G11 (1MS) EL&S: Earth's History & Geologic Time (2)

Petrology

a branch of geology that deals with the origin, composition, structure, and classification of rocks (study of rocks).

Stratigraphy

the study of rock layers (strata) and layering (stratifications).

Paleontology

- studies the life prior to, sometimes at the start of, the Holocene epoch (the current geologic epoch).

- study of fossils

Geologic Time Scale

- represents the interval of time occupied by the geologic history of the Earth

- time frame within which events of the geologic past are arranged.

Precambrian eon

- comprises about 88% or a total span of roughly 4.1 Gya

Phanerozoic eon

- most recent eon, an interval of time from 542 Ma to the present.

1. Principle of Original Horizontality

2. Principle of Superposition

3. Principle of Cross-Cutting Relationships

4. Principle of Inclusion

4 Basic Principles of Relative Order of Geologic Events

Principle of Original Horizontality

- sedimentary rocks are placed horizontally or nearly horizontal layers

- any deviation from horizontality indicates that deformation occurs after the deposition (layers started off horizontally; if they're not horizontal, then that's evidence something happened after they were formed).

Principle of Superposition

- in sedimentary rocks the layer at the bottom is the oldest, and the one at the top is the youngest

Principle of Cross-Cutting Relationships (Intrusion)

- states the dikes are younger than the rock they cut across

Principle of Inclusion

- if you see pieces (fragments) of one rock inside another rock, then those fragments must be older than the surrounding rock that contains them.

Relative Dating

- places events or rocks in their chronological sequence without knowing their actual age

- one can find out which layer was formed first relative to the other layers.

- cannot tell exactly how many years have passed since the event happened.

- qualitative

Absolute dating

- places actual ages of rocks and events

- the technique used is based on the decay rate of certain radioactive isotopes within fossils, rocks and artifacts.

Half-life

- amount of time necessary for one-half of the nuclei in a sample (parent) to decay to a stable isotope (daughter)

Fossil

a remnant or trace of organisms of past geologic age.

- True Form Fossils

- Mold Fossils

- Cast Fossils

- Trace Fossils (Ichnofossils)

4 Types of Fossils

True Form Fossils

entire animals or plants trapped or preserved in ice, tar, or other material

Mold fossils

- hollow impressions

- reflects only the shape and surface marking of the organisms

Cast Fossils

- created when mold fossils get filled with mineral

- the minerals harden and form a replica of the original fossil.

Trace fossils (ichnofossils)

- impressions on rocks that showed various activities

- can be footprints, eggs, droppings, or nests of animals

William Smith

- main proponent of using fossils to understand the relative ages of rock layers

- Father of Stratigraphy

Principle of Fossil Succession

- states that fossil organisms succeed one another in a definite and determinable order and, therefore, any time period can be recognized by its fossil content.

(aka. the digger we deep, the farther back in time we see)

Index fossils

- a distinctive fossil used to compare the relative ages of fossils

Ammonites

the most widely used index fossils, as they are abundant and can be easily identified at the the species level.