Two More Rock Types to Collect
Igneous rocks are born from fire. The other two types have different origin stories. Sedimentary rocks are made from bits and pieces glued together. Metamorphic rocks are rocks that got squeezed and cooked into something new. Think of it as a rock makeover show.
Built Layer by Layer
Sediment is loose material like sand, mud, pebbles, and bits of shells. Over time, sediment piles up, usually at the bottom of lakes, rivers, and oceans. Then two things happen:
- Compaction: layers on top press down and squeeze out water and air.
- Cementation: minerals dissolved in water act like natural glue and stick the pieces together.
Press plus glue equals rock. It can take thousands to millions of years. Patience is not optional.
Three Kinds of Sedimentary Rock
- Clastic: made from broken pieces of other rocks. Sorted by size: conglomerate (big rounded pebbles), sandstone (sand), and shale (super-fine mud).
- Chemical: forms when minerals come out of water, usually as it evaporates. Rock salt and some limestone form this way. The Great Salt Lake is making it right now.
- Organic: made from the remains of living things. Coal is squished ancient swamp plants. Some limestone is made of billions of tiny shells.
Reading the Layers
Sedimentary rocks are history books. They form in layers called strata, and the oldest layer is usually on the bottom. They can hold clues like ripple marks from ancient beaches, mud cracks from dried-up lakes, and, best of all, fossils. Almost every fossil ever found came from sedimentary rock. Want dinosaurs? Dig here.
The Grand Canyon is basically a giant stack of sedimentary layers, nearly 2 billion years of history you can see from the rim.
The Rock Makeover
Metamorphic means "change of form." These rocks start as igneous, sedimentary, or even other metamorphic rocks, then get transformed by:
- Heat: from nearby magma or from being buried deep.
- Pressure: from tons of rock on top or from crashing tectonic plates.
- Hot fluids: that carry new chemicals in and out.
Here's the key: the rock doesn't melt. If it melts, it becomes magma and starts over as igneous. Metamorphism is more like baking cookie dough. It changes, but it doesn't turn back into liquid.
Two Ways It Happens
- Contact metamorphism: magma bakes the rock right next to it. Small area, lots of heat.
- Regional metamorphism: huge areas get squeezed and heated when plates collide and mountains form. This is the big one.
Foliated vs. Non-Foliated
- Foliated: minerals get squished into flat layers or bands, like a stack of pancakes. Shale becomes slate (used for old chalkboards and roofs), then schist (sparkly), then gneiss (pronounced "nice," with striped bands). Yes, the rock is literally named nice.
- Non-foliated: no layers. Marble comes from limestone and is used for statues and fancy floors. Quartzite comes from sandstone and is super tough.
Parent Rock Cheat Sheet
- Shale → slate → schist → gneiss
- Limestone → marble
- Sandstone → quartzite
- Granite → gneiss
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Sedimentary rocks form from sediment through compaction and cementation.
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They come in three kinds: clastic, chemical, and organic.
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Sedimentary rocks form in layers and hold nearly all fossils.
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Metamorphic rocks form when heat, pressure, or hot fluids change a rock without melting it.
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Foliated rocks have layers (slate, schist, gneiss); non-foliated don't (marble, quartzite).
A cliff shows three layers. The bottom is shale with fish fossils. The middle is sandstone with ripple marks. The top is limestone full of seashells. Tell the story: what was this place like long ago, and how did it change? Which layer is oldest, and how do you know?
HS-ESS-1.6* (using ancient Earth materials as evidence of Earth's history), HS-ESS-2.5 (water's role in Earth materials and surface processes).
Write your answer in your notebook or class assignment, then check it off here.