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All work is due on April 12!
- Tues 4/5: class
- Thurs 4/7: no class
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There are three types of rock- Igneous, Sedimentary, and Metamorphic.
- Igneous: These form from molten rock materials: either
lava or magma. An example is granite.
- Sedimentary: These form by cementing together lots
of rock particles. Some of these rocks can have horizontal layers, like
sandstone, or can form from organic processes, like coral (limestone).
- Metamorphic: These form when a rock is re-heated
and partially re-molten altered in some way. Metamorphic rocks can be
made from Igneous rocks, Sedimentary rocks, or even other Metamorphic
rocks. Marble forms from limestone, Gneiss forms from Granite, and Schist
might form from Slate, which forms from Shale, a sedimentary rock.
How can you recognize each of these types of rock?
These may have many interlocking crystals, or may be glassy. They will
not have any layers or bands. We'll talk more about these later.
These may be composed of particles- grains of sand, silt, or clay, or
even gravel. The particles would be cemented together, and so are likely
to break free if you rub a face of the rock. These 'clastic' sedimentary
rocks tend to be softer, or more fragile than Igneous or Metamorphic rocks.
If it is possible to see evidence that some sort of life process made
the rock, then it is likely that the rock is an organic sedimentary rock,
such as coal or organic limestone, which is often made from coral.
Metamorphic rocks start as one type of rock and end up as another after
re-heating. This reheating may partially melt some of the minerals and
not others. When the rock cools down, these minerals may get a chance
to re-crystalize. Igneous rocks often metamorphose so that you get 'bands'
of a mineral. These bands are less distinct than a sedimentary layer,
and they are composed of a mineral, not sedimentary particles. Sedimentary
rocks may metamorphose so that you get shiny mica platelets forming. This
is called 'schist'. Rocks with lots of schist can be called 'schisty'-
but you should only use that term when you are with friends.
Oh, by the way- Baltimore County is filled with metamorphic rocks. The
marble used in the Washington Monument came from Cockeysville, and all
of the stone houses around here are made from a stone called Quartzite,
which in turn came from sandstone. And have you ever gone walking at Oregon
Ridge park and noticed how shiny the dirt trails are? Really- the soil
is filled with lots of shiny mica platelets. You guessed it: you are standing
in schist.
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