Lab 1 Flashcards
What are strata?
- Contain planet’s history
- “rock layers” you may have seen in cliff faces or along highway road cuts
What are strata typically made of?
Most of those strata (sometimes called “beds”) are usually composed of sedimentary rocks.
Stratigraphy
the branch of geology concerned with studying strata as they occur through geological time, their composition, and how to correlate them between different areas
The Principle of Superposition
- “what’s on top is youngest”: over time, sedimentary layers (strata) are successively deposited on top of each other
- Assuming that strata have not been tilted or folded
The Principle of Original Horizontality and the “Way Up”
- When sediments are deposited, they form horizontal layers.
- Strata that are not horizontal have probably been tilted or folded due to tectonic processes
- This can cause problems determining in which direction the strata are getting younger, often referred to as the “Which way is up?” issue.
Solutions to the way-up problem: Fossils
- Firstly, we know that the type of fossils we find in rocks change over time, and we can use those changes to determine in which direction the strata are getting younger.
- Plant roots and rootlets (small roots) can be fossilized.
- As roots grow down into the soil, they indicate the direction that was originally up (and somehow tilted)
Solutions to the way-up problem: Graded bedding
- Grading bedding is a sedimentary structure that forms when sediment settles out of flowing water.
- Initially, only the largest and heaviest (coarse) sediment grains can be deposited when the flow is rapid.
- Over time, the flow wanes (gets slower) and progressively finer sediment is deposited. This produces a sedimentary layer that is coarse at its base and becomes finer towards the top, indicating the order in which the sediment was deposited and the direction of the way up.
Solutions to the way-up problem: Mudcracks
- Mudcracks
- Mudcracks are features that form when mud dries and shrinks. In cross-section the cracks “V” downwards, another way up indicator.
Crosscutting relationships
- Like in craters, cross-cutting relationships are also fundamental in stratigraphy
- The principle of cross-cutting relationships states: “A geologic feature which cuts another is the younger of the two features.”
Rules of Inclusions
- Inclusions (one rock type contained in another) are older than the rock they are embedded in
- Practically, the younger rock CONTAINS the inclusions
When does Thermal Contact Metamorphism occur?
- When magma or lava comes into contact with rock, the heat from the molten mass passes into the surrounding rock, causing the rock’s minerals to change.
- This is called thermal contact metamorphism.
Thermal Contact Metamorphism - Lava Flow
On the surface
As a lava flow moves over the surface, only the underside of the flow will cause thermal metamorphism in the rock it is flowing over
Thermal Contact Metamorphism - Sill
A sill is intruded BETWEEN strata, meaning contact metamorphism will occur on either side of the sill.
Conformities
- Sedimentary record can contain gaps where sediments were not deposited or when parts of the record have been eroded away.
- Where each layer is deposited regularly, with younger layers on top of older ones.
- This type of arrangement is termed conformable as the strata “conform” to the regular manner in which sedimentary layers are deposited.
- The angled conformable strata are separated from the horizontal conformable strata by a gap in the stratigraphic record
What can gaps in the sedimentary record represent?
AKA Unconformities
May represent times when sediment was not being deposited OR when strata were eroded, and new sediments were deposited on top of the erosion surface.
KNOW HOW TO IDENTIFY AGES OF SEDIMENTS
Angular unconformity
Types of Conformity
- Younger sediments rest upon the eroded surface of tilted or folded rocks.
- This unconformity develops when strata are tilted or folded at depth and, after this, are elevated and ultimately eroded at the surface. Following erosion, new strata are deposited on top of the erosion surface.
Nonconformity
Types of Conformity
- This develops between sedimentary strata and older ERODED igneous or metamorphic rock.
- Strata are deposited on top of the eroded igneous or metamorphic rock.
Disconformity
Types of Conformity
- Sedimentary Strata on either side of the erosion surface are parallel.
- A visible, irregular, or uneven erosional surface marks contact between younger and older beds. The older rocks have been exposed and eroded (but not tilted or folded), and new strata are deposited on the erosion surface.
Paraconformity
Types of Conformity
- All strata are parallel, but the erosion surface is FLAT, or there was a very long period (millions of years) of no sediment accumulation before sedimentation resumed.
- This can make identifying paraconformities difficult
How are unconformities still useful?
Significance of Unconformities
- They often represent episodes of terrestrial (land) conditions that followed the withdrawal of oceans from continents due to falling sea levels or periods of tectonic activity, causing crustal uplift and erosion.
- EX: the Great Unconformity (most famously exposed in the Grand Canyon but found in other parts of North America
