1. How the Earth’s Rocks Were Formed
Chapter 5
How the Earth’s Rocks Were Formed

2. 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

3. Igneous Rock

4. Uniformity of Process Modern geology began in 1795
This is when Scottish geologist James Hutton described a new concept
called uniformitarianism

5. 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

6. 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
The geological process now at work were also active in the past
The present physical features of Earth were formed by these same processes, at work over long periods of time

7. 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

8. 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

9. 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).

10. 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.

11. 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

12. 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

13. 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

14. Recognizing Igneous Rock
Volcanic rock example:
Scoria – formed from lava slowly cooling

15. 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

16. Kinds of Magma There are 2 general kinds that are most common Felsic
Mafic

17. 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

18. Kinds of Magma Felsic Granite is a common felsic rock
It contains light coloured minerals like quartz and orthoclase
Most plutonic rocks are felsic

19. 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

20. 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.

21. 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).

22. 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

23. Textures of an Igneous Rock
Coarse-grained texture
Fine-grained and smooth 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
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

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

25. Porphyritic Texture How are these rocks formed?
There are 2 stages of cooling
Magma is at a large depth, and here is cools slowly so large crystals can form of one mineral, the rest remains liquid
As the magma gets closer to the surface the rest of the magma cools into fine grained rock around the larger crystals

26. 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

27. 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

28. Families of Igneous Rock
Granite Family
Granite Family
Course grained granite
Fine-grained rhyolite

29. Families of Igneous Rock
Granite Family
Granite Family
Glassy obsidian

30. 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

31. Families of Igneous Rock
Gabbro Family
Gabbro Family
Course-grained gabbro
Fine-grained basalt

32. Families of Igneous Rock
Gabbro Family
Glassy example:
basalt glass

33. Families of Igneous Rock
Diorite Family
Composition and colour between the granite and gabbro families

34. Families of Igneous Rock
Diorite Family
Diorite Family
Diorite: Course-grained
Has less quartz than granite
Has less dark plagioclase than gabbro
Andesite: fine-grained

35. Families of Igneous Rock
Some igneous rock do not fit into any of these 3 families.

36. Families of Igneous Rock
Don’t Fit
Don’t Fit
Granodiorite
Course grained rock
Composition between granite and diorite
Pyroxenite – nearly all pyroxene
Coarse, dark and heavy
May be like rock in Earth’s mantle

37. Families of Igneous Rock
Don’t fit
Don’t Fit
Dunite – almost all olivine
Coarse, dark and heavy
May be like rock in Earth’s mantle
Peridotite – a mix of pyroxene and olivine
Coarse, dark and heavy
May be like rock in Earth’s mantle

38. 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

39. 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

40. Description of Common Igneous Rock
Granite
Felsite is the general name
for light-coloured,
fine grained granite
Rhyolite is an example of a
fine-grained, light gray to
pink rock

41. 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

42. Obsidian

43. 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

44. Pumice

45. 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

46. Basalt

47. 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

48. Gabbro

49. Descriptions of common Igneous Rocks
Diabase
Has a composition simular to gabbro
It has a finer texturethan gabbro, but courser than basalt

50. Diabase

51. Descriptions of common Igneous Rocks
Basalt Glass
Is like obsidian, but has a mafic composition

52. Descriptions of common Igneous Rocks
Scoria
Is full of holes like pumice
Is made of denser material than pumice, so not all scoria floats

53. Class/Homework
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