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2/7/2013 Telling Time THE ROCK RECORD Chapter 32 Finding Age with Relative Time • Relative Time: • Places events in a sequence but does not identify their actual date of occurrence • Ex: You went to Disneyland in grade 5 • Absolute Time: • Identifies the actual date of an event • Ex: You went to Disneyland on Tuesday, May 4th, 2012 Geologic Timetable • The Law of Superposition: • in a sequence of undisturbed rocks, the oldest rocks will be at the bottom and the youngest will be on top • Geologic Timetable: • a summary of the major events of Earth’s history preserved in the rock record. Fossils are an important part of history • The Law of Cross-Cutting Relationships: • an igneous rock is younger than the rocks it has intruded, or cut across • Era: the longest segments of geologic time. • Archean Era (4-5 billion years ago) • Proterozoic Era (2.5 bya) – No life • Paleozoic Era (570 million years ago) – Plants & Animals • Mesozoic Era (250 mya) – Dinosaurs • Cenozoic Era (65 mya) – Current era • The Law of Included Fragments: • the pieces of one rock found in another rock must be older than the rock in which they are found • Unconformity: • The place in the rock record where layers of rock are missing Geologic Timetable How Fossils are Formed • Periods: • Eras are divided into smaller segments called periods • Fossil: • any evidence of earlier life preserved in a rock. Evidence can be shells, bones, footprints, petrified trees, leaf impressions • Epoch: • periods are divided into smaller segments called epochs • Ways fossils are preserved: • Original remains (Ex: Woolly mammoth) • Amber – a hardened resin from pine trees • Replaced remains • soft parts have disappeared and the hard parts are replaced by mineral material (calcite, silica, pyrite). • Usually the work of underground water • Ex: petrified wood. 1 2/7/2013 How Fossils are Formed Fossils as Evidence for Evolution • Ways fossils are preserved: • Through molds and casts • The fossil record indicates that the first organisms were • a bone will leave a hollow depression in a rock • Trace fossils • includes any impression left in the rock by an animal (trails, footprints, tracks, burrows, borings) simple in structure • As time passed, life forms increased in size and complexity • The rock record shows that through time many kinds of organisms disappear and are replaced by new and different organisms • Evolution: • the process of change that produces new life forms over geologic time Index Fossils and Key Beds Rock Correlation • Index Fossils (a.k.a. guide fossils): • animals evolve over time, so some fossils are typical of a particular time segment of Earth’s history • Correlation: • the matching of rock layers from one area to another • Three characteristics: 1. easily recognizable 2. widespread in occurrence 3. limited in time • Key bed: • is a single rock layer that has the same characteristics as an index fossil. • Ex: a large volcanic eruption represents a single instant in time, but is unique and widespread • Simplest and most direct method of correlation is following an outcrop • Outcrop: • the part of a rock layer that can be seen at Earth’s surface • Another method is to match similar rock characteristics • Color, composition, appearance • The best method is to use index fossils Other Use of Fossils • Indicators of past climate • coral reefs today are only found in shallow, warm water. When a rock containing fossil coral is found, we can assume the area was once warm and shallow water • Oil exploration and microfossils • Microfossil: fossils so tiny that you need a microscope • Ex. Foraminifera and diatoms • Used to correlate the layers of rock 2