Download the rock record - Sardis Secondary

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.
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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
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