lower crust and the upper mantle: dense, iron- and magnesium-rich rocks such as peridotite, hornblende gabbro and websterite. Ichinomegata was the first volcano anywhere recognized to have erupted andesite carrying mantle xenoliths. That discovery turned the lake into a destination for geologists from many countries, who examine the fragments as direct samples of rock that normally lies tens of kilometers underground.

The official designation cites both the occurrence of distinctive rocks and minerals and the features left behind by volcanic activity. Ichinomegata is valued as a textbook maar, as the sole representative of its kind in Tohoku, and as a natural drilling core into the deep Earth.

What to Look For When You Visit

Ichinomegata is best seen from above. The Hachibodai observation point, set on high ground behind Toga Bay, looks down on both Ichinomegata and neighboring Ninomegata at once. From the deck the circular shape of the craters reads clearly, something almost impossible to grasp from the shoreline.

The name Hachibodai, meaning roughly "platform of eight views," was chosen in 1952 by Prince Takamatsu. The former town of Kitaura ran a public contest to name the lookout, and the prince selected the winning entry. The panorama takes in the volcanic peaks of Honzan, Shinzan and Mount Kanpu. Centuries earlier the Edo-period traveler Sugae Masumi paused here, sketching the same lakes and bay. Visitors today see a view that has changed remarkably little.

Look at the water and consider what lies beneath it. The floor of Ichinomegata holds varves: sediment laid down in fine paired layers, one couplet per year, accumulated without interruption for tens of thousands of years. Cold winters and the sheltered, oxygen-poor depths of the crater keep the layers from being stirred up. Only a few lakes on Earth preserve such a record. By reading the varves, scientists can reconstruct year-by-year shifts in climate and environment, which is why the lakebed is treated as a scientific archive in its own right.

The lake also remains a working part of local life. Ichinomegata still serves as a water source for the city of Oga.

Around Ichinomegata

The maars sit within Oga Quasi-National Park and form one of the headline sites of the Oga Peninsula and Ogata UNESCO Global Geopark. Reaching the area takes some planning. It lies well beyond the towns, and most visitors arrive by car or tour bus.

Cape Nyudozaki, at the very tip of the peninsula, is a short drive away. Its black-and-white striped lighthouse stands on a windswept grassland where a marker traces the 40th parallel of north latitude. Toga Bay, just below Hachibodai, is itself a volcanic feature: a crater flooded by the sea rather than by rain. The Oga Aquarium GAO overlooks the bay and makes an easy stop for families.

The peninsula is also home to the Namahage, the straw-caped ogre figures who visit households on New Year's Eve. Travelers with extra time can climb Mount Kanpu, a smooth grass-covered dome with a wide view over the rice plains, or visit Shinzan Shrine, closely tied to the Namahage tradition.