Formation of Stratified Rocks and Dating

• Stratification is the layering that occurs in most sedimentary rocks and in those igneous rocks formed at the Earth's surface
• Stratigraphy-The study of layered sedimentary rocks and their:

  ● Composition

  ● Origin

  ● Age relationships

  ● Geographic extent
• STRATIFIED IGNEOUS ROCKS: Stratification in a succession of lava flows in Oregon
• STRATIFIED METAMORPHIC ROCKS: Stratification in Siamo Slate, in Michigan
• STRATIFIED SEDIMENTARY ROCKS: Stratification in sedimentary rocks consisting of

  alternating layers of sandstone and shale, in California
• SOIL DISPLACEMENT: earthen materials are worn away and transported by

  natural force.
• WEATHERING: breaking down of rocks.
• LITHIFICATION: Process formed from the deposition of multiple layers of

  sediments.
• NATURAL BEDDING

  When stone is built on its natural bed, its sediment remains on the same

  plane as it was formed.

  • It allows compressive strength.

  Ex: ashlar, lintols, etc
• JOINT/EDGE BEDDING

  The stone is built on its edge bed at 90 degrees to the plane when it was

  formed.

  • Ex: archstones, overhanging stones, etc.
• CROSS BEDDING

  • is layering within a stratum and at an angle to the main bedding plane.
• RELATIVE: Know order of events but not dates
• ABSOLUTE: Know dates
• Example of Relative dating is bedrock in wisconsin formed before glaciers came
• Example of Absolute dating is glaciers left Wisconsin about 11,000 years ago
• RELATIVE DATING

  tells us the sequence in which events occurred, not on the numbers of how long ago

  they occurred

  • Chronological Order
• LAW OF SUPERPOSITION

  The law of superposition states that in an undisturbed sequence of

  sedimentary rocks, each bed is older than the one above it and younger than

  the one below it.
• LAW OF ORIGINAL HORIZONTALITY

  The principle of original horizontality means that layers of sediment are

  generally deposited in a horizontal position
• LAW OF LATERAL CONTINUITY

  It states that layers of rocks are continuous until they encounter other solid

  bodies that block their deposition acted upon by agents.
• LAW OF CROSS-CUTTING RELATIONSHIP

  It states that the body of rock cuts through another body of rock then it must

  be younger in age.
• GEOLOGICAL COLUMN

  Geological column is an ideal sequence of rock layers that contain all the

  known fossils and rock formations on Earth arranged from oldest to youngest
• UNCONFORMITY: sequences of strata represent times erosion that encompass long

  periods of geologic time.
• ANGULAR UNCONFORMITY

  Forms when rock deposited in horizontal

  layers is folded or tilted and then eroded.

  When erosion stops, a new horizontal

  layer is deposited on top of a tilted layer.
• DISCONFORMITY

  Layers of sediments are uplifted without

  folding or tilting and are eroded.

  Eventually, the area subsides and deposition resumes.
• NONCONFORMITY

  exists between sedimentary rocks and metamorphic or igneous rocks when the

  sedimentary rock lies above and was deposited on the preexisting and eroded

  metamorphic or igneous rock.
• PARACONFORMITY

  There is no evidence of a gap in time, because the planes above and below the gap are

  parallel and there is no evidence of erosion.
• INDEX FOSSILS

  • Fossils of widely distributed organisms that lived

  during only short time period.
• Fossils are the preserved remains or traces of living

  things.

  1. Fossils provide evidence of how life has

  changed over time.

  b. Fossils also help scientists infer how Earth’s

  surface has changed.

  c. Fossils are clues to what past environments

  were like.
• CORRELATION

  is the process of matching up rocks in different areas.

  The brachiopod Lingula is not useful because, although

  it is easily identified and has a wide geographic extent.

  The brachiopod Atrypa and trilobite Paradoxides are

  well suited for time-stratigraphic correlation

  • They are guide fossils
• HOW DO FOSSILS FORM?

  ● Most fossils form when living things die and are buried by sediments. The sediments

  slowly harden into rock and preserve the shapes of the organisms.
• Scientists who study fossils are called Paleontologists.
• MOLD FOSSIL

  Forms when sediments bury an organism, and the sediments change into

  rock.

  • The organism decays leaving a cavity in the shape of the organism.
• CAST FOSSIL

  • Forms when a mold is filled with sand or mud that hardens into the shape of

  the organism.
• PETRIFIED (PERMINERALIZED) FOSSIL

  Also known as permineralized fossils

  • Forms when minerals replace the structure of an organism.
• TRACE FOSSIL

  Forms when the mud or sand hardens to stone where a footprint, trail,

  droppings, or burrow of an organism was left behind

  • Helps to give clues to an animals diet and lifestyle.
• PRESERVED FOSSIL
• ABSOLUTE DATING

  ● Process of establishing the age of an object by determining the numbers of years it

  existed.

  ● It is the most precise.

  ● It gives an actual number to the age
• RADIOMETRIC DATING- isotopes of some elements are unstable and

  breakdown in the process called radioactive decay. The decay or breakdown

  of unstable isotopes allowing us to determine the age of the rocks.
• Carbon Dating. It is a method for determining age by comparing the

  amount of carbon- 14 to the amount of carbon-12 in a sample.
• TREE RINGS- before radiocarbon dating came onto the field, it was one of

  the most reliable forms of dating for those areas that had sufficient data to

  create.