Introduction - Big Concepts in Geology
1) Scientific Method
2) Geologic Time
3) Interior of the Earth
4) Plate Tectonics
1)
-
Scientific Method
Ask question about natural world
Collect data
Propose hypothesis to explain data
Testing of hypothesis - If hypothesis fails any tests = rejected
Hypothesis repeatedly confirmed = Theory
Durable Theory = Scientific Law
2)
-
Geologic Time
How long did it take for the sediments to be deposited?
Can be determined using Relative Dating - Law of Superposition
Time to Deposit Sediments at a rate of 1 mm/year
Convert 25’ to equivalent number of millimeters
(25’ x 12 inches/foot x 2.54 cm/inch x 10 mm/cm)
= ~ 65,000 mm
= 65,000 years
Age of Earth = 4.6 Billion Years = 4,600,000,000 Years
Age established using Absolute Dating
Radioactive Decay of Uranium to Lead - 238U > 206Pb
Carbon 14 Dating - produced by cosmic radiation in upper
atmosphere
- half of 14C decays to 14N in 5,730 years
- method good for material up to ~50,000 years old
(e.g. 15,000 year old moccasins from Missouri cave)
-
- Ice Layers in Glaciers
- glaciers in Greenland, Antarctica are 3,000 m thick (2 miles)
- glaciers are built up by annual deposition of snow
- there is a nearly continuous record over 65,000 years
and a composite record extending back 165,000 years
3)
-
Interior of the Earth
How do we know what the interior of the Earth looks like ?
Seismic Waves from big Earthquakes (+ atomic explosions)
Seismic Waves can be used to study the interior of the Earth just
like ultrasonic waves are used to study the human body
- Refraction and reflection of seismic waves are caused by changes
in velocity with depth and the existence of layers
4) Plate Tectonics
- Plate Tectonic Theory is a unifying theory that explains most of
the geological features and events on Earth.
- It is as important to Geology as the Theory of Evolution is to
Biology
- Sea Floor Spreading - proposed after Asthenosphere discovered
- There are three types of plate boundaries
- 1) Divergent / Spreading
- 2) Convergent / Subduction
- 3) Transform Fault
Minerals
- Why are Minerals Important to Us?
- Civilization as we know it would not exist without Agriculture and
Minerals
-
Minerals are used for:
Building construction (cement, stone)
Metals (steel, copper, aluminum)
Glass production (bottles)
Medicine (toothpaste, kaopectate)
Food (salt, salt substitute, cereal)
Industry (polish lenses, mirrors)
Gems (diamonds)
A mineral is:
(1)
(2)
(3)
(4)
naturally occurring
inorganic solid substance,
has an orderly internal arrangement of atoms
particular physical properties and chemical composition
N.B. - This definition is not the same as the one in your book
ATOMS + ELEMENTS
Basic Atomic Structure
- all matter is composed of atoms
- atoms are the smallest part of an element
- atoms are made up of neutrons, protons and electrons
- There are 92 naturally occurring elements on earth
- Is that all there is to atoms? No – there are Quarks, Gluons and
other subatomic particles
-
Atomic Number = # protons in element (defines element)
Atomic Weight = # protons + # neutrons
Isotopes - # neutrons vary
Electrons = electric charge + chemical behavior of elements
- Atoms commonly have too few or too many electrons
- Most atoms have positive or negative charge and are called Ions
Common Ions:
Na 11P (+) 11N (0) 10e (-)
Cl
17P (+) 17N (0) 18e (-)
Na+ Cation
Cl- Anion
Na+ + Cl- = NaCl (halite/rock salt) electrically neutral compound
Compounds - formed by bonding of oppositely charged ions
- organic - formed by life processes
- inorganic - all others
- Atoms are most stable when the outer most electron shells
(electron orbits) are filled to capacity with electrons. Filling
the shells can be done by the Bonding atoms together.
- How can we fill the outer electron shell?
- We can fill the shells by donating, exchanging, or sharing (with all)
an atom
Bonding types
- Ionic bonds (donate one e-) - Oppositely charged Na + Cl ions join
together to make NaCl (halite).
- Most minerals have Ionic Bonds
- Covalent bonds (share an e-) - shared electrons orbit both nuclei
(the electrons are not lost or gained by an ion)
- Metallic bonds (share e-’s with all) - each atom contributes 1 or
more electrons to form a “sea” of electrons that move around
freely (weaker bond)
- Intermolecular bonds - due to uneven distribution of electrons
- van der Walls bonds - electrostatic bonds (very weak)
Identification of Minerals
- The internal atomic structure and chemical composition determine
mineral properties.
I.
Crystal Form - External shape reflects internal arrangement of
atoms – Euhedral, Subhedral, Anhedral
- Crystal Size
- Polymorphs - Minerals with the same chemical
composition but with different crystal structure
2. Hardness - the measure of resistance of a smooth mineral surface
to abrasion – Mohs Hardness Scale
3. Specific Gravity
- the ratio between weight of a mineral and the weight
of an equal volume of water at 4°C
- Specific Gravity increases with the atomic weight of
the elements in a minerals as well as their density
(packing of atoms).
