Download Taikos g. 2, LT-21371, Vievis, Elektrėnų Municipality

The Lithuanian Musuem of Geology
The exposition of boulders of the Lithuanian Museum of Geology
Audioguide
Welcome to the Lithuanian Museum of Geology! Its start dates back to 1980, when the drill-core
storage facility and museum expositions were opened here. Until 1989, the drill-core storage
facility functioned as a separate unit of the Institute of Geology, whereas in 1992 it was
combined with the exposition of Vilnius. In 2000, the drill-core storage facility of Vievis was
conferred the status of an independent institution – the Lithuanian Museum of Geology.
The museum’s collection is comprised of the core drilled from over 1,100 bore-wells since 1953
and stored in the storage facility. Core is a column of a cylinder-shaped rock obtained by drilling
in a column-drilling method. It serves as a geological chronicle of the place of drilling and as the
main source of the geological information. The following expositions are also displayed in the
museum’s building: the exposition of polished boulders, the exposition of the minerals of
Lithuania, the memorial exposition of Professor Juozas Dalinkevičius, the paleontological
exposition of Alfonsas Žukelis and the exposition of the currently forming rocks.
The visitors are invited to explore the open-air exposition of boulders situated in the park of the
area of 4 ha that surrounds the building of the Lithuanian Museum of Geology and was designed
in accordance with the design of the landscape architects of the Kaunas Botanical Garden. There
are pathways stretching among the bushes and trees near the two ponds and leading you to the
boulders brought here from various places of Lithuania and arranged in groups and displayed on
the grass-plots. These witnesses of the continental glaciations were brought by the glaciers from
the Scandinavian countries, the bottom of the Baltic Sea, Aland Islands and other places 20-12
thousand years ago. The boulders are grouped in accordance with the petrographic classification.
Lithuania is a typical region of continental glaciations characterised by the relief formed by the
ice and melt water as well as by the diverse composition of soil and boulders that are commonly
referred to the stones. They are composed of the igneous rocks that formed in Scandinavia and
Finland hundreds of millions or even billions of years ago and appeared in our country relatively
recently from the geological point of view – a million years ago, during the Pleistocene Epoch of
the Quaternary Period. It has been established that the oldest rock of Lithuanian is one billion
and eight hundred million years old.
The boulders brought by the glaciers to our land are of various sizes. They constitute more than
fifty kinds of rocks differing in their mineral composition and structure. Geologist Rimvydas
Tarvydas distinguishes 43 kinds of boulders characteristic to Lithuania. The collections of rocks
stored in the museum are used when teaching students, familiarising the pupils with the science
of rocks, which is called the petrography.
Please take a look at the open-air exposition of the museum. The first object that should catch
your eye is the large metal tower. It is related to the core stored in the museum – this is a device
used to drill rocks from the depths of the Earth. This equipment allowed us to reach the rocks
lying in the depth of two kilometres. The column of rock pulled out every ten metres, i.e. the
core, provides the geologists with the information on the composition of the Earth beneath our
feet and opens the layers of the Earth’s “cake”. Its definition derived from the German word
“Kern”, which means the “core”. The process of drilling is a very long and expensive work.
1
The Lithuanian Musuem of Geology
Now, please take a closer look at the boulders displayed in the open-air exposition. They are
grouped based on their petrograhic composition. Igneous rocks are marked by the audio-guide
numbers from 1 to 12, sedimentary rocks – from 13 to 15, metamorphic rocks – from 16 to 22,
metasomatic rocks – from 23 to 24 and ultrametagenetic rocks – from 25 to 30.
The first exhibit is displayed in the group of igneous rocks situated at closest distance to the
drilling tower, on its southern side.
(1) Pyroxenite
It has been established that the boulders of igneous origin found in Lithuania make up
approximately 60% of all boulders. This exposition is also characterised by the abundance of
these boulders – you can actually find 141 boulders. Igneous rocks are formed through the
cooling and crystallisation of the liquid igneous melts – the magmas. Some rocks are formed at
a great depth – these are called the intrusive rocks, others are formed when magma erupts onto
the surface of the Earth and slowly cools here; these are called the extrusive rocks.
