Interpreting Geologic Time “The present is the key to the past” Earth Science Mr. Cloud

Mr. Cloud at the Hall of Earth located at the AMNH!

Two Ways To Date Rocks 1. Relative Age: correlation 2. Absolute Age: “My age is…”

Principle of Superposition In an undisturbed sequence of sedimentary rocks , each bed is older than the one above it and younger than the one below.

Principle of Superposition

Principle of Cross-Cutting Relationships When a fault cuts through other rocks, or when magma intrudes and crystallizes, we can assume the fault or intrusion is younger than the rocks affected.

Igneous Intrusion Igneous intrusion: Crawford Notch, White Mountains

Faults

Unconformity An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. “A missing page from a book.” There are three types: angular unconformity, disconformity, an nonconformity

Angular Unconformity

Disconformity

Nonconformity

Correlation process by which rocks or geologic events from different places are determined to be the same or similar age

3 Methods of Correlation 1. Similarities in Rocks: “Walking the Outcrop” 2. ***Index Fossils*** 3. Volcanic Ash/ Meteorite Deposits

“Walking the Outcrop” Permian reef in Guadalupe Mountains National Park, Texas

***Index Fossils***

Volcanic/Meteoritic Deposits Chicxulub Crater: ~ 65 million years old Although the Chicxulub crater is buried beneath layers of sediments, gravity and magnetic field data have revealed its three-dimensional structure.