4. Cleavage
- tendency of a mineral to break parallel to planes of
weak bonding
5. Fracture - Breakage other than along cleavage planes
(e.g. breaking of bonds in a mineral that the same
strength of bond in all directions)
6. Color - may vary widely (due to trace element content)
- NOT VERY DIAGNOSTIC
7. Streak - color of a finely powdered mineral
- Although the color of a mineral varies,
the streak is constant
8. Luster - appearance of light reflecting from a mineral surface
(reflects the bonding)
9. Other Properties
a) diaphaneity - the ability to transmit light
b) tenacity - resistance to bending, breaking, crushing,
bending, or tearing
c) taste
d) odor
e) reaction to acid
f) magnetism
g) radioactivity
h) fluorescence
i) X-ray diffraction
Silicates
- Independent Tetrahedra - Si:O = 1:4
- Single Chain - Si:O = 1:3
- Double Chain - Si:O = 1:2.75
- Sheet - Si:O = 1:2.5
- Framework - Si:O = 1:2
Common Non-Silicates
- sulfides, oxides, carbonates, sulfates, native metals
Mineral Composition of Continental Crust
Feldspars
58 %
Pyroxene + Amphibole
13 %
Quartz
11 %
Mica + Clay
10 %
Most common elements in the crust of the Earth – Si, O
Most common element in the entire Earth – Fe
IGNEOUS ROCKS
- Rock - A consolidated mixture of minerals (not fixed)
- Igneous rocks form from magma, a hot, molten rock material
• liquid  pasty
• contains molten silicates, dissolved water + gases
• magma crystallizes in Earth to form igneous rocks
• lava crystallizes on surface of Earth to form
igneous rocks
- Igneous rocks are the most abundant rock type on Earth
CLASSIFICATION OF IGNEOUS ROCKS
1 - Mineral Content
2 - Color (Felsic or Mafic)
3 - Specific Gravity
4 - Texture - size, shape, arrangement + distribution of minerals
- Phaneritic - coarse-grained
- Aphanitic – very fine-grained
- Glassy
- Porphyritic
- Pegmatitic
- Pyroclastic
Intrusive (plutonic) and Extrusive (volcanic) Rocks
Intrusive (plutonic) - magma intruded into existing rocks
- Types - Concordant and Discordant
- Concordant - parallel to layers of pre-existing rocks
- laccolith - lens-shaped
- sill – tabular
- Discordant - cross-cuts layers in pre-existing rocks
- batholith - huge (100’s km)
- stock - pipe-shaped
- dike - tabular
Extrusive (volcanic) - magma extruded on Earth’s surface = lava
- Types - Flows and Volcanoes
Igneous Rock Classification – Fig. 3.4
Why Do Igneous Rocks Have Different Compositions?
A. Magmatic Differentation
B. Assimilation
C. Magma Mixing
Order of Crystallization
There is a definite temperature range over which certain minerals
crystallize out of a magma.
- As magma cools, minerals with simple silicate structures
(less polymerized - isolated SiO4 tetrahedra) crystallize first.
- as crystallization proceeds, magma becomes enriched in SiO2
and other elements (K, Na, Al). The later minerals contain
more Si, and are more highly polymerized silicates (e.g.
framework silicates).
- Bowens Reaction Series - started with a Basalt
Why do Magmas Form?
- lower pressure
- water addition
Where do Magmas Form?
1) Mid-Ocean Ridge -melting of ferromagnesian silicate minerals in
Upper Mantle produces Fe + Mg-rich magma at a 20 to 30 km depth
- Cooling of a Fe + Mg-rich magma causes a rock rich in
ferromagnesian silicate minerals to form (e.g. Basalt (Gabbro)).
2. Subduction Zones
- Melting of continental crust + ocean crust + sediments + mantle
+ H2O produces a magma that is poor in Fe + Mg
- This process forms igneous rocks rich in non-ferromagnesian
silicates (e.g. Granite)
3) Hot Spots
- probably caused by a stationary plume rising through the mantle
- Magma produced at depths of 30 to 50 km by decreasing pressure
as plume rises
Volcanoes and Volcanic Activity
I . Modern occurrences
- Circum-Pacific (“Ring of Fire”)
- Mediterranean Region
- Mid-Ocean Ridges (Atlantic and Pacific)
- Rift Valleys / Fractures in the crust (East Africa)
- “Hot Spots”
- anomalous unexplained areas (Hawaii)
II. Volcanic Products
- tephra - pyroclastic material
- lava - pahoehoe - fluid lava that cools to a “ropy” texture
- Lava - aa - thick viscous lava that cools with a rough,
jagged, angular texture
- pillow lava - extruded beneath sea, ~1m in diameter
III. Rock Types
- ash - <1 mm
- tuff - fine-grained rock of volcanic fragments, <4mm
(characteristic of ash falls and is generally welded)
- breccia - angular fragments, > 4 mm
- pumice - low density rock formed by frothing of lava
by escaping gases
- scoria - vesicular basalt flow (cavities form by expanding
gas)
- Blocks - Coarse, angular
- Bombs - Hot lava thrown into the air and cooled to form
spiral-shaped bombs
IV. Eruptive Styles
- quiet and effusive
- explosive
- dissolved gases (H2O,CO2) can’t escape causing
pressure buildup + leads to an explosion
- explosive nature depends on viscosity of the magma
and this in turn is a function of its SiO2 content
1) explosive  siliceous rocks such as rhyolite, obsidian, tuff
+ pumice (high SiO2 content - low viscosity)
2) less explosive  andesite (Mt. St. Helens)
(intermediate SiO2 content + viscosity)
3) least explosive  basalt (Hawaii)
(low SiO2 content
+ high viscosity)
Types of Volcanoes
I. Basalt flows (through fissures)
II. Shield Volcanoes - basaltic lava erupts from a single vent
or fissure.
III. Composite Cone / Strato- Volcano - alternating lava flows
and beds of pyroclastic rocks
IV. Cinder Cone
Volcanic Features
Crater - depression above vent caused by lava cooling and sinking
back into vent after eruption
Plug Dome - viscous lava that solidified above volcanic vent
Caldera - collapse feature formed around vent after the magma
chamber is emptied
Columnar Jointing –
Lava Caves and Tubes –
VOLCANIC ACTIVITY ALERT CODE
- Green, Yellow, Orange and Red