GE: Najstarší kameň v Bratislave EarthCache

🏁 Úvod

🔍 Otázky pre zalogovanie earth cache

1. Popíšte vlastnými slovami zvlnené vzory alebo vrásy na migmatite – aký majú tvar, veľkosť, ako sa farebne odlišujú jednotlivé vrstvy.
2. V migmatite sa nachádzajú drobné granáty. Odhadnite veľkosť najväčších granátov, ktoré môžete na fasáde nájsť a popíšte ich farbu.
3. Podľa Vášho pozorovania granátov a dole uvedeného popisu typov odhadnite, aký typ granátov sa nachádza na obklade budovy. Zdôvodnite toto tvrdenie.
4. K logu priložte fotku Vás alebo Vášho osobného predmetu (SWG, papierik s geocaching menom) na mieste EarthCache.

📍 Najstarší kameň v Bratislave

🪨 Bengal Blue migmatit

Bol privezený z lomu v Indii. Tento kameň je asi najstarší v Bratislave – má približne 3 miliardy rokov (3000 miliónov!). Vznikol v dobách, keď sa Zem ešte len formovala. Dotkli ste sa už niečoho tak pradávneho?

Migmatit je druh horniny, ktorá vzniká kombináciou rôznych typov hornín alebo minerálov počas geologických procesov. V hlbokých vrstvách zemskej kôry bol vystavený teplu a obrovskému tlaku, čo spôsobilo, že sa niektoré časti kameňa začali taviť a premiešavať s okolitou magmou. Po ochladení horniny sa z nej vylúči živec a kremeň, následkom čoho vzniknú v hornine svetlé prúžky. Tento proces sa nazýva migmatitizácia.

Na hornine si všimnite zvlnené vzory. Tieto kamenné "vrásky" sú výsledkom deformácií, ktoré nastali, keď bol kameň v hĺbke zohriaty na vysoku teplotu do plastickeho stavu a zároveň naň pôsobili okolité horniny tlakom dôsledkom čoho vznikli deformácie, ktoré nazývame vrásy. Vrásnenie migmatitu prebiehalo veľmi dlhú dobu, hornina sa preto prevrásnila mnohonásobne, preto pôvodné zoradenie vrstiev už nie je na prvý pohľad zrejmé. 

V migmatite na obklade budovy možete voľným okom pozorovať granáty – drobné, tmavofialové minerály, ktoré sa tvorili v extrémnych podmienkach vysokej teploty (okolo 600 až 800 °C) a vysokého tlaku počas veľmi dlhej doby rádovo až miliónov rokov, keď hornina bola hlboko pod zemským povrchom. Počas kryštalizácie granáty pomaly rastú a môžu dosiahnuť rôznu veľkosť v závislosti od toho, ako dlho zostane hornina vystavená vhodným podmienkam.

💎 Granáty

Granáty sú skupinou minerálov, ktoré sa vyskytujú v širokej škále chemických zložení, no všetky majú podobnú kryštalickú štruktúru. Všetky granáty majú základný chemický vzorec X₃Y₂(SiO₄)₃, kde "X" a "Y" predstavujú rôzne kovové prvky, ktoré sa môžu v rôznych granátoch meniť. Granáty patria k najstarším známym nerastom a drahým kameňom, sú cenené pre krásu a lesk. Používajú sa na výrobu šperkov, typický známy je u nás český granát.

Základné typy granátov sú
Almandín: Fe₃Al₂(SiO₄)₃ - farba: hlboká červená až fialovočervená.
Pyrop: Mg₃Al₂(SiO₄)₃ - farba: červená až oranžovočervená.
Spessartín: Mn₃Al₂(SiO₄)₃ - farba: oranžová, oranžovožltá až červenohnedá.
Grossulár: Ca₃Al₂(SiO₄)₃ - farba: zelená, žltá, hnedá až bezfarebná.
Andradit: Ca₃Fe₂(SiO₄)₃ - farba: zelená, žltá, hnedá až čierna.
Uvarovit: Ca₃Cr₂(SiO₄)₃ - farba: smaragdovo zelená.

