Earth’s Past

Determining Relative age Relative age – the age of the object in relation to the age of other objects – No actual age in years given Law of superposition – undeformed rock is younger at the top and older at the bottom – Layers of the rock – called strata

Determining the Absolute Age Absolute age – the numerical age determined by an absolute dating process Some estimated ages can be found for features 10,000 – 20,000 years old – Rates of erosion, rates of deposition, and varve count (layers deposited in lakes)

Most accurate is radiometric dating Radiometric dating – determine age using the percentages of parent isotope to daughter isotope Based on half life – time needed for half the parent to decay into the daughter isotope Each isotope has a different half-life – Most commonly used is carbon 14 – turns into nitrogen 14 in 5730 years

The Fossil Record Fossil – the remains of a plant or animal Study of fossils – called paleontology Become fossils by: – Mummification, amber, tar seeps, freezing, petrification (minerals) Types of fossils: – Trace fossils – tracks, footprints – Index fossils – fossils that only appear in a specific rock layer meeting specific conditions (used to absolute age)

Fossil Formation Example

100 million years ago Scallops and other invertebrates inhabit the sea in this area. When they die, they are covered by sand that settles on them. 75 million years ago Pressure compacts the loose sandy sediments into hard sandstone. Dinosaurs now live, and die, in the locality. Like the scallops before them, their bodies are covered by sand..

50 million years ago There is a tremendous volcanic eruption in the area. The ash thickly covers the land, and for a while the land cannot support much life. 20 million years ago A Parahippus, a 3-toed horse common at the time, dies at the locality. Like the other creatures before it, its body is buried in sand and sediments by wave action.

3 million years ago The massive force of an earthquake causes layers of sediments to tilt and push upward. Today Forces of erosion, such as water moving against rock for millions of years, gradually wears away the land surface and a few of the rock layers beneath it. The fossilized bones of the horse, hidden for millions of years, are exposed above the ground.

Geologic column – ordered arrangement of rock layers that is based on relative ages of the rocks – Used to estimate the ages of rock that cannot be dated radiometrically Find similarities in rock and fossils of the layers to determine where it fits in time With this a Geologic Time Scale was created

Geologic Time Scale Contains blocks of times – Eons: largest unit of geologic time – Eras Periods Epochs Given an age, a name, and organisms of that time frame

Evolution/Change in fossils Records have shown that over time organisms have changed Over the years many strategies have been used to determine how the various organisms are related to each other

Strategies to Determined Relatedness Fossil record – organisms and their order of existence Comparative anatomy – structures in common Embryology – how long they look similar during development Biogeography – common locations of the same fossils Molecular biology – the similarities in genetics and chemistry Observed evolutionary changes – similarities seen externally

Index Fossils Rocks that occur only in rock layers of a particular geologic age – Be found in rock scattered over a large region – Clearly distinguished features – Organism must have lived during a certain time period – Fossil must occur in fairly large numbers in the rocky layer

Early Fossils in the Fossil Record

Comparative Anatomy

Embryology

Molecular Biology