>>Today we will discuss sedimentary rocks. They form from breaking apart or from weathering
of other types of rocks, such as igneous, metamorphic, or even other sedimentary rocks.
It's those external processes that cause the weathering of the rocks and break the rocks
up. So think a little bit about what will actually break a rock up. Think about potholes.
What actually causes potholes? Well, it has to do with ice and water. When water freezes
what happens? It expands. There's a 9% increase. That expansion can crack rock and creates
potholes.
Also, biologic factors can also contribute to breaking of a rock. Think of a tree up
on a crag in a mountain and the roots breaking the rock apart. But there's another part of
weathering, and that's called chemical weathering, and that's caused by water. You may not know
this, but water is an acid. It's a weak acid, but it's still an acid. It has a pH of 5.7.
Why? Because as the rain falls, it absorbs CO2 in the atmosphere, and that makes the
water slightly acidic. And this water, this weak acid causes rocks and minerals to break
apart. Now please note I'm not talking about acid rain here, which is caused by pollutants
that we put into the atmosphere like sulfuric acid. I'm talking about natural good old rain.
Even before the Industrial Revolution, rainwater was acidic.
So have two types of weathering: mechanical weathering, which is caused principally by
water freezing, and chemical weathering caused by water and the acid nature of rainwater.
So what is produced by weathering of rocks? What happens when a rock is weathered? Let's
start with a very common rock like a granite. It has several minerals like quartz, feldspar,
and maybe some biotite mica. So what's going to happen when we weather a granite? We'll
start off with the quartz. Remember quartz is a very hard mineral. It's seven on the
Mohs scale. So what's going to happen to quartz? It's going to form quartz sand, and quartz
sand produces what's called a clastic sedimentary rock. Feldspar's another important mineral
in granite. The acid water causes the decomposition of the feldspar, and it causes clay to form,
and also you produce ions like sodium and calcium in solution. They basically come out
of the feldspar and are carried away by the water, and I think a good analogy for this
is to think about a drip coffee maker. If you put coffee into the drip coffee maker,
the coffee is like the feldspar. So what happens? The water goes through the coffee and takes
out the coffee, which is a sodium and calcium, and takes it away and you drink it. What's
left behind are the grounds, which are like the clay. So what happens to the sodium and
the calcium when they're carried away in the water? They're carried in the river. They
go to the ocean, and what happens in the ocean? The sodium can combine with chlorine, and
what does it form? Salt. What about the calcium? The calcium is going to form or be used by
various animals and plants to form their shells, like corals and clams, and they produce limestones,
which another very common sedimentary rock. So we have two types of sedimentary rocks:
the clastic sedimentary rocks, which are particles of like sand or clay, and then the chemical
sedimentary rocks, which are produced by those ions in solution. Now let's look at some pictures
of the different rock types. Here I have a picture of a clastic rock. This is the Navajo
Sandstone out in Zion National Park. Look at the large cross bedding in this picture.
This was produced in a dune setting on a aeolian or wind dune. You could also find sand grains
on beaches, as you can see from this picture of a beach, and here's a picture of a ripple
mark caused by water flowing over the sand. Clastic rocks form in many different environments,
from rivers to the oceans to the deep ocean. So where do carbonates form? They form in
warm, shallow water like in southern Florida and in the Bahamas, and here are a few pictures
of students investigating carbonates forming in southern Florida.