How the Earth’s Rocks Were Formed

How the Earth’s Rocks Were Formed

Chapter 5

I will be able to …

Explain the principle of uniformitarianism and relate it to the formation of igneous, sedimentary, and metamorphic rocks.

Discuss the differences between plutonic and volcanic igneous rock.

Define rock texture and list some factors that control the texture of igneous rock.

Name and describe members of the granite, diorite, and gabbro igneous rock families

Igneous Rock

Uniformity of Process

Modern geology began in 1795 This is when Scottish geologist James

Hutton described a new concept called uniformitarianism


Before Hutton goelogists thought Earth was formed by spectacular events

and catastrophes They believed these catastrophies

formed mountains, canyons, waterfalls and almost all land forms


James Hutton’s ideas were different. He came to the conclusion, “the

present is the key to the past” This statement has 2 key concepts

1. The geological process now at work were also active in the past

2. The present physical features of Earth were formed by these same processes, at work over long periods of time


For example: a river valley Otheres would describe this as a sudden

split of the Earth’s crust Hutton explained it as the slow steady

wearing away of the land by the river over thousands of years

Three Groups of Rock

A rock is defined as a group of minerals bound together in some way.

Using Hutton’s principle, and studying different rocks around the Earth goelogists have concluded all rock from the earths crust is formed in one of 3 ways


Igneous Rock – formed by the cooling and hardening of magma from inside the earth. Magma is hot molten rock (under

surface).


Sedimentary rock –formed by hardening and cementing of layers of sediments. Sediments may consist of rock

fragments, plant and animal remains, or chemicals that form on lake and ocean bottoms.


Metamorphic Rocks – formed when rocks that already exist are changed by heat and pressure into new rocks. Garnet is an example

Recognizing Igneous RockPlutonic, or intrusive, igneous rock Rocks that form underground from

cooled magma These rocks are seen on the surface

only after the rocks that cover them are cleared away.

Example: Granite

Recognizing Igneous RockVolcanic, or extrusive, igneous rock Magma that pours onto the earths crust during a volcanic eruption is called lava

These rocks are formed when the lava cools

They can also be formed out of the dust and ash of a volcanic eruption

Recognizing Igneous Rock Volcanic rock example:

Scoria – formed from lava slowly cooling

Recognizing Igneous Rock Plutonic and volcanic rocks are

simular in mineral composition, but volcanic rocks lack mineral grains

Example: obsidian – volcanic rock formed from lava cooling rapidly

Kinds of Magma

There are 2 general kinds that are most common1. Felsic2. Mafic

Kinds of Magma

Felsic when hardens forms rocks with

mainly light coloured minerals high-silica Does not have much clacium, iron or

magnesium Thick and slow flowing magma

Kinds of Magma

Felsic Granite is a common felsic rock

It contains light coloured minerals like quartz and orthoclase

Most plutonic rocks are felsic

Kinds of Magma

Mafic Low percentage of silica Higher percentage of calcuim, iron,

and magnesium Magma is hotter, thinner, and more

fluid than felsic When solid forms rocks with dark

ferromagnesian minerals

Kinds of Magma

Mafic An example is basalt

It contains a number of dark minerals like hornblende, augite, and biotite

Most volcanic rock are mafic.

Textures of an Igneous Rock Texture is another way to group

rocks. A rock’s texture depends on the size,

shape, and arrangement of it’s mineral crystals.

Igneous rock textures range from glassy-smooth (obsidian), to course grained

(granite).

Textures of an Igneous Rock Crystal size is most important for

texture.

This depends on how fast the magma hardens. The amount of gas dissolved in the

magma

Textures of an Igneous RockCOARSE-GRAINED TEXTURE Magma deep within

the crust cools slowly Creates large mineral

grains of fairly uniform size

Plutonic rock has this texture

Granite is an example

FINE-GRAINED AND SMOOTH TEXTURE Magma that reaches

the Earth’s crust as lava hardens quickly

This rock has tiny crystals

The crystals are too small to see without a microscope

Volcanic rock like basalt is fine-grained; obsidian is glassy

Porphyritic Texture

Porphyry (POR fur ee) rock has 2 distinctly different textures.

