Unit 8: Earth’s History
Review
 What is the age of the rock compared to the
surrounding rocks?
 Relative Age
 The law that states that, in horizontal layers of
sedimentary rock, each layer is older than the layer
above it and older than the layer below it.
 The Law of Superposition
 What are the fossils of organisms that were widely
distributed but only lived during a short period of
time?
 Index Fossils
 What type of age do index fossils reveal?
 Relative Age
 What do we call the process that occurs when atoms
of an unstable element break down to form atoms of
another element?
 Radioactive Decay
 What enables geologists to determine the absolute
age of rocks?
 Radioactive dating
 Radioactive dating works best with what type of
rocks?
 Igneous
 Which time period makes up 7/8 of Earth’s history?
 Precambrian Time
 The Cambrian Explosion began which era in which
many new invertebrates lived in the sea?
 Paleozoic Era
 During the Devonian Period animals that could live
on land began to evolve. What are animals called
that spend part of their lives in water and part on
land?
 Amphibians
 What caused the mass extinction of the dinosaurs
and other organisms at the end of the Cretaceous
Period?
 KT Asteroid
 Hutton’s idea the geologic forces that occur today
are the same geologic processes had operated in the
past.
 Uniformitarianism
 The Earth is believed to be approximately how old?
 4.6 billion years old.
 A gap in the geologic record where some rock layers
have been lost because of erosion
 Unconformity
 Geologists use which radioactive element to
determine the age of organic remains, such as bone.
 Carbon-14


Magma that melts it’s way across other rock
layers then hardens underground to form igneous
rock.
Intrusion
 Which rock layer is most likely to contain fossils?
 Sedimentary Rock
 The time it takes for half of a radioactive substance,
such as Potassium-40, to change into a new stable
substance.
 Half-life