東北地方の火山フロントに沿って玄武岩質-安山岩質成層火山群が形成されるに先立ち, 400-100万年前に大量のデイサイト-流紋岩質火砕岩流が噴出した。これらの岩石は過斑晶質であって通常20-47%の石英,斜長石,輝石,普通角閃石と酸化鉄鉱物を斑晶として含む。黒雲母流紋岩のみは輝石を欠くが,上記の鉱物組み合わせの外にサニディン,カミングトナイトと黒雲母を含む。輝石温度計から推定されるこれらの岩石のマグマは980-910°Cであって,同じ地域の第四紀成層火山のデイサイト質岩とほぼ同じ温度を示す。
これらのデイサイト-流紋岩質火砕岩流の主成分と非調和元素の量は珪酸の増加あるいは分別作用の進化と共に規則的に変化し,典型的なカルクアルカリ系列安山岩-デイサイトの経路を辿る。
これらのデイサイト-流紋岩質マグマは上部マントルから由来した典型的な島弧カルクアルカリ苦鉄質マグマの高い分別結晶作用によって生成した最晩期の分化物であり,はんれい岩質の下部地殻の部分融解によって生成したものでは無いと考えられる。

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Various metamorphic rocks such as amphibolites (garnet-cpx amphibolite, schistose amphibolite, kyanite-bearing amphibolite), pelitic and basic schists (epidote amphibolite), and ultramafic rocks (spinel lherzolite, pyroxenite, hornblendite) are exposed in the Yoshimi Hills, to the east of the Kanto Mountains. Schistose amphibolites, which have the largest exposure in the Yoshimi Hills, and pelitic schists were moderately suffered mylonitization.
The chemical composition of metamorphic minerals such as garnet, cpx and hornblende in the garnet-cpx amphibolite, and garnet in the schistose amphibolite suggests that these rocks were formed under the amphibolite to the granulite facies. The garnet-biotite temperature after Pigage and Greenwood (1982) estimated on the pelitic shcists is more than 800°C, considerably higher than the temperature (610°C) estimated on the pelitic schists in the highest metamorphic grade zone of the Sanbagawa belt in the Shikoku region.
K-Ar ages of hornblende are 63.5 Ma for the garnet-cpx amphibolite and 61.0 Ma for the kyanite-bearing amphibolite, whereas K-Ar age of muscovite for the pelitic schist is 34.1 Ma. Form these age data, even though taking a possibility of rejuvenation into consideration, it is difficult to correlate the metamorphic rocks of the Yoshimi Hills to the metamorphic rocks of the Kurosegawa belt. Considering the occurrence of tectonic blocks of various amphibolite masses in the Sambagawa belt in the Shikoku region, it is possible to correlate the metamorphic rocks of the Yoshimi Hills to those in the Sambagawa belt. However, further study is necessary before we come to decide the belongings of the metamorphic rocks in the Yoshimi Hills.

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Volcanic fragments were dredged from the eastern summit of the Futago Knoll situated in the northeastern part of the Japan Sea. The dredged fragments include mainly basaltic rocks and a small amount of andesite, dacitic welded tuff, sandstone, and granite. The presence of abundant angular and unsorted basaltic fragments indicate that the knoll is an in situ volcanic body.
K-Ar age of the basaltic andesite was dated as 5.7 ± 0.3 Ma. The dredged samples of the Futago Knoll are similar to the rocks of the nearby Shakotan Peninsula of the same age, suggesting that the knoll is underlain by continental crust.

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