Rocks in the Kvarken Area

This page contains information about the bedrock in the Kvarken area. Being a geological text, some terms may appear unfamiliar. Hence it is suggested that when in doubt, readers should refer e.g. to Wikipedia for explanations of these terms.

Introduction

The geological history of the Kvarken area began in an orogeny some 1880 million years ago when Finland was located close to the Equator. The orogeny took pushed sediments, formed in shallow seas with imbedded volcanic layers deep into Earths crust where they re-crystallized as mica gneisses, vein gneisses, amphibolites. They also mixed with granodioritic melts.

Some 1800 million years ago a new heat pulse influenced bedrock development by creating granitic melts in the Eastern parts of Kvarken and pegmatites (a coarse grained granitic rock) in the western parts. A tectonically quiet time followed and lasted until 1570 million years ago when the Rapakivi magmas rose and began to crystallize in the upper parts of the crust. During Rapakivi magmatism also diabases and gabbros intruded the bedrock.

The last bedrock forming process took place 1270 million years ago when olivine diabases crystallized in the crust. After this, erosion and sedimentation have been dominant. The Jotunian sandstone (age 1400-1200 million years) is a remnant of these processes.

Although many of these geological events happened deep within the Earths crust, thanks to erosion they can be observed at outcrops today.

Igneous Rocks

Diorite and Gabbro

Diorite was formed deep in Earths crust. Its composition is close to gabbro although it contains less heavy elements, such as iron and magnesium. Some quartz can be found in diorites whereas in gabbros it’s absent. The formation of diorite is typically associated with granite or gabbro intrusions. Intrusions with dioritite-gabbro composition are fairly common in the Central Ostrobothnia region.

Due to the high amount of mafic minerals (containing e.g. iron and magnesium) such as olivine or pyroxenes, gabbro is almost black in color. The exact age of the gabbro is yet to be determined
but it’s been thought to be associated with Rapakivimagmatism.

Gabbro is only scarcely seen in the Kvarken area a few outcrops being visible on Tistonskär, South of Raippaluoto.

Granodiorite

The granodiorite found around the Kvarken area is commonly called Vaasa granite. The term Vaasa granite however offers little hint of the tectonic setting it was formed in and hence a more appropriate name for it is granodiorite. The Vaasa granite typically contains large feldspar phenocrysts and to a lesser extent garnet, cordierite, sillimanite and also quartz. The rock was formed in metamorphic conditions by the melting of sedimentary rocks about 1890 to 1880 million years ago. Also, fragments of other rocks such as gneisses can be seen in the Vaasa granite.

The rock is best observed between the areas of Raippaluoto—Vöyri and Maalahti—Pietarsaari in bedrock and boulders. Vaasa granite is also widely used as a dimension stone e.g. in the oldest arch bridge of Finland in Mustasaari or in the stone base of the Court of Appeal.


Rapakivi

The formation age of finnish Rapakivi varies somewhat being generally about 1600 millions years ago. Large, round felsdspar crystals (2-5 cm) surrounded by plagioclase rings are very typical for Rapakivi. In Finland, the Rapakivi formations have crystallized close to the surface (the magmatic origin being the lower crust from where it has risen along cracks to the upper parts of the crust) and hence cut sharply into the surrounding bedrock. Large planar plutons that formed the Rapakivi were some 10 km in size. Known Rapakivi formations include e.g. Åland, Vehmaa and Laitila. In the Vaasa region Rapakivi can be seen in Fjälskär. Rapakivi is also found in other countries such as Tanzania, India and China.

Homogenous color and good durability have made the Rapakivi a widely used dimension stone. The name Rapakivi refers to the finnish words “rapautua” which means erosion and “kivi” which simply means rock. The term might refer to the fact that Rapakivi erodes quite fast if being exposed (unprotected) to the elements.

Pegmatite

Pegmatites crystallized from melts with high H2O content that were left over from crystallization of other rocks. They are very coarse-grained (graine size >3cm) ja light in color. The composition is granitic meaning that their main minerals include feldspar, plagioclase and quartz. Also dark minerals such as muscovite and/or biotite are present. Rare minerals like tourmaline or beryl are commonly found in pegmatites. In the Vaasa region pegmatite is quarried in Kaatiala (Kuortane).

Diabase

Diabase is a very dark rock and its origin in Finland is commonly associated with the formation of the Rapakivi. Diabases are formed e.g. by mantle derived mafic melts which mix with country rock while rising in the crust (where they crystallize as dikes). It’s commonly thought that the mafic melts have warmed lower parts of Earth’s crust and hence have produced the granitic melts which formed the Rapakivi.

Youngest dikes are dated to be 1260 million years old which means that like Rapakivi, they cut shaprly into the surrounding bedrock. Diabase consists mainly of plagioclase, hornblende and pyroxenes.

There are four parallel diabase dyke swarms in the Vaasa region and they can be observed e.g. on the Moikipää islands.

 

Metamorphic rocks

Gneiss and Mica Gneiss

Gneisses are typically composed of fairly coarse-grained quartz and feldspars. In addition, gneisses often contain biotite, hornblende and muscovite. Their texture is typically foliated or contains lensoid structures. Gneisses are divided into subcategories such as garnet gneisses, vein gneisses or eye gneisses.

Finnish gneisses are often folded. The folding is closely related to the orogeny which stated 1880 million years ago. This is when sedimentary stones were pushed deep into Earth’s crust and thus metamorphized into mica gneisses, vein gneisses and amphibolites (a dark massive stone that contains amphiboles such as hornblende).

Sedimentary Rocks

Sandstone

Sandstones form when materia eroded from bedrock becomes bedded. During time, this material cements and ultimately turns into sandstone. It’s by such process, that the sandstone seen in the Vaasa region was also formed. Sandstones are mainly composed of quartz and in addition contain feldspars, fragments of other stones, clay minerals and carbonates. In Vaasa, the sandstone begins roughly from the left side of the diabase dykes continuing sub-aquatically all the way to the coast of Sweden.

References

Breilin, O., Kotilainen, A., Nenonen, K., Virransalo, P., Ojalainen, J. and Stén, C-G. 2004: Geology of the Kvarken Archipelago. Appendix 1 in the application for nomination of the Kvarken Archipelago to the World Heritage list.

Lehtinen, M., Nurmi, P. and Rämö, T. 1998: Suomen kallioperä: 3000 vuosimiljoonaa. Suomen geologinen seura. Helsinki. 375 p.

Lehtinen, M., Nurmi, P.A. and O.T. Rämö. 2006: Precambrian Geology of Finland, Volume 14. Elsevier Science. The Netherlands. 750 p.

www.geologia.fi

Photos: Mikko Turunen (kiviopas.fi/geologia.fi)