🏁 Introduction

🔍 Tasks

1. Describe the wavy patterns or folds on the migmatite in your own words—what shape, size, and color differences do you notice between the layers?
2. The migmatite contains small garnets. Estimate the size of the largest garnets visible on the facade and describe their color.
3. Based on your observation of the garnets and the provided description of types below, identify the type of garnets present in the building's cladding. Justify your answer.
4. Attach a photo of yourself or a personal item (e.g., SWG, a slip of paper with your geocaching name) at the EarthCache site with your log.

📍 The Oldest Stone in Bratislava

🪨 The Bengal Blue migmatite

It was imported here from a quarry in India. This stone is perhaps the oldest in Bratislava—it is approximately 3 billion (3,000 million!) years old, dating back to the time when the Earth was still forming. This 3-billion-year-old piece of rock is freely accessible here. Have you ever touched something so ancient?

Migmatite is a type of rock formed by the combination of various rocks or minerals during geological processes. Deep within the Earth's crust, it was exposed to intense heat and pressure, causing some parts of the rock to melt and mix with surrounding magma. Upon cooling, minerals like feldspar and quartz crystallized, forming light-colored bands within the rock. This process is known as migmatization.

Observe the wavy patterns on the rock. These "wrinkles" are the result of deformations caused by heat and pressure when the rock was deep underground in a plastic state. Over time, these deformations created the folds we see. The folding of migmatite occurred over a very long period, with the layers being folded multiple times, making the original arrangement of layers no longer apparent at first glance.. 

In the migmatite cladding of the building, you can observe garnets with the naked eye—small, dark purple minerals that formed under extreme conditions of high temperature (around 600–800°C) and pressure over millions of years while the rock was deep beneath the Earth's surface. During crystallization, garnets grow slowly and can reach varying sizes depending on how long the rock remained under suitable conditions.

💎 Garnets

Garnets are a group of minerals that occur in a wide range of chemical compositions, all sharing a similar crystalline structure. All garnets have the general chemical formula X₃Y₂(SiO₄)₃, where "X" and "Y" represent various metallic elements that differ among garnets. Garnets are among the oldest known minerals and gemstones, valued for their beauty and brilliance. They are often used in jewelry, with the Czech garnet being particularly well-known in the region.

Basic types of garnets areAlmandine: Fe₃Al₂(SiO₄)₃ - color: deep red to purple-red
Pyrope: Mg₃Al₂(SiO₄)₃ - color: red to orange-red
Spessartine: Mn₃Al₂(SiO₄)₃ - color: orange, orange-yellow, to reddish-brown.
Grossular: Ca₃Al₂(SiO₄)₃ - color: green, yellow, brown, to colorless.
Andradite: Ca₃Fe₂(SiO₄)₃ - color: green, yellow, brown, to black.
Uvarovite: Ca₃Cr₂(SiO₄)₃ - color: emerald green.

🪨 The Bengal Blue migmatite

It was imported here from a quarry in India. This stone is perhaps the oldest in Bratislava—it is approximately 3 billion (3,000 million!) years old, dating back to the time when the Earth was still forming. This 3-billion-year-old piece of rock is freely accessible here. Have you ever touched something so ancient?

Migmatite is a type of rock formed by the combination of various rocks or minerals during geological processes. Deep within the Earth's crust, it was exposed to intense heat and pressure, causing some parts of the rock to melt and mix with surrounding magma. Upon cooling, minerals like feldspar and quartz crystallized, forming light-colored bands within the rock. This process is known as migmatization.

Observe the wavy patterns on the rock. These "wrinkles" are the result of deformations caused by heat and pressure when the rock was deep underground in a plastic state. Over time, these deformations created the folds we see. The folding of migmatite occurred over a very long period, with the layers being folded multiple times, making the original arrangement of layers no longer apparent at first glance.. 

In the migmatite cladding of the building, you can observe garnets with the naked eye—small, dark purple minerals that formed under extreme conditions of high temperature (around 600–800°C) and pressure over millions of years while the rock was deep beneath the Earth's surface. During crystallization, garnets grow slowly and can reach varying sizes depending on how long the rock remained under suitable conditions.

🧾 Zdroje informácií a referencie / Sources of information