Igneous rocks have many different mineral compositions. The most common minerals are
feldspars, quartz, amphiboles, pyroxenes, micas, and the less common include olivine, nepheline,
magnetite, apatite and other minerals.
In 1982, Gediminas Motūza M.D. offered the classification of chemical igneous rocks for the
exposition of boulders. According to this classification, the igneous rocks are divided into
ultrabasic, basic, intermediate, acidic and alkaline rocks. We will present you with these groups
in the course of telling about individual boulders.
Pyroxenite is attributed to the igneous ultrabasic rocks. They are called like that because of very
low silica content (40-45%). These rocks are usually of dark colour and relatively high density.
Ultrabasic rocks do not contain light minerals and the dark minerals are dominated by olivine,
pyroxenes and magmatite. These rocks are not very common on Earth’s crust, therefore, you can
find only two examples of these boulders in our exposition. Pyroxenites do not contain quartz
and olivine. The composition is dominated by pyroxene. They have coarse-crystalline or
medium-crystalline structure. Pyroxenites are used for chippings and as a decorative stone.
The next exhibit illustrates the group of igneous basic rocks. This is gabbro-diabase. You will
find it in the group of boulders situated near the junipers.
(2) Gabbro-diabase
The basic rocks have 40-55% silica content. They are characterised by dark grey, greenish grey,
bluish grey and black colours. The predominant light minerals include basic plagioclases –
labradore, anortite, bitovnite; the most common dark minerals include augite, hyperstene,
hornblende, the less common is olivine. Basic rocks can be intrusive – abyssal and veined as
well as extrusive. The Vievis exposition contains twenty intrusive abyssal boulders of the basic
origin.
Basalts are the most common extrusive rocks. Basalt is usually grey to black in colour and has a
vitrophyric or cryptocrystal and less commonly porphyritic structure and a smooth texture. The
exposition of boulders does not contain basalts, however, it contains rocks highly influenced by
the secondary processes – the so-called diabases or basalt porphyrites. On the surface of some
boulders, one can see the flutes that have been formed through the process of lava flowing aslope
and gradually cooling. Due to the dominating uralite and chlorite, diabases are of dark green
colour and are attributed to the phase of green stones.
(3) Gabbro
Gabbro is an intrusive rock of dark grey to black and sometimes greenish or bluish grey colour.
It is composed of light minerals – plagioclase from anortite to labradorite and mineral of the
2
The Lithuanian Musuem of Geology
group of pyroxenes or amphiboles. Gabbro has a coarse-crystal granular structure and a smooth
texture. It can also be ophytic with diabase in structure. These rocks are widely spread in all
stages of the development of deep igneous rocks, but do not form such massive solids as
compared with the granites. It is an excellent constructional material. The forms of the
characteristic stratification of gabbro rocks include stock, pebble and dikes. Magnetitic ores of
titan are genetically related to gabbro intrusives. Sometimes, they contain vanadium impurities.
The sulphidic nickel and copper ores are also related to the basic intrusions.
Next to this boulder is another variety of gabbro – norite.
(4) Norite
Norite is a mafic intrusive igneous rock composed of plagioclase and orthopyroxene. The name
of rock derived from the name of the Norwegian mythical hero Noro. It is found together with
gabbro in the layered intrusive bodies, which also contain platinum. Such large bodies are found
in South Africa, Greenland and the United States of America. Norite is also found in the world’s
second-largest deposit of nickel located in the state of Ontario. This igneous rock is also quite
common in the lunar soil samples.
Now, let us move to the next group of igneous rocks – medium-acidity boulders. They are
exhibited near the gates to the museum. The next exhibit of our excursion is marked with number
5 in the audio-guide. This is diorite.
(5) Diorite
As compared with basic boulders, medium-acidity boulders contain more silica and less dark
minerals. They are grouped into rocks with plagioclases and rocks with potassium-sodium
feldspars. In our exposition of boulders, you can find the following varieties of medium-acidity
rocks: diorites, granodiorites, metadiorites and andesitic porphyrites.
Diorites are intrusive abyssal rocks characterised by grey, dark grey or greenish grey colour.
They have a granular structure and a smooth texture. The mineral composition of diorites
includes plagioclases and hornblende and less commonly pyroxene and biotite.
