Download G1100 Ch. 3 The Rock Cycle

Geology 1100
Lecture 04 Mon. Jan. 30, 2017
• Review Chapter 3
• Read Ch. 4 for Wednesday and Friday
• Labs start this week
• Today:
– Review final Mineralogy slides from last week
– Chapter 3: Rocks and the rock cycle
1
New Material: Chapter 3
Rocks and the Rock Forming Processes
• Analogy I used earlier – in reading history of Earth
– Minerals are like letters of the alphabet
– Rocks are like words
– Rock formations are the sentences
• What are some common rocks (made from minerals)?
• How do they form?
2
Virtual field trip: Grand Canyon
• Types of rocks:
– Upper rocks have many layers
– Inner gorge rocks do not have layers
3
Characteristics of Rocks
• Visible crystals in some of the unlayered rocks
– Garnet, Mica
• Fossils in some of the layered rocks
4
Observations on a beach
• Sand made up of a mixture of rounded minerals and shell
fragments
– quartz, dark magnetite, shells (of calcite or aragonite)
• Nearby sea cliffs made up of beach components, cemented
together by calcite
5
Observations at a hot springs
6
Observations at a hot springs
• Chemically precipitated calcite
• Thin layers of very small crystals
• Sponge-like holes
7
Solidified Lava
• A few large (mm) crystals:
• Blocky green olivine
• Rectangular plagioclase feldspar
• Many smaller interlocking crystals
• more olivine and plagioclase plus
pyroxene and other dark minerals
8
Blocks blown out of volcano
• Larger crystals often found
• Crystals often arranged in bands, flow-related structures
9
How and where do rocks form?
• Most rocks are aggregates of mineral grains
• Many rocks originate from observable processes at the Earth's surface
• Some rocks not related to surface processes are formed deeper in the Earth
10
Classifying Rocks
• Descriptive Classifications
(observable characteristics)
vs.
Genetic Classifications
(processes which formed them)
• Genetic classification gives more insight – if you know how to properly
decipher processes of origin based on appearance
11
3 Main Rock Classes
• Sedimentary
• Igneous
• Metamorphic
12
Sedimentary Rocks
• Deposition or preciptitation of mineral grains formed by
breakdown (weathering) of previous “surface” rocks
• Two main types of Sediments
– Clastic Sediments
– Chemical Sedimentary Rocks
(grains from previous rocks)
(precipitation of completely
dissolved minerals)
• For clastic sediments you need to
cement grains together: lithification
• Most easily recognized by “bedding”
Layering as conditions
change slightly
13
Clastic vs. Chemical
14
Igneous Rocks
• Solidified from magma
(more general term for “lava” – the latter usually only refers to molten
rock at the surface of the earth.)
• Two general kinds:
– Volcanic Rocks
– Plutonic Rocks
Erupt at the surface
Also called “extrusive” igneous rocks
Solidify at depth
Named for “Pluto” – god of the underworld
Also called “intrusive” igneous rocks
• Volcanic rocks are usually layered but intrusive rocks usually are not
• Large crystal grains in plutonic rocks come from slow cooling
15
Volcanic
vs.
Plutonic
16
Metamorphic Rocks
• Greek: meta (change) + morphe (form)
• Rocks which have been significantly changed by
heat and pressure without completely melting (but they may get “soft”)
• High temperatures and pressures at depth (and the fluids which may be
present there) cause chemical reactions and cause rocks to deform and
slowly “flow”
– Example: Quartzite: Quartz sandstone which has been heated so
much the grains are partially melted together – not just cemented.
– Flow patterns may superficially resemble bedding:
• bands of dark vs. light minerals
• Are not continuous and of uniform thickness over long distances
• Often show complex distorted flow patterns
– Often occur at convergent plate boundaries where surface rocks are
17
buried to great depths
Metamorphic Rocks
18
Metamorphic
Rocks
• “Massive” metamorphic
rocks – but at a fine scale
they still show banding
19
Neptunist vs. Vulcanist Debate
• In late 1700's and early 1800's large debate about relative
importance of sedimentary processes from “Primordial Ocean”
vs. volcanic processes of melting
• Clearly the “primitive” rocks below the stratified upper ones are
somehow formed differently
– Neptunists believed they were a different kind of
precipitation from a massive “primordial” ocean
• Least soluble minerals (silicates) precipitated first
• The water somehow eventually evaporated to space
• Abraham Werner (1749-1817)
– Vulcanists believed heat, metamorphism, and melting of older
rocks created the deep “primitive” unlayered rocks
• James Hutton (1726-1797)
20
Neptunist model
21
Success of the Vulcanist Model
•
Evidence of “primitive” rocks injected as liquid into the stratified rocks – so
those “primitive” rocks had to come AFTER the stratified rock
•
Better observations of volcanic areas and volcanic rocks
– Most have nothing to do with burning coal deposits as Neptunists claimed
•
But Vulcanists didn't get everything right:
– They claimed sediments were “lithified” by heat –
but in most cases they ARE cemented by chemical precipitants
22
The Rock Cycle
•
Sedimentary rocks deposited from breakdown products of preexisting
igneous, metamorphic, or other sedimentary rocks
•
Metamorphic rocks form from heat and pressure acting on preexisting
igneous, metamorphic, or other sedimentary rocks
•
Igneous rocks form from “complete” melting of previous rocks –
even if those rocks may have been from somewhere deep within the Earth 23
Plate Tectonics indirectly drives
the Rock Cycle
•
Plate tectonics uplifts some rocks and buries others – causing both erosion of
the uplifted ones and metamorphism of the buried ones.
•
Plate tectonics opens ocean basins which can act as places for sediments to
collect
•
Volcanoes at mid-ocean ridges and subduction zones creates igneous rocksif
those rocks may have been from somewhere deep within the Earth
24
Next time:
• Begin reading Chapter 4: Igneous Rocks
– Wednesday: Sections 4.1-4.6
– Friday: Sections 4.7-4.10
• Labs start this week – be sure you have bought the lab packet
25