Grade 8 Geologic History

Big Idea: Evidence of the dynamic changes of Earth's surface through time is found in the _______________ ______________________.

geologic record

Uniformitariansim

a principle that states that geologic change is gradual (present is a clue to the past) such as rivers and mountains changing

Catastrophism

a principle that states that geologic change occurs suddenly such as volcanoes, tsunamis, and earthquakes

Modern Geology

science that combines ideas from both Uniformitarianism and Catastrophism : Actualism this idea was proposed by James Hutton

Relative dating

process of comparing rock layers with others in a sequence to determine their age

Principle of Superposition

A principle that states that younger rocks lie above older rocks if the layers have not been disturbed; younger over older

Principle of Original Horizontality

principle that states all sedimentary rocks are originally deposited in flat, level layers

Principle of Cross-cutting

principle that states in order to be cut, the rock must exist first

Geologic Column

created by using a combination of relative dating, absolute dating, and fossil evidence which allows geologists to connect different rock units from different areas

Unconformity

a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time; represented by missing layers of rock

Nonconformity

sedimentary rock on top of igneous rock

Disconformity

missing flat parallel layer of sedimentary rock

Angular Unconformity

horizontal rock on tilted rock

Intrusion

molten rock that squeezes into existing rock and cools;

Tilting

slanting of rock layers; occurs as mountains are formed

Absolute dating

any method of measuring the age of an event or object in years (used on rocks)

Radioactive decay

breakdown of unstable atoms into stable forms; process always occurs at a steady rate; the rate of decay for a radioactive element in the rock is used in absolute dating

Radiometric dating

a method of determining the absolute age of an object by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope

Half-life

the time needed for half of a sample of a radioactive substance to undergo radioactive decay

Fossil

the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock

Index fossil

a fossil that is found in the rock layers of only one geologic age and that is used to establish the age of the rock layers.

A fossil of an organism that: lived a short time period of time, was widespread (worldwide), was well-defined in time.

Methods of radiometric dating

method is picked based on materials available and the useful ranges; to determine the age of some of Earth's oldest igneous rocks the Uranium-Lead method would be used.

Divisions of geologic time

eons- largest

era

periods

epochs - smallest

Is it difficult to get absolute ages on sedimentary rocks? Why?

Yes, because they are made from pieces of other rocks that were weathered and eroded away. Igneous rocks are used to give a good absolute date.

deposition

the process in which material is laid down (dropped down) at a location

erosion

transportation of soil and sediment from one location to another

Age of Earth

4.6 billion years old; in comparison Earth's history is a long time while human history represents a short time.

In radioactive decay, a parent atom decays into a ______________ atom.

daughter

Isotope

Atoms of the same element that have different numbers of neutrons