The Jormua Ophiolite ( Finnish: ''Jormuan ofioliitti'') is a

remnant Remnant or remnants may refer to: Religion * Remnant (Bible), a recurring theme in the Bible * Remnant (Seventh-day Adventist belief), the remnant theme in the Seventh-day Adventist Church * ''The Remnant'' (newspaper), a traditional Catholic n ...

of ancient

oceanic Oceanic may refer to: *Of or relating to the ocean *Of or relating to Oceania **Oceanic climate **Oceanic languages **Oceanic person or people, also called "Pacific Islander(s)" Places * Oceanic, British Columbia, a settlement on Smith Island, ...

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...

near the village of

Jormua Jormua is an old village on the shore of the lake of Oulujärvi in the town of Kajaani, Finland. The village has a population of about 750. The Jormua Ophiolite The Jormua Ophiolite (Finnish language, Finnish: ''Jormuan ofioliitti'') is a Ophio ...

close to the geographical centre of

Finland Finland, officially the Republic of Finland, is a Nordic country in Northern Europe. It borders Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bothnia to the west and the Gulf of Finland to the south, ...

. The rocks of the Jormua Ophiolite formed about 1,950 million years ago in the Paleoproteozoic Era. The conditions under which the ocean crust rocks of Jormua formed was likely similar to present-day

Red Sea The Red Sea is a sea inlet of the Indian Ocean, lying between Africa and Asia. Its connection to the ocean is in the south, through the Bab-el-Mandeb Strait and the Gulf of Aden. To its north lie the Sinai Peninsula, the Gulf of Aqaba, and th ...

. Thus, a linear sea of this type is thought to have existed between two

continental Continental may refer to: Places * Continental, Arizona, a small community in Pima County, Arizona, US * Continental, Ohio, a small town in Putnam County, US Arts and entertainment * ''Continental'' (album), an album by Saint Etienne * Continen ...

landmasses in Finland. At some point this sea closed and the ophiolite was obducted. The Jormua ophiolite is the best preserved one along a larger chain of ophiolites that occur within the Kainuu

Schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

Belt. The Jormua Ophiolite is broken into 4 geographical blocks termed as the: eastern block, central block, northern block, and western block. The eastern and northern blocks are most similar to one another containing mantle peridotites intruded by gabbro stocks and basaltic sheeted dikes. These are overlaid by a metabasaltic pillow lava layer which is directly overlain by black turbitic shales. These are best associated with an extrusive seafloor sequence. The central block lacks extrusive rocks, gabbro rocks and dikes but otherwise resembles the eastern and northern blocks. The main difference being EMORB dikes. The western block resembles the central block except its bounded by older

archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...

basement rocks and its mantle peridotites are in general less depleted.Peltonen, P., & Kontinen, A. (2004). The Jormua Ophiolite: A Mafic-Ultramafic Complex from an Ancient Ocean-Continent Transition Zone. In ''Developments in Precambrian Geology'' (Vol. 13, pp. 35–71). Elsevier. In review, the crustal rocks of the ophiolite include

pillow lava Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of discontinu ...

hyaloclastite Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass (from the Greek ''hyalus'') fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin marg ...

, sheeted dykes,

gabbro Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

plagiogranite Trondhjemite is a leucocratic (light-colored) intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites that occur in the oceanic crust or in ophiolites are usually called pla ...

and

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

cumulate rock Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group ...

. Mantle rocks exposed in the ophiolite are the

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...

varieties of

lherzolite Lherzolite is a type of ultramafic igneous rock. It is a coarse-grained rock consisting of 40 to 90% olivine along with significant orthopyroxene and lesser amounts of calcic chromium-rich clinopyroxene. Minor minerals include chromium and alumi ...

harzburgite Harzburgite, an ultramafic, igneous rock, is a variety of peridotite consisting mostly of the two minerals olivine and low-calcium (Ca) pyroxene ( enstatite); it is named for occurrences in the Harz Mountains of Germany. It commonly contains a ...

and

dunite Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, wit ...

References

Geology of Finland Ophiolites Paleoproterozoic magmatism Paleoproterozoic volcanism Suture zones {{OuluProvince-geo-stub