Sediments and Sedimentary Rocks
Sediments - Material (such as gravel, sand, mud, and lime) that is transported by wind, water, ice, or gravity;
material that is precipitated from solution; deposits of organic origin (such as coal and coral reefs).
Sediments are created by:
1. The weathering of rock
2. The secretions of organisms or decomposition of organic matter.
3. Chemical precipitation
Sedimentary Rock – Rock formed by the accumulation and consolidation of sediment.
Lithification – the process of converting unconsolidated sediment clasts into sedimentary rock one of two ways.
1. Cementation – gluing the clasts together. This is accomplished as water, saturated with dissolved ions begins to precipitate those
ions in the pore spaces between the clasts
2. Compaction – Clasts are squeezed together, and water is expelled.
Cemented ( breccia, conglomerate, sandstone, coquina), Compacted ( siltstone, shale, limestone, coal)
Types of Sediments
There are two main types of Sedimentary rocks
1. Clastic – broken fragments of rock produced by weathering. Clasts range in clast size from largest boulder to microscopic clay
particle. They are classified according to size of clast and are found everywhere on the Earth’s surface.
2. Nonclastic - not formed from other rocks or rock particles.
a. Biochemical – composed of remains of plants or animals.
Terrestrial Sediments - mainly plant matter (coal)
Marine Sediments - mainly carbonates (limestone, coquina).
b. Chemical – formed by minerals precipitating from solution. Inorganic process only, no biological activity involved.
Terestrial (Evaporites); Gypsum - CaSO4 . H2O, Halite -NaCl
Marine (Whitings); Carbonates - CaCO3
Sediment Texture
In General, the farther clasts are transported, the rounder, more sorted, and smaller they become.
Texture - Size, shape, and distribution of particles that collectively make up a rock
Sorting – separation of sediments by grain size and density. This is a function of
1. Water
2. Wind
3. Glaciers
- Poorly sorted – sediment with a wide range of grain sizes.
- Well sorted – sediment with a small range of grain sizes.
Roundness – the shape of sediment grains.
Related to the distance sediment has been transported.
Depositional environment for each sedimentary rock.
Near Source (Breccia, Conglomerate)
Downstream (Arkose sandstones)
Swamp (Coal)
Beach (Quartz sandstone, Coquina)
Playa lakes/basin (evaporates: gypsum, chert)
Offshore (Siltstone, Shale, Limestone)
Metamorphism and Metamorphic Rocks
Metamorphism – mineralogical, chemical, and physical changes that occur in solid rocks.
Occurs at depths greater than where sedimentary rocks form.
Solid state recrystallization – changes that occur without the rock melting (rocks that melt are igneous).
Factors influencing Metamorphism
1. Temperature
Below about 150oC, most minerals are stable (little or no metamorphism).
Above 150oC, reaction rate increases as temperature increases, new minerals begin to form.
Above 600oC, some minerals begin to melt (transition to igneous rocks).
2. Pressure
As pressure increases, pore spaces reduced and density increases, pore fluids are expelled.
Differential stress - pressure is greater in one direction than in another.
Foliation – a metamorphic rock texture due to alignment of minerals as a result of differential stress.
Slaty cleavage – foliation that develops at low temperature and pressure (metamorphosed shale).
3. Pore Fluid
As pressure increases, pore fluids are expelled (water and carbon dioxide).
Pore fluids increase the rate of metamorphic reactions by:
storing ions involved in reactions.
moving ions from one place to another.
4. Time
Solid state recrystallization is a slow process.
In general, size of minerals increases with time.
Metamorphic Processes
1. Burial Metamorphism
(foliated textures)
Most common type, occurs where crust is greater than 5 km thick.
Occurs over a range of temperatures and pressures.
Low grade to high grade metamorphism
2. Regional Metamorphism
(foliated textures)
Associated with convergent plate margins.
Folding and faulting increase thickness of the crust.
Occurs over a range of temperatures and pressures.
Low grade to high grade metamorphism.
3. Contact Metamorphism
(nonfoliated textures)
Occurs in rocks around a magma body.
High temperature (heat from magma).
Low pressure (occurs at shallow depths in the crust).
Metamorphic Rock Textures
Foliated – minerals are aligned upon a plain. Caused by differential pressure and heat
Nonfoliated – no alignment of minerals. No differential pressure, just heat.
The development of foliated metamorphic rocks from low grade through high grade metamorphism.
This would occur during regional Metamorphism (mountain building). Starting with the sedimentary rock – shale….
1. The clay minerals align making the rock harder. This is foliation and the rock is now SLATE.
2. The atoms rearrange into new minerals like muscovite mica. This gives the rock a shiny surface. The minerals are still too small to
see. This rock is now PHYLLITE.
