Unit 9-1: The Rock Cycle Part 1

Unit 9-1: The Rock Cycle Part 1

The rock cycle begins as magma cools and hardens

into rock. This type of rock is known as Igneous Rock.

Extrusive Igneous Rock:Forms at surface when

a volcano erupts.

Intrusive Igneous Rock:Forms when magma cools

below the surface of the Earth.

So there’s basically two types of igneous rock,

depending on where the magma cooled!

Two types of lava/magma

Mafic Magma: -Low silica content, but high calcium, iron and magnesium.

-Hotter and more fluid than Felsic.-Forms darker minerals and rocks when it cools.

-Ex. Horneblende, Augite, Basalt.-Mostly makes up extrusive igneous rocks.

Felsic Magma: -High silica content.-Thick and slow flowing.

-Forms light colored minerals and rocks.-Ex. Quartz, Orthoclase Feldspar, Granite.-Mostly makes up intrusive igneous rocks.

Igneous rocks can be glassy-smooth in texture, or they can be rough and

coarse in texture.

The texture of the rocks depends on thesize, shape, and arrangement of its grainsor crystals (from the minerals that make

up the rock.)

Some igneous rocks, like obsidian, contain grains that are too small to

be seen by the naked eye.

The size of grains is dependent on how fast the magma that formed the

rock cooled. The faster the magma/lava cools, the smaller the

crystals.

Another factor is the percentage of dissolved gas in the magma. Gas keeps the magma hotter for

longer.

Another factor is the order in which the crystals form. This is

because different rocks melt/solidify at different

temperatures.

So what you’re telling me is thata rock’s physical properties aredependent on its composition,

AND how/where it cooled?

-Some igneous rocks have two distinct textures.

-When large crystals are surrounded by a fine grained or

glass rock (called a groundmass), the rock is a porphyry.

Now this rock has fine mineralgrains and large mineral grains.So how do we figure out where

it formed?

-A porphyry forms in two stages. -First, it begins to form deep in the earth, where it cools slowly.

-This allows the large crystals to form. -Then the magma slowly rises to the surface.

-Once it breaks to the surface, it cools very quickly. -This forms the groundmass of very small crystals.

-Now there are three families of igneous rocks, based ontheir composition. Each family has members based on

the grain size.-The families are the granite family, the gabbro family, and

the diorite family.-There is a fourth igneous rock type called granodiorite.

Now, let’s take a look at these individually.

The Granite FamilyThe Granite Family-Made of high-silica magmas.-Made mainly of orthoclase

feldspar and quartz.-There can also be plagioclase

feldspar, mica, and horneblende.

-Rocks in this family are usually light in color.

Based on grain size: -Granite is coarse-grained.-Rhyolite is fine-grained.

-Obsidian and Pumice are glassy.-All three have the same chemical structure.

The Gabbro FamilyThe Gabbro Family-Rocks in this family have

low silica content.-Made mainly of plagioclase

feldspar and augite.-Can also contain olivine,horneblende, and biotite.

-All minerals in this family (except feldspar) contain

high levels of iron.-These rocks are usually

dark in color.

-In this family, gabbro is coarse-grained.-Basalt is fine-grained.-Basalt glass is glassy.

-Diabase is somewhere between basalt and gabbro in texture.

The Diorite FamilyThe Diorite Family-Rocks in the diorite family have a texture that is somewhere in

between the granite and gabbro family.

-They are made of plagioclase feldspar, horneblende, augite

and biotite.-They are a mix of dark and

light colors.

-Diorite is the coarse-grained rock in this family.-Andesite is the fine-grained rock in this family.

-Obsidian without quartz in it is the glassy rock in this family.

GranodioriteGranodiorite

-Between the coarse textured rocks in the granite and diorite family is

granodiorite.-It shares in compositions somewhere

in between granite and diorite.

Three other coarse grained rocks are of interest as well:-Pyroxenite is almost entirely pyroxene.

-Dunite is almost all olivine.-Periodotite is a mixture of olivine and pyroxene.

Hey, now that we’ve looked at how igneous rocks form and their

classifications, can we look at some of the properties of the individual rocks themselves?

I think that if rocks formed at different locations and have different compositions, then they should have different

properties.

-Granite is made from quartz, orthoclase feldspar, and at least one other mineral, such as mica or horneblende.

-The quartz looks like gray or smoky glass.-The feldspar is usually white, gray, or pink, and is

noticeable by its flat fracture planes.-The mica can be easily broken away, while the

horneblende cannot.

-Granite can come in many different colors.

-Granite is the most common continental igneous rock.

-It can be found in the Rockies, the Adirondacks, the Black Hills, etc.

-Granite is an intrusive rock, so it is only exposed to the surface after a

great amount of erosion.

-Obsidian is formed by very rapid cooling of lava.

-It is dark brown to black in color, and is glassy in texture.-Obsidian is a very hard rock,

but also very brittle.-When broken, it has very sharp

edges.

-Pumice is made by very rapid cooling of lava, when dissolved

gasses cannot escape.-It looks like a sponge, but this

rock is glass, like obsidian.-Because of the trapped gas,

pumice is light enough to float.

-Basalt is the igneous rock of the ocean floor.

-This is the type of rock that forms from surface lava flows that cool moderately slowly.-Fine grained, colors range from black to dark green.

-Diabase is similar to basalt.-Made from the same material

as basalt, but with larger grains.

-Also known as dolerite or trap rock.