Porphyritic Texture

How are these rocks formed? There are 2 stages of cooling

1. Magma is at a large depth, and here is cools slowly so large crystals can form of one mineral, the rest remains liquid

2. As the magma gets closer to the surface the rest of the magma cools into fine grained rock around the larger crystals

Families of Igneous Rock

Igneous rock is grouped into families according to mineral composition

Granite Family Gabbro Family Diorite Family And some igneous rocks don’t fit into

any of these families


Granite Family Forms from felsic magmas Consist mainly of orthoclase feldspar

and quartz▪ These are light in colour, so the rocks in this

family are usually light-coloured other minerals likely present are

plagioclase feldspar, mica, and hornblende

Rocks in this family will have different textures, but simular chemical compositions


GRANITE FAMILY

Course grained granite

GRANITE FAMILY

Fine-grained rhyolite


GRANITE FAMILY

Glassy obsidian

GRANITE FAMILY


Gabbro Family Forms from mafic magma Made mainly of dark plagioclase feldspar

and augite Other likely minerals are olivine,

hormblend, and biotite. These are generally darker and more

dense than those found in the granite family


GABBRO FAMILY

Course-grained gabbro

GABBRO FAMILY

Fine-grained basalt


Gabbro Family Glassy example:

basalt glass


Diorite Family Composition and colour between the

granite and gabbro families


DIORITE FAMILY

Diorite: Course-grained Has less quartz than

granite Has less dark

plagioclase than gabbro

DIORITE FAMILY

Andesite: fine-grained


Some igneous rock do not fit into any of these 3 families.


DON’T FIT

Granodiorite Course grained rock Composition between

granite and diorite

DON’T FIT

Pyroxenite – nearly all pyroxene Coarse, dark and heavy May be like rock in

Earth’s mantle


DON’T FIT

Dunite – almost all olivine Coarse, dark and heavy May be like rock in

Earth’s mantle

DON’T FIT

Peridotite – a mix of pyroxene and olivine Coarse, dark and heavy May be like rock in

Earth’s mantle

Description of Common Igneous Rock Granite

Made of:▪ quartz – looks like tiny chips of cloudy or grayish

glass▪ orthoclase feldspar – often has smooth cleavage

surfaces and is white, gray, pink or orange▪ and one other mineral like mica (shiny black flakes)

or hornblende (tiny dull black chunks or sticks) Ranges in colour from light to medium gray

and pinks Have course-grained texture


The most common continental igneous rock Occurs in thr Rockies, the Aairondacks of

New York State, the Black Hills in South Dakota, the White Mountains of New Hampshire, and in many other mountain areas

It is plutonic, and when it appears on the surface it shows errosion removed thousands of meters or rock since the magma cooled


Felsite is the general name for light-coloured, fine grained granite

Rhyolite is an example of a fine-grained, light gray to pink rock

Description of Common Igneous Rock Obsidian

Volcanic glass of the granite family▪ Contains many of the same minerals and

other light coloured rocks like granite Usually dark brown or black▪ This is due totiny amounts of dark coloured

minerals scattered throughout the rock Is hard and brittle It has a conchoidal, or shell-like, fracture

Obsidian

Descriptions of common Igneous Rocks Pumice

Formed from felsic lava that hardened while steam and other gases were still bubbling out of it

It looks like a sponge with many small holes in it

Because of all the air holes, it is sometimes light enough to float on water

Pumice

Descriptions of common Igneous Rocks Basalt

The most common rock in the gabbro family A fine-grained rock Ranges in colour from dark green to black Basalt is the igneous rock on the ocean floor On land it is the most common rock formed

from flows of lava Large areas occur in the lava flows of:

Iceland, Hawaiian Islands, and Columbia and Snake River in western United States

Basalt

Descriptions of common Igneous Rocks Gabbro

Has about the same composition as basalt

It is cooled slowly deep underground, so it is course-grained

Very dark in colour

Gabbro

Descriptions of common Igneous Rocks Diabase

Has a composition simular to gabbro It has a finer texturethan gabbro, but

courser than basalt

Diabase

Descriptions of common Igneous Rocks Basalt Glass

Is like obsidian, but has a mafic composition

Descriptions of common Igneous Rocks Scoria

Is full of holes like pumice Is made of denser material than pumice,

so not all scoria floats

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How the Earth’s Rocks Were Formed

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