Chapter 5. Explain the principle of uniformitarianism and relate it to the formation of igneous,...

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

Uniformity of Process

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

Uniformity of Process

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

Uniformity of Process

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

Three Groups of Rock

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

surface).

Three Groups of Rock

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.

Three Groups of Rock

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

Recognizing Igneous Rock

Plutonic, 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 Rock

Volcanic, 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

Families of Igneous Rock

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

Families of Igneous Rock

GRANITE FAMILY

Course grained granite

GRANITE FAMILY

Fine-grained rhyolite

Families of Igneous Rock

GRANITE FAMILY

Glassy obsidian

GRANITE FAMILY

Families of Igneous Rock

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

Families of Igneous Rock

GABBRO FAMILY

Course-grained gabbro

GABBRO FAMILY

Fine-grained basalt

Families of Igneous Rock

Gabbro Family Glassy example:

basalt glass

Families of Igneous Rock

Diorite Family Composition and colour between the

granite and gabbro families

Families of Igneous Rock

DIORITE FAMILY

Diorite: Course-grained Has less quartz than

granite Has less dark

plagioclase than gabbro

DIORITE FAMILY

Andesite: fine-grained

Families of Igneous Rock

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

Families of Igneous Rock

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

Families of Igneous Rock

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

Description of Common Igneous Rock

Granite 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

Description of Common Igneous Rock

GraniteFelsite 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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