Diorites have a number of varieties. We would like to familiarise you with several of these
varieties. The granodiorite marked by number 6 in the audio-guide is placed between the two
large boulders.
(6) Granodiorite
Same as diorite, granodiorite is a rock that was formed through the solidification of magma deep
down in the Earth’s crust. This is an intermediate rock between granite and diorite. It is largely
composed of quartz, pyroxene, hornblende and plagioclase. This rock is used as a decorative
material in the construction sector. It is worth to mention one of the world’s famous
granodiorites – the Rosetta Stone that was discovered by the French in 1799. This stone was
useful in decoding the ancient Egypt hieroglyphs as it contained the same text carved in the
classical Greek language, hieroglyphs and the Demotic script.
If you move further along the group of boulders situated by the bushes, you will find the next
variety of diorite – quartz diorite.
(7) Quartz diorite
Quartz diorite is an igneous rock of the intermediate composition characterised by greenish grey
colour and composed of plagioclase, hornblende, biotite, pyroxene and quartz. It rarely forms
separate solids and is usually found together with diorite. It is common in Germany, the United
States of America, Norway and Italy. Quartz diorite is related to the gold and iron ore
3
The Lithuanian Musuem of Geology
mineralisation. It is used as a decorative stone and in a form of chippings for concrete and road
building.
The group of igneous acidic rocks also includes the rapakivi granite. It is displayed in front of
the quartz diorite.
(8) Rapakivi granite
Rapakivi is an exclusive variety of biotite-hornblende granite. Its composition is distinguished by
very coarse (of the size of several centimetres) reddish porphyric crystals of potassium feldspar.
Usually, they are oval in shape and have thin edgings of grey plagioclase. In Finnish, the name
of rock means the “crumbly stone”, because the component minerals expand and contract
differently under heat and cold conditions. In other words, the minerals have different heat
expansion coefficients. Therefore, rapakivi cracks and decays more quickly than any other rock
under temperature fluctuations.
Several large plutonic rocks of rapakivi granite are in Finland, Sweden and at the bottom of the
Baltic Sea. A great number of boulders of this granite, including the famous Puntukas, have been
brought by the glaciers to Lithuania.
Another igneous acidic rock is the pegmatite. You will find this rock by walking towards the
pond.
(9) Pegmatite
The veined variety of granite is called the pegmatite. This rock is characterised by a unique
structure, mineral content and minerals related to it. Pegmatite is composed of the same minerals
that are characteristic to granite, mostly plagioclase, microcline, quartz and micas – black biotite
and light muscovite. In addition, pegmatites usually contain rare minerals and jewellery
gemstones – tourmaline, topaz, fluorite, beryl, mountain crystal, emeralds and aquamarines.
Pegmatite is of very coarse granularity. It is composed of the crystals that are of the size of
several tenths of centimetres or even of the size of several metres. Another feature of pegmatite’s
composition is the fancy ingrowths of quartz in microcline crystals reminding of the Arabic or
Hebrew script. Due to this, such texture is called graphic or pegmatitic. Describing this feature,
the French scientist Haüy came up with the term “pegmatite”. Pegmatites form the bodies of the
shape of veins. Their thickness usually ranges from several tenths of centimetres to several tenths
of metres and the length – from several tenths of metres to several kilometres.
(10) Granosyenite
Alkaline rocks have higher content of potassium and sodium than of aluminium. The silicon
content is too low for the formation of the feldspar-type aluminium silicates, therefore, alkaline
rocks are characterised by nepheline, whereas the effusive varieties – by leucite. Alkaline rocks
are usually characterised by light colours and low density. They are very rare among the igneous
rocks. There are 6 boulders attributed to this group in the exposition. These are not the typical
alkaline rocks as they are composed of grano-syenites. Their composition is intermediate
between granite and syenite, therefore, these rocks are sometimes called the quartz-syenite. The
quartz content does not exceed 15%.
And now, let us take a look at a yet another pegmatite marked by number 11. This rock is
interesting due to clearly visible pegmatitic veins that have been formed in the course of millions
of years.
(11) Pegmatite
The major group of metasomatic rocks is composed of the boulders of pegmatitic rocks. There
are 16 of them in our open-air exposition. Usually, these are the granites with a piece of
4
The Lithuanian Musuem of Geology
pegmatitic vein or a vein crossing the boulder. Pegmatite is a veined variety of the acidic rock.
