Download Obsidian Cooling Rate

Rocks &
the Rock Cycle
The underlying principle for
understanding all things geological is:
Minerals and rocks are stable only
under the conditions at which they form.
Change the conditions and the rocks
will change to adapt to the new
conditions.
Let’s start at the very beginning…
Mineral (review) – Naturally occurring
inorganic solid materials with definable
chemical compositions and an orderly
arrangement of atoms within a lattice.
Rock - an aggregate of one or more
minerals
Let’s start at the very beginning…
Three main rock types
~ Igneous
~ Sedimentary
~ Metamorphic
Igneous Part
of the Rock
Cycle
Pyroclastic
material
Lava
Igneous Rocks
• All igneous rocks
– cool and crystallize from magma (inside)
– or solidify from lava (outside)
– or consolidate from pyroclastic materials (outside)
• Magma is molten material below the surface
• Lava is molten material on the surface
• Pyroclastic materials
– particles such as volcanic ash, volcanic bombs,
tephra, etc.
Classification of Igneous Rocks
Igneous rocks are classified by
composition and texture.
Composition = what it’s made of
Texture = how it’s put together.
Composition
Composition – These categories are based on silica content.
“Granitic” igneous rocks - (AKA - Felsic igneous rocks) Light-colored
igneous rocks made from felsic (light-colored) minerals such as quartz,
micas, and feldspars.
“Basaltic” igneous rocks - (AKA - Mafic igneous rocks) Dark-colored
igneous rocks made of mostly mafic minerals such as olivines, pyroxenes,
amphiboles, and Calcium-rich plagioclase.
The terms the book uses for these rocks (Granitic and Basaltic) are a poor
choice, as we will see….
Texture
Texture - the size, shape and arrangement of crystals, grains
and other components of the rock. Texture is controlled by the
rate of cooling and crystallization.
• Extrusive (“outside earth”) – cools quickly to instantaneously. Crystals
do not have time to grow to visible size.
• formed at the surface from lava or pyroclastic materials
• Example: Basalt, Pumice
• Intrusive (“inside earth”) – cools slowly over loooooong periods of time,
allowing crystal growth to occur. Produces crystals visible to the
naked eye.
• formed below the surface
• Example: Granite
So why are the book terms of “Granitic” and “Basaltic” not good terms to use?
Differences in Texture
Extrusive texture
Rapid cooling
Vs.
Intrusive texture
Slow cooling
Extrusive Textures
Glassy texture
Extremely fast cooling
Vesicular texture
Created by escaping
gasses trapped in
cooling lava
Rainbow Obsidian
The iridescent, rainbow-like sheen of
rainbow obsidian is caused by
inclusions of magnetite nanoparticles.
Say what?
Tiny pieces of the mineral magnetite
are trapped in the molten rhyolite
magma when it cools very fast.
Spear tips made from
rainbow obsidian
Common Intrusive
Igneous Rocks
Granite
Slow cooling magma
allows crystals to grow
to visible size
Gabbro
Felsic
Composition
Mafic
Basalt
Common Extrusive
Igneous Rocks
Fast cooling, typically in the
form of lava, prevents crystals
from growing to a visible size
Rhyolite
Mafic
Composition
Felsic
Rhyolite
Extrusive texture
(cooled quickly)
Felsic composition
(light in color)
When rhyolite lava
cools extremely
quickly, it forms
obsidian.
Intrusive texture
(slow cooling)
Felsic
composition
(light in color)
Slow cooling
allows the
crystals time to
grow to a visible
size
Granite
Scoria
Vesicular
texture
(fast cooling)
Holes are
produced by
gas
escaping
rapidly as it
cools.
Glassy
texture
(cooled
extremely
fast)
Obsidian
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Famous Igneous Rocks!!
Igneous rocks - Review
Igneous rocks – form from cooled molten rock
Magma
Lava
Textures – 2 types, know how each type forms
Extrusive
Intrusive
Composition – 2 types
Felsic
Mafic
Sedimentary
Rocks
Sediment is created through the process of weathering…
Sediment
Loose particulate material or residue (clay, sand, gravel, etc.) from a
specific source (biochemical, chemical, detrital).
Uplift
Weathering
Lithification
The
Sedimentary
Cycle
Transport
Deposition
Weathering = Making sediments
Uplift
Two types of weathering:
Weathering
The
Sedimentary
Lithification
Cycle
Transport
Chemical – decomposing or
dissolving of Earth material
Deposition
Physical – cracking, scratching, abrasion,
or other physical breaking up of Earth material
(clasts, rock fragments, mineral grains from inorganic
sources; peat, shell gravel from organic sources)
Chemical
Weathering
Physical Weathering
Weathering
Transport
Moving sediments from where they were
formed to where they are deposited
Transported by
wind, water, and/or ice
Transport
by Wind
Transport
by Water
Transport
by Ice
Deposition
Sediments are ‘deposited’ somewhere like
a lake, river, or other area where the wind,
water, or ice will not move them any
further.
Alluvial Fans
Braided
Stream
Deltas
Meandering Rivers
Lithification = “Turned to stone”
Once deposited, sediments turn to
sedimentary rocks by one or more of
these processes:
• Compaction
• Cementation
• Precipitation / Evaporation
METAMORPHIC ROCKS
“Change of form”
Metamorphic
Rocks
Metamorphic rocks are formed when heat
and pressure changes another rock
The rock that is changing does not melt
during metamorphosis
What would we call it if it did melt?
What can be changed?
Metamorphic rocks can be formed from:
 Igneous rocks
 Sedimentary rocks
 Other metamorphic rocks
Metamorphic Rock
Classification
Metamorphic rocks are broken out into two main
groups, foliated and non-foliated.
Foliated – Have bands of different colored
minerals
Non-foliated – No layering or banding present
Where on Earth?
The Rock Cycle – A Different View