Aging The Earth: Making Inferences to Learn About the Earth’s Past and Geologic Time

Law of Uniformitarianism Thought of by late 18th Century scientist James Hutton – who is considered to be the “Father of Modern Geology” The law states that: “The present is the key to the past.” This means that by studying and observing the present conditions of earth, past events and time order {known as Geologic Time} can be inferred.

GEOLOGIC TIME IN ROCKS IS DETERMINED BY 1. RELATIVE AGE - THE AGE OF A ROCK COMPARED TO THE AGES OF OTHER ROCKS (Sedimentary rock layers). 2. ABSOLUTE AGE-THE NUMBER OF YEARS SINCE THE ROCK FORMED (Igneous Rock formations).

RELATIVE AGE

Can you order the following people in order from oldest to youngest? B. Can you order the following people in order from oldest to youngest? A. Youngest ____ Oldest D. C. F. E.

RELATIVE AGE: Reading the Rock Record CLUES: Law of Original Horizontality Law of Superposition Fossil Evidence (faunal succession & index fossils) Law of Unconformity Law of Cross-Cutting Law of Included Sediments

Law of Original Horizontality Layers (strata) are deposited in nearly horizontal layers Each new layers is parallel to the Earth’s surface and is laid on top of the older one. Strata that are folded must have been disturbed by an earth event.

LAW OF SUPERPOSITION In an undisturbed sequence of sedimentary layers, the youngest layer is at the top, and the oldest layer is at the bottom Which layer is the youngest?

Fossil Evidence Geologists use clues that fossils give to help age the earth. There are two types of clues that fossils can provide. They are: 1 – Faunal Succession 2 – Index Fossils

FAUNAL SUCCESSION Faunal Succession suggests that as time goes by, life forms change from simple to more complex (sometimes appearing to go from smaller to larger). If fossils that show faunal succession are found in strata, then the youngest (more recent) version of the species is found in upper strata.

FAUNAL SUCCESSION

INDEX FOSSILS Geologists look at index fossils as clues to help age rocks because the organism that made the fossil only lived for a brief period of time, but was very common. If a geologists notices one of these fossils in a rock layer, then they have an idea of how old that layer is based on the life of that organism and infer how old other layers are relatively.

INDEX FOSSILS

Law of Unconformity A gap in geological time caused by the eroding away of an unknown amount of strata. Usually, an unconformity is noticeable where faulted or folded strata are chopped and horizontal strata layer on top.

Generating Unconformities

LAW OF CROSS-CUTTING Anything cutting across a rock layer is younger. Cross cuts can be: intrusions, extrusions, faults or folds Intrusions and extrusions are caused by molten igneous rock. Faulting and folding are caused by earth events such as earthquakes.

Fault Fold Faults & Folds Strata that show faults and folds have experienced events such as earthquakes or the pressing together of plates causing the original horizontality to become distorted. Fault Fold

Intrusion – magma cutting into sedimentary rock layers { younger than the layers it cuts into – the layers had to be there first } A C D B

Extrusion – lava exited out on top of sedimentary rock layers { younger than the layers it cuts onto – the layers had to be there first }

Law of Included Sediments Pieces of rock found in other layers are ALWAYS OLDER than the layer they are in. For example: sediments (eroded pieces of rock) that make sedimentary rock come from rocks that are older than the sedimentary rock they make.

ABSOLUTE AGE

ABSOLUTE AGE: Using the properties of the elements that make rocks to determine their age usually through radioactive dating. Radioactive dating- comparing the mass of the rock’s original element to the new element created by decaying.

RADIOACTIVE AGE DATING Radioactivity refers to internal changes of the atom’s nucleus One element can transform into another If an element loses or gains a proton it changes into another element

RADIOACTIVE DECAY Half-life- The time required for one half of the original mass of an element to undergo radioactive decay

Using radioactive dating to compare ages Radioactive dating only works with igneous rock. Igneous intrusions and extrusions can be given an absolute age. This age is useful when determining the ages of rock layers – the last layer it cuts into is of close to the same age.

A C D B 4.8 MYO