Relative and Absolute Dating in Geology

• Relative dating in geology determines a structure's approximate age based on its physical relationship to surrounding geologic structures.
• It is a qualitative process that sequences geologic events.
• Geologists use their understanding of rock formation to determine the order of events and relative age.
• Examples include determining geologic events like floods and desertification, documenting changes over time via fossils, and determining environmental changes based on geologic formations.

Relative Dating vs. Absolute Dating

• Relative dating determines approximate age based on surrounding materials.
• Absolute dating determines age based on radioactive isotope decay using half-lives.
• Carbon-14, with a half-life of 5730 years, is used in dating artifacts.
• Uranium-238, with a half-life of 4.5 billion years, is another isotope used in absolute dating.
• Absolute dating can determine the relative ages of other formations.
• Landscapes features like fault lines or igneous intrusions can be dated using both methods and absolute dating determines a quantitative age.

Relative Dating Principles

• Relative dating relies on logic and a solid understanding of geology.

Principle of Uniformitarianism

• The processes seen today are the same as those in the past.
• Understanding present-day rock formation, erosion, and deposition provides clues about past events.

Principle of Superposition

• Higher rock layers are younger than lower layers with the oldest rocks at the bottom.
• The Grand Canyon displays layers with the oldest at the base.

Principle of Original Horizontality

• Original rock layers lie horizontally.
• With the oldest rocks on the bottom, relative dating can determine surrounding rock ages.
• If sedimentary layer A is above C, and B is below C, then B is the oldest, followed by C, then A.
• The principle applies to sedimentary rocks.

Principle of Lateral Continuity

• Rocks form in layers, so layers separated by a valley can be matched and considered the same layer.
• Rock layers disrupted by processes can be matched to the other side.

Principle of Cross-cutting Relationships

• Rock formations that cut across other rocks are younger than the rocks they cut across.
• Sedimentary rock can form around other completed structures, meaning that the completed structures must be older.
• Fault lines or igneous intrusions cutting across rock strata must be younger than that set of rock.

Principle of Inclusions

• Any inclusion within sedimentary rock must be older than that rock.
• It's formation was completed before the completion of the surrounding rock.
• The principle indicates that these inclusions must predate the encompassing rock.

Principle of Baked Contacts

• Magma heats surrounding rocks as it cools into igneous rock.
• Rocks with heat markings surrounding igneous rock must be older than the igneous rock.

Relative Dating Unconformities

• An unconformity is an interface between discontinuous rock layers.
• They disrupt the picture of what happened throughout the rock's geologic history.
• Disconformities occur between parallel lines.
• Nonconformities occur on top of igneous or metamorphic rocks.
• Angular unconformities occur on top of shifted rocks.
• Angular unconformity is where layers underneath are shifted at angles before new rock layers form on top.

Relative Dating in Planetology

• Relative dating can estimate a planet's surface age.
• A surface with more impact craters is probably older than one with fewer, assuming equal asteroid impact consistency.
• This is relative dating without an exact age, but it establishes a sequence of older and younger planets.
• Absolute dating of asteroids and meteorites can provide more precise ages.

Relative dating in geology determines age based on physical relationships. Absolute dating uses radioactive isotope decay to determine age. Methods like Carbon-14 and Uranium-238 are used in absolute dating to determine the age of geological formations and artifacts.