The Ancient Gneiss of Milford Sound
Tasks:
- Observe the rock faces around Milford Sound. What colors and textures can you see in the gneiss? Based on these observations, what minerals might be present in the rock?
- Look closely at the banding in the rock. Are the layers straight, wavy, or folded? What might this tell you about the forces that shaped them?
- How does the gneiss influence the overall shape of the fjord? Consider its resistance to erosion compared to other rock types.
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Milford Sound is famous for its breathtaking fjords, towering cliffs, and dramatic landscapes. But beneath this beauty lies an ancient geological history—one written in the gneiss that forms the foundation of the region. Gneiss is a metamorphic rock that has undergone extreme heat and pressure deep within the Earth's crust, giving it its characteristic banded appearance.
In this EarthCache, you will explore:
- What gneiss is and how it forms
- The minerals that make up Milford Sound’s gneiss
- How geological forces shaped this landscape over millions of years
What is Gneiss?
Gneiss is a high-grade metamorphic rock, meaning it formed under intense heat and pressure. It originates from pre-existing rocks (either sedimentary or igneous) that were transformed deep underground through regional metamorphism.
Key Characteristics of Gneiss:
- Foliation: Gneiss displays a banded texture due to the separation of minerals into light and dark layers.
- Minerals: Typically contains quartz, feldspar, and mica, with possible traces of hornblende, garnet, or other minerals.
- Formation: The gneiss in Milford Sound formed over 300 million years ago as part of the broader Fiordland metamorphic complex.
Gneiss in Milford Sound
The cliffs surrounding Milford Sound are primarily composed of metamorphic gneiss and schist, shaped by tectonic forces, heat, and pressure over millions of years. Key geological events include:
- Metamorphism: The original rocks were deeply buried and subjected to extreme conditions, transforming into gneiss.
- Tectonic Uplift: Over time, tectonic forces pushed these rocks back to the surface.
- Glacial Erosion: During the Ice Age, massive glaciers carved out the deep fjords we see today, exposing the ancient gneiss.
The result is the dramatic, steep-walled fjords of Milford Sound, where the resistant gneiss formations rise sharply from the water.
Identifying Gneiss in the Field
When observing the rock formations in Milford Sound, look for:
- Banded or layered appearance with alternating light and dark minerals.
- Hardness and durability, as gneiss is more resistant to erosion than softer rocks.
- Glacial striations or polished surfaces, showing past glacial movement.
https://de.wikipedia.org/wiki/Gneis
https://www.mineralienatlas.de/lexikon/index.php/RockData?lang=de&rock=Gneis
https://de.wikipedia.org/wiki/Milford_Sound/Piopiotahi