What is the process of formation of pegmatites? These rocks are formed through the rapid
cooling of magmas. Although pegmatites required fast-cooling conditions, they can form
extremely large crystals even of the length of up to several tenths of centimetres. The rock lying
deep down in the Earth cracks in case of immediate opening of the Earth and hot mineral mass
penetrates into its gaps. It cools and solidifies quite fast. In this way, the ingrowths of coarsecrystalline pegmatite veins are developing in the rock. In the course of time, the old rock of fine
granularity starts to decay and the young strong veins thrust up to the surface forming interesting
shapes of boulders.
(12) Red Baltic porphyry
Extruding onto the Earth’s surface, granitic magma causes the formation of several types of
volcanic rocks. The varieties of clearly porphyric composition are the porphyries. In these
varieties, the phenocrysts are composed of quartz, potassium feldspar, plagioclase, whereas the
non-crystalline or extremely fine-crystalline part can be grey, reddish, red and brown and
sometimes almost black.
The red variety of quartz porphyry, where potassium-sodium feldspars are dominated by the red
ortoclaz and microcline, is called the red Baltic porphyry. The phenocryst content is composed
of orthoclase or microcline and quartz. This rock has a smooth texture. When recognising these
rocks, it is very important to sport the inserts of quartz. The deposits of porphyry are located in
western Finland, Aland Islands and central Sweden, therefore, the boulders of these rocks are
quite common in Lithuania and can be found in the roadsides and fields. According to the
finding place, it is possible to identify the direction of the moving of glacier, therefore, such
boulders are called the inherent. This rock came from the bottom of the Baltic Sea.
Another group of petrographically classified rocks includes the sedimentary rocks. The
sedimentary boulders are known to constitute less than 1% of all rocks. They are not resistant to
environmental impact, therefore, are prone to fast decay. In this open-air exposition of boulders,
you can find the dolomites, limestone and sandstones attributed to this group.
They are displayed in the southern part of the territory, near the groups of boulders of igneous
and metamorphic rocks.
(13) Dolomite
Dolomite is composed of the mineral having the same name, is similar to limestone and is
characterised by yellow or grey colour and granular texture. It is harder and more durable than
limestone, although is also usually spongy, contains cavities as it is formed not only through
direct sedimentation of dolomite in the water, but also of limestone under the influence of
saltiness penetrating to the layers of the limestone from the water bodies. The easiest way to
distinguish dolomite is to apply the reaction with hydrochloric acid – the dolomite is
characterised by a weak reaction, whereas the powder of dolomite and limestone are
characterised by a strong reaction. This rock is formed in supersaturated lagoons through the
dripping of dolomite crystals. It is completely water-insoluble. The name “dolomite” is in
honour of the French geologist de Dolomieu who investigated the mountains of western Italy and
later named them the Dolomite Alps.
In Lithuania, dolomites are mostly found in the northern part of the country. This rock is
considered to be inherent to this place. Its chippings are used for building roads, blocks are used
in the construction sector and for the production of lime and finishing materials. The dolomite
was used for the finishing of the facades of the Building of Government and the Vilnius Bus
Station.
5
The Lithuanian Musuem of Geology
Nearby is another dolomite. If you take a closer look, you will see the process of layer-by-layer
formation of the sedimentary rocks. The rocks are forming with one deposit covering another.
Interesting is the fact that millions of years have to pass between the formation of one layer and
another. The surface of this dolomite reflects the bottom of the sea of the Devonian Period.
Please take a closer look at the white and greyish spots and you will see that the surface of rock
is carved with worm paths, so-called the fucoids, and the remains of the dead shells. Therefore,
although the geology is a science of inanimate nature, it is also closely related to the animate
nature.