3. The minerals grow large enough to see. New minerals grow like garnets, boitite mice, quartz, feldspar. Foliation is visible. This
rock is new SCHIST.
4. Minerals segregate to zones of varying pressure within rock giving the rock a “banded” appearance. This rock is now GNIESS.
The protolith (unmetamorphosed parent rock) of some metamorphic rock can still be identified.
Marble – limestone
Quartzite – quartz sandstone
Slate -- shale
Space and Earth Systems
Big Bang - Reigning Theory of the formation of the Universe
Nebular Theory - Origin of the Solar System
a. Swirling Cloud of Star Dust
b. Rotating Cloud Flattens into a Disk
c. Condensation and Accretion
Terrestrial planets (rocky) - Mercury, Venus, Earth, Mars
Jovian planets (gaseous) - Jupiter, Saturn, Uranus, Neptune
Pluto - the odd ball
Planetary Differentiation - As the Earth formed, it separated into layers based in the density of minerals that formed. The heavy
ones sank to the inside forming the core. The core is primarily metal. The heavy silicates formed the mantel and the light silicates
formed the crust.
Three distinct layers based of their physical properties are: crust, mantle, and the core.
Uniqueness of Earth - Earth is the only planet in our solar system:
1. Where water is known to exist near the surface in solid, liquid, and gaseous forms.
2. Where regolith, loose layer of material covering the Earth, is formed by dynamic processes.
3. Where life, as we know it exists.
System Science
1. Systems are defined by how energy and matter cross the boundaries.
2. The boundaries of a system can be defined however you want.
3. Portions of the universe that can be separated from the rest for the purpose of studying changes in them.
Three Types of Systems
Isolated
- no mass
- no energy
transfer
transfer
Closed
- no mass
- energy
transfer
transfer
Open
- mass
- energy
transfer
transfer
Relative / Absolute Dating
Principle of Uniformitarianism - "The present is the key to the past."
James Hutton - 1770;
Geologic processes operating today are similar to those that have operated throughout Earth's history.
Relative Dating - The age of a rock, fossil, or other feature measured relative to another. Absolute Dating
(numerical dating) - The age of a rock in years.
Object of Relative Dating
•To place a geologic event in between two (or more) other geologic events.
•Does not tell the age of the event.
•Keep putting events in time, in relation to other events until the sequence spans geologic
history.
•Some events can been seen throughout the world.
Stratum –
distinct layer of sediment
Strata –
plural of stratum
Stratigraphy – the study of sedimentary layers
Rules / Principles
1.Original Horizontality - Water-borne sediments are deposited in horizontal layers.
2.Superposition – when several layers of sedimentary rock are exposed, the oldest layer is at the bottom
of the pile and the youngest layer is at the top..
3.Unconformities - A surface within a sequence of layers where no deposition, and possibly erosion,
took place.
a. Disconformity
b. Angular Unconformity
c. Nonconformity
4.Inclusions - If a rock fragment is found within another rock type, the fragment is older.
5.Cross-cutting Relations - A rock layer is always older than a feature that cuts through it.
6.Fossil Correlation a. Based of evolution
b. Also relies of superposition
c. When a species becomes extinct, it does not reappear.
Radioactive decay - The spontaneous disintegration of the nucleus of an atom.
Two types of radioactive decay.
Alpha – Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (two
protons and two neutrons bound together into a particle identical to a helium nucleus) and transforms (or
'decays') into an atom with a mass number 4 less and atomic number 2 less.
Beta – In nuclear physics, beta decay is a type of radioactive decay in which a beta particle (an electron or a
positron) is emitted.
Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different
numbers of neutrons.
Half-life.
The half-life is the amount of time it takes for half of the atoms in a sample to decay. The halflife for a given
isotope is always the same; it doesn't depend on how many atoms you have or on how long they've been sitting
around.
This is a sample calculation which determines the age of a given sample.
You have a rock sample containing the radioactive isotopes “A” and “B”. Isotope “A” decays into isotope “B”.
The half-life of this radiogenic pair is 22,000 years. You determine the weight of each isotope in your sample.
Your objective is to determine how old the sample is?
Weight of isotope “A” is 60g. (parent)
Weight of isotope “B” is 420g (daughter)
First, combine the weights of the samples. 480g. this is the weight of isotope “A” when the when the sample
was created. Next, determine how many half-lifes must have passed if the sample began with only isotope
“A”.
Parent Isotope
480
240
120
60
30
15
Daughter Isotope
0
240
360
420
450
465
Number of Half-lifes
1
2
3
4
5
Age of sample
22000
44000
66000
88000
110000
This is a quick method.
A = 480 240 120 60
Half-lifes
1
2
3
3 x 22,000 = 66,000