Traces of Glaciers on Earth's Oldest Rocks EarthCache

Traces of Glaciers on Earth's Oldest Rocks

Den geologiske situation omkring Nuuk

Bjergarter fra den prækambriske periode (4,5 milliarder til 570 millioner år siden) kan findens i den vestlige og sydlige del af Grønland. Disse gamle granitter og gnejser var dækket af sedimenter, der blev erodet væk over millioner af år, så det prækambriske grundfjeld, dannet på overfladen af gletsjerne, nu dominerer de isfrie områder i Grønland.

I området med den såkaldte "gamle kerne" i det sydlige Grønland er klipperne 2,7 til 3,7 milliarder år gamle; de ældste kendte sten på jorden blev fundest nordøst for Nuuk. I området for earthcachen du sådan en gammel gnejs, der krydses af talrige letterer kvartsårer.

Glaciale striber

Glaciale striber eller striae er ridser eller udhulninger skåret ind grundfjeldet ved glacial slid. De er en række lange, lige, parallelle linjer eller riller, der er ridset på en grundfjeldsoverflade af klippefragmenter, der er fastgjort i bunden af en gletsjer. Disse ridser og udhulninger blev først genkendt som et resultat af en gletsjer i bevægelse i slutningen af det 18. århundrede i Schweiz/Europa. De schweiziske alpinister bemærkede også, at hvis disse striber er synlige i dag, må gletsjerne også være på vej tilbage.

Glaciale striber er sædvanligvis multiple, lige od parallelle, hvilket repræsenterer gletsjerens bevægelse ved hjælp af klippefragmenter og sandkorn, indlejret i bunden af gletsjeren, som skærende værktøjer. Store mængder af groft grus og kampesten, der bæres med under gletsjeren, giver den slibende kraft til at skære gennem-lignende gletsjerriller. Finere sedimenter også i bunden af den bevægende gletsjer gennemsøger og polerer grundfjeldets overflade yderligere og danner en gletsjerbelægning. Is i sig selv er ikke et hårdt nok materiale til at ændre formen på sten, men fordi isen har sten indlejeret i den basale overflade, kan den effektivt slibe grundfjeldet.

De fleste gletsjerstriber blev afsløret ved tilbagetrækningen af gletsjere siden det sidste istidsmaksimum eller den nyere lille istid. Ud over at angive glacialisens strømningsretning, kan dybden og omfanget af forvitring af striberne bruges til at estimere varigheden af klippens eksponering efter gletsjeren.

EarthCache-stedet

Den meget gamle gnejs, der allerede er beskrevet, kan findes i EarthCache-området, og klippens overflade har her en enestående ejendommelighed: Der gletsjerstriber, der er synlige på overfladen, løber faktisk parallelt med adskillige kvartsårer, der er indlejret i gnejsen. Forskellen mellem grunfjeldet (gnejs) og kvartsårerne, som efterfølgende er dannet ved migration af opløsninger til sprækker i den metamorfe gnejs, kan her iagttages særligt godt. På grund av denne ekstraordinære situation og muligheden for at lære noget om stedets geologi, blev denne placering valgt til en EarthCache.

Det særlige: Kvartsårerne, der løber parallelt med gletsjerstriberne i gnejsen, rager lidt ud fra grundfjeldet. Årsagen til dette ligger i den forskellige modstand mod glacial slid. Tænk over det, og besvar derefter spørgsmålene om at logge EarthCache.




The geological situation around Nuuk

Rocks from the Precambrian period (4,5 billion to 570 million years ago) can be found in the west and south of Greenland. These ancient granites and gneisses were covered by sediments that were eroded away over millions of years, so that the Precambrian bedrock, formed on the surface by the glaciers, now dominates the ice-free areas of Greenland.

In the area of the so-called "old core" in the south of Greenland, the rocks are 2,7 to 3,7 billion years old; the oldest known rocks on earth were found northeast of Nuuk. In the area of the earthcache you will find such an ancient gneiss that is crossed by numerous lighter quartz veins.

Glacial Striations

Glacial striations or striae are scratches or gouges cut into bedrock by glacial abrasion. They are a series of long, straight, parallel lines or grooves scratched onto a bedrock surface by rock fragments lodged in the base of a glacier. These scratches and gouges were first recognized as the result of a moving glacier in the late 18th century in Switzerland/Europe. The Swiss alpinists also noted that if these striations are visible today that the glaciers must also be receding.

Glacial striations are usually multiple, straight, and parallel, representing the movement of the glacier using rock fragments and sand grains, embedded in the base of the glacier, as cutting tools. Large amounts of coarse gravel and boulders carried along underneath the glacier provide the abrasive power to cut through-like glacial grooves. Finer sediments also in the base of the moving glacier further scour and polish the bedrock surface, forming a glacial pavement. Ice itself is not a hard enough material to change the shape of rock but because the ice has rock embedded in the basal surface it can effectively abrade the bedrock.

Most glacial striations were exposed by the retreat of glaciers since the Last Glacial Maximum or the more recent Little Ice Age. As well as indicating the direction of flow of the glacial ice, the depth and extent of weathering of the striations may be used to estimate the duration of post-glacier exposure of the rock.

The EarthCache site

The very old gneiss already described can be found in the area of the EarthCache and the surface of the rock has an unique peculiarity here: The glacial striations visible on the surface of the rock run parallel to numerous quartz veins embedded in the gneiss. The difference between the bedrock (gneiss) and the quartz veins, which subsequently formed through the migration of solutions into fissures in the metamorphic gneiss, can be observed particularly well here. Due to this exceptional situation and the opportunity to learn something about the geology of the place, this location was chosen for an EarthCache.

The special feature: The quartz veins, which run parallel to the glacial striations in the gneiss, protrude a little from the bedrock. The reason for this lies in the different resistance to glacial abrasion. Think about it and then answer the questions about logging the EarthCache.


Your task to log the EarthCache:

Answer the following questions via message in English or German via my geocaching profile:

1.) Take a closer look at the gneiss and the embedded quartz veins, feel the surface and describe the differences in terms of structure and surface!
2.) How far do the quartz veins protrude from the gneiss on average?
3.) Describe in your own words why you think the quartz veins protrude from the gneiss!
4.) When you look at the glacial striations - in which direction did the glacier that left the surface traces move?
5.) Optional: Post a photo with your log, showing you and/or your GPS near the location!


After you've sent me the message with your answers, feel free to log! If there's something wrong, I'll contact you via message!

The most exciting way to learn about the Earth and its processes is to get into the outdoors and experience it first-hand. Visiting an Earthcache is a great outdoor activity the whole family can enjoy. An Earthcache is a special place that people can visit to learn about a unique geoscience feature or aspect of our Earth. Earthcaches include a set of educational notes and the details about where to find the location (latitude and longitude). Visitors to Earthcaches can see how our planet has been shaped by geological processes, how we manage the resources and how scientists gather evidence to learn about the Earth. To find out more click HERE.