(14) Stromatopora limestone
Limestone is the most common sedimentary carbonate rock in Lithuania. It is usually of white to
grey in colour but can be of brown, reddish, dark grey to black colour due to impurities. This
rock is characterised by various granularity, aphanitic structure, massive, layered and lumpy
texture. Limestone usually contain fossils – remains of organisms that sometimes occupy even
the major part of the limestone. Based on the predominating remains of organisms, the coral,
articulate-brachiopod, foraminipheral, bryozoan, lily of the sea-crinoidean and other types of
limestone are distinguished. Limestone is largely composed of mineral calcite the main sources
of which are the aforementioned marine organisms. The skeletal fragments of these organisms or
shells are visible in the rock. The exhibited limestone also contains various remains of cnidaria
and the stem of the lily of the sea.
Crushed limestone is used for building roads, the blocks of limestone – for building
constructions. Limestone is used as a raw material in the production of cement, lime and the
calcification of acid soils. In Lithuania, the most important deposits of limestone are formed in
the northern part of the country, mostly in the city of Naujoji Akmenė.
The next exhibit of our excursion is displayed in the nearby group of boulders. You will find it
by walking towards the oak-wood.
(15) Conglomerate
The rock composed of the boulders reinforced with particles of cement and pebble is called the
conglomerate, whereas the rock composed of gravel is called the gravelite. The reinforcing
materials of conglomerate usually include calcite, clay, quartz or iron hydroxides. These
materials can deposit from the water, through the layering of rocks at the bottom of the water
body or can be drifted by the water floating and oozing through the rocks.
(16) Amphibolic epidotic gneiss with garnets
A certain part of the exposition is occupied by the metamorphic rocks divided into smaller
subgroups. They constitute the majority of boulders found in Lithuania. Let us take a closer look
at some of them.
Metamorphic rocks are formed through the endogenous processes acting on the sedimentary,
igneous and previously formed metamorphic solid rocks. These processes cause changes in their
mineral and chemical composition, structure and texture. The term “metamorphism” is derived
from the Greek word “metamorfoz” meaning the “change”.
Metamorphic rocks have a diverse chemical composition that depends on the composition of
primary rocks. They can also be composed of one or several minerals. The main rock-forming
minerals include quartz, feldspars, micas, pyroxene and amphiboles.
The metamorphism of rocks is caused by various reasons – temperature, pressure, composition
of gaseous and aqueous solutions of rocks. Metamorphic rocks are very common on the Earth
and can be divided based on various features. At the moment, the majority of scientists use the
classification based on the metamorphic facies. By composing the classification of boulders, the
geologist Gediminas Motūza divided the group of metamorphic rocks into the following facies:
6
The Lithuanian Musuem of Geology
greenschist, epidotic-amhibolite, amphibolite, granulite and eclogite. The open-air exposition of
boulders exhibits only those boulders that reflect the epidotic-amphibolite, amphibolite and
granulite facies.
The metamorphic rocks of the epidotic-amphibolite facies are composed of plagioclases,
hornblende, epidote and can also contain garnets. Epidotic-amphibolite facies are formed under
the medium temperature and pressure conditions.
Gneiss is a metamorphic rock formed from the sedimentary or acidic igneous rocks in the course
of regional metamorphism. It has an exclusive texture with clearly visible layers. Gneiss is
characterised by durability and is composed of quartz, plagioclase and feldspar.
The next boulder marked by number 17 in the audio-guide is displayed nearby. You will find it
by moving in the direction of the museum’s building.
(17) Crystalline mica schist
Mica schist is composed of micas – light muscovite or black biotite and sometimes – both. Micas
entirely cover the surface of schist planes and seems as they are dominating, although the rock
actually has high quartz, plagioclase, microcline and garnet content. Based on the mineral
composition, these rocks can have more precise names, i.e., component minerals can be listed in
an increasing order, for example: garnet-muscovite-feldspar-quartz schist. Crystalline mica schist
has a fine medium-grained texture, which can also be porphyric quite often. It is composed of
larger grains of garnet or other minerals – of the size from several up to 20 millimetres. The
minerals forming the schist texture that is characteristic to the rock are of the shape of extended
plates, for example, micas are arranged in one direction along the long axes and the planes of flat
minerals are more or less parallel.
Now, let us take a walk towards the building of the museum to explore to the group of three
large boulders.
(18) Biotite plagiogneiss
This is a medium-metamorphic rock that has been formed during the regional metamorphism in
a very deep, almost non-aqueous environment.
