Greywacke facts for kids
Greywacke is a special kind of sandstone. It's known for being very hard and usually dark. Imagine a rock made of different bits and pieces! Greywacke contains grains of quartz and feldspar. It also has tiny pieces of other rocks. All these parts are stuck together in a fine, muddy material called a clay matrix.
You often find greywacke in very old rock layers, especially those from the Palaeozoic Era. This time period was hundreds of millions of years ago. The grains in greywacke can be as small as sand or as big as gravel. More than 15% of the rock's total volume is made of the fine, muddy material that holds it all together.
Contents
How Greywacke Forms
For a long time, scientists wondered how greywacke was made. Usually, when a river flows into the ocean, it drops heavier sand and gravel first. Lighter mud settles later. So, it was a puzzle why sand, gravel, and mud would all be mixed together in one rock.
Underwater Avalanches
Now, geologists have a good idea of how greywacke forms. They believe it's created by powerful events like underwater submarine avalanches. These are also called strong turbidity currents. Think of a giant, fast-moving underwater landslide!
Mixing Sediments
These powerful currents stir up all sorts of sediment from the ocean floor. They mix sand, gravel, and mud into a thick, soupy mixture called a slurry. When this slurry settles, it creates rocks like greywacke. These rocks often show special features from how they were laid down.
Where Greywacke is Found
The idea of turbidity currents makes sense because of where we find greywacke. It's often located at the edges of continental shelves. These are the parts of continents that extend under the ocean. You can also find greywacke at the bottom of deep oceanic trenches. It's also common at the base of mountain ranges that are still forming. Sometimes, it appears alongside dark shales, which are rocks that form in very deep parts of the sea.
Images for kids
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Photomicrograph of a feldspathic wacke (sandstone). Top image is in plane polarized light (PPL); bottom image is in cross polarized light (XPL). Blue epoxy fills pore spaces.
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Photomicrograph of a lithic wacke (sandstone). Top image is in plane polarized light (PPL); bottom image is in cross polarized light (XPL). Blue epoxy fills pore spaces.
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A greywacke rock
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Example of greywacke cliffs, Mangaweka, Nthrn.Manawatu, New Zealand
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Wall of greywacke at Möhne Reservoir dam, Germany
See also
In Spanish: Grauvaca para niños