(19) Amphibolic gneiss
The stones with various impressions are quite common in Lithuania. People usually see the
shapes of goat hoof, hand of the devil, etc. Various myths and legends accompany such stones,
however, the geologists can easily explain the origin of these impressions. The glaciers are
composed not only of ice but also of dirt and boulders. As the warming climate caused glaciers
to melt, the boulders of various shapes and sizes started to move from their places, rub one to
another and were washed with water and dulled by the pebble. Under the influence of external
forces, even the smallest stone of pebble passing to the hollow of the boulder erodes the holes of
different depths and shapes. Such holes are also visible on this boulder with hand-shaped
impressions.
(20) Cataclasite
Metamorphism is conditioned by various geological processes that are reflected by the features
of the texture and composition of the rocks formed during these processes. For example, the
rocks are crushed in the breaks between the moving blocks of the Earth’s crust as if in the quern.
In the abyssal areas of breaks, in the depth of several kilometres, where the temperature reaches
several hundred degrees, the rocks suffer plastic deformations, their grains are extended,
recrystallised and arranged in a certain tendency. Such rocks are called the mylonites. They are
characterised by the extended remains of crystals of the shape of lens or eyes. In the smaller
7
The Lithuanian Musuem of Geology
depths, where pressure is a more influencing factor than temperature, the rock is simply crushed
into fine fragments without tendentious arrangement. Not only the rocks but also their
component minerals are influenced by the crushing and deformations.
On the other side of the pathway you will find a similar rock – breccia.
(21) Breccia
Same as cataclasite, breccias represent the group of rocks of cataclastic metamorphism. Breccia
is the same cataclasite composed of clearly visible fragments of the size of several centimetres or
even larger. Usually, the large fragments are “sunk” in the mass of fine-ground particles.
Now, please cross the trench to explore the field of boulders of metasomatic rocks hiding under
the birches.
(22) Secondary quartzite
Metasomatism is an alternation of one mineral to the mineral of different chemical composition
caused by the influence of chemically active solutions. Adinoles, skarnes, secondary quartzites,
greisens, pegmatites and gneisses are attributed to metasomatic rocks. This secondary quartzite is
grey in colour and fine-grained in texture.
Please come closer to the next rock of metasomatic origin marked by number 23. You can find it
in the nearby group of boulders.
(23) Cordierite-garnet-biotite gneiss
This small boulder is one of the varieties of the group of metasomic rocks composed of
cordierite, garnet and biotite minerals. The rock is covered with schist and is characterised by
medium granularity. Large and transparent crystals of cordierite are considered as precious
stones. According to the legends, cordierite was used by the Vikings in the navigation as due to
strong pleochroism it could be used to determine the position of sun even under overcast
conditions. The crystals of garnet contained in the gneiss are also considered as the gemstones.
Let us go back over the trench to explore the groups of boulders situated along the fence on the
western side.
(24) Amphibolite-biotite plagiogneiss
Ultrametagenic boulders are the secondary rocks formed of igneous and metamorphic rocks in
the depth of several kilometres under the conditions of high temperature, pressure and hot
solutions rising out of the depths of the Earth. These factors condition the alteration of chemical
and mineral composition of these rocks and in even cause the melting in some spots.
Ultrametagenic boulders are classified into homogeneous and massive rocks. Amphibolitebiotite plagiogneiss belongs to the group of homogeneous rocks.
Gneiss, which is attributed to massive ultrametagenic rocks, is displayed in the group of boulders
situated near the pond.
(25) Garnet-cordierite gneiss with xenoliths
The exposition also contains a lot of granitoids, including some large boulders. This group is
composed of biotite-granite-gneisses, garnet-biotite plagiogneisses, pegmatoidic microclinic
granites, biotitic garnets, plagioclase-microclinic granites, garnet-cordierite gneiss with
xenoliths, etc. In Greek, “xenolith” means the “foreign rock”. These are the fragments of rocks
pulled by the magma. If the rock containing xenoliths solidified in the great depth, it is usually a
8
The Lithuanian Musuem of Geology
very alternate rock containing an intrusive. Xenoliths are also common in lava. In such case,
they are the fragments of the volcanic channel. Xenoliths are of various sizes – from the size of
small microscopic crystals to the size of several metres or even larger.
Xenoliths are important for the investigation of the depths of the Earth, as this is the only way to
see and receive samples from the depth of several tenths to several hundreds of kilometres.
Sometimes alkaline basalts contain xenoliths that are brought from the depth of 60-80 km,
whereas the xenoliths found in kimberlites are brought from the depth of 100-200 km.
And now let us take a closer look at migmatites. The exhibit number 26 is nearby. You will find
it by walking towards the line of big trees.
(26) Biotite-microclinic plagio granitogneiss
Migmatites are mixed rocks formed of primary metamorphic rocks mixed with younger quartzfeldspar material. They are of very diverse shapes – striped, spotted, usually with elaborately
curly veins.
The largest group of exhibited migmatites is composed of substratum migmatites. These are
extremely fine-striped gneisses. Their two phases are distinguished by clear layers that are
parallel to the area covered with schist.
The temperature rising in the depths of the Earth eventually causes the melting of rocks. It does
not happen immediately: first of all, the less temperature-resistant part starts to melt and the
other, less fusible, retains its solid state. In this way, the inhomogeneous formation is developed
from the remains of primary not yet melted rock and the melt solidified and decrystallised in the
shape of veins, stripes and irregular spots. Such inhomogeneous rock is called by the Greek word
“migmatite” with the meaning of “mixture”. If the melting reaches 25-30% of the volume of
thickness, it functions as a liquid, plastic material and flows onto the Earth’s crust from highpressure places to low-pressure places. During the movement, the entire material tends to felt
and mixes with other substances, therefore, migmatites are characterised by diverse composition.
Melting occupies large volumes in the depths of the Earth, whereas migmatites compose the
major part of the upper crust of the continents. Due to rising and erosion, the parts of the abyssal
crust, including the migmatites, appear on the surface of the Earth. As a result, they are very
common here. Migmatites constitute the major part of boulders found in Lithuania.
The boulder-forming rock belongs to the variety of migmatites, which is characterised by the
net-like weaving of the granitic veins.
(27) Microclinic garnet-biotite-plagiogranite gneiss
This is the example of another textured variety of migmatites – lens-shaped migmatite with the
characteristic lens-shaped accumulations.
The next migmatite marked by the audio-guide number can be found by walking towards the
museum’s building.
(28) Amphibolite-biotite plagiogneiss
This boulder is attributed to porous migmatites. They are characterised by the mineral
accumulations of the shape of large eye, are composed of quartz and feldspar and are oriented in
the direction of layering. Sometimes, the flaser textures (the films of the shape of needles or
leaves) are visible around the “eyes” - as if the eyelids.
(29) Ptigmatite
This is a morphologic variety of migmatites characterised by ornately layered veins of granitic or
aplitic substance. These were formed due to ranging movements in a semi-melted or soft
9
The Lithuanian Musuem of Geology
environment. Due to the irregular stratification and texture of the containing rocks, ptigmatites
are different from the layered migmatites that are characterised by regularly stratified granitic
veins.
(30) Shadow migmatite
Shadow migmatite or nebulite is a textured variety of migmatites. It is characterised by diffusive,
indistinct and uneven distribution of older and usually darker and fine-grained rocks. They are
sort of dissolved in the granitic material and become unrecognisable and appear as if the
accumulations of clouds. These are the intermediate rocks between the granitoids and
migmatites.
It was the final stop of our excursion around the park of field boulders. We are expecting to
supplement the Vievis exposition of boulders with historical/archaeological boulders. At the
moment, four millstones are displayed near the entrance to the storage facility. The exact
purpose of use of one of them is not fully known. Juozas Lisauskas, the person who donated this
object to the museum, made an assumption that this could be the prototype of the wheel as it is
composed of medium-cemented sandstone and the surfaces of rubbing are uneven and have
indentations. The bowl-shaped stones are also displayed nearby.
We also invite you to visit the drill-core storage facility and the expositions of polished boulders
and component minerals of rocks displayed in the museum’s building. The colour photographs
on the walls of the third floor corridor show portray the images of the largest boulders in
Lithuania – the natural wonders, as well as the schematic map of the largest spread of glaciers
and the map of the locations of geological monuments in Lithuania.
Thank you for visiting our museum! See you next time!
10