Introduction to Planet EarthIntroduction• Geology defined. – Physical versus Historical geology.• History of Geology.– Catastrophism and Uniformitarianism.• Divisions (spheres) of the Earth– biosphere, atmosphere, lithosphere and hydrosphere• Origin of the solar system explained by the solar nebular hypothesis.• Earth as a Dynamic Planet• Layers of the Earth.– Crust (oceanic, continental), Mantle, Core (inner, outer).– Lithosphere, asthenosphere.• Geologic Time.• The Rock Cycle.– Igneous, Sedimentary and Metamorphic Rocks.• A Scientific Theory.

Geology DefinedGeology is the study of the earth with two main subdisciplines: Physical Geology and Historical

Geology

Physical Geology Earth materials, changes in the earth’s surface and interior, why things happen, and how

changes affect people. Examples: petrology/mineralogy, seismology, hydrology, volcanology, structural geology,

environmental geology, meteorology. Time scale is on the order of hours, days, years or millions of years.

Historical Geology Examine earth’s history, origin and development over time and/or the changes in life on earth

over time (or both!). Earth history extends back to 4.6 billion years (radiometric dating of earth, moon and meteor

rocks). Time scale is millions to billions of years.

Geology as a Scientific DisciplineGeology is a non-fundamental science, that is, it requires usage of many different scientific

disciplines to understand how the earth works. You will find we will need to learn about CHEMISTRY, BIOLOGY, MATH and PHYSICS.

Changes on the earth impact humans, and we cause changes on the earth. Because the earth and people interact with one another, SOCIAL SCIENCE and POLITICS may also be considered!

History of GeologyObservations of the earth extend back to Greek civilization (philosophers). Philosophers OBSERVED

changes on the earth (geologic phenomena) but lacked the science to EXPLAIN their observations. The best explanation at for ancient philosophers was to associate earth changes caused by the many gods in their religion. Who are Neptune and Hades and what would they be responsible for?

During the 1600’s Archbishop James Ussher (sometimes spelled Usher) was credited with a very important job. He is the first person to try to date the age of the earth. Using the chronology of the Bible, James Ussher back calculated the date the earth was formed using genealogy (family history) in the old testament. His calculation dated the earth to be approximately 6000 years old, formed in October 4004 BC (I think he got down to roughly 10:30AM).

The train of thought during the 1600’s (on how earth operates) is called CATASTROPHISM. Any changes on earth are the result of world-wide catastrophes. These changes happen FAST! Think about it. Earthquakes, volcanoes, tsunamis, hurricanes. Big changes on the earth over a short period of time. This is catastrophism. Archbishop Ussher would be considered a catastrophist (according to him, earth is YOUNG).

James Hutton (1700’s) comes along with an entire new way of thinking. He knows soil (dirt) comes from weathered rock. Hutton estimated the time it takes rock to turn into dirt (estimated soil formation at thousands of years to produce a centimeter). Now looking at layers of sedimentary rocks (hundreds of meters thick), he did some calculations and decided that the earth had to be much older than 6000 years to produce all these sediments. Thanks to Hutton, we have a new way of looking at how the earth operates.

Hutton suggested that changes on the earth occurs SLOWLY. Many, many tiny changes (i.e. wind blowing grains of sand, water washing dirt in my backyard, waves striking a beach) will result in a BIG change over a long period of time. The present is the key to the past. This is the catch slogan for uniformitarianism. We assume that earth’s physical and chemical laws are uniform. Therefore we can look at earth processes today and infer the geologic past.

Earth as a Dynamic SystemThe earth is composed of many variables none of which are isolated from one another. A change in

one component will often result in a change in another component. The earth is complex and cycles and processes operate at widely varying rates.

THE EARTH’S SPHERESATMOSPHERE

Gaseous envelope of air that shields and insulates the earth. Composed of many gasses—78% Nitrogen, 21% Oxygen, 1% Argon and 0.03% Carbon

Dioxide (also trace gasses and water vapor). The atmosphere sustains life on earth, protects it from intense heat, plays an integral role in

weather and most importantly interacts with the hydrosphere.

HYDROSPHERE Contains all water on the earth. 71% of Earth’s surface is covered by water. Where is the other 29%? We will see that the hydrosphere plays an IMPORTANT role in shaping the earth.

BIOSPHERE Includes all living components of the Earth. Each and every organism has an important role

(job) in their environment. All organisms have very specific interactions with each other and their environment.

LITHOSPHERE Refers to the ‘rocky’ portion of the earth—including rocks, minerals, soils, sediments,

chemicals. Any component that is inorganic (not living).

Give examples of how each of these four divisions or ‘spheres’ interact with one another? You will find that separation of these ‘spheres’ is impossible and that they are all interrelated in complex ways!

Origin of the Earth The Solar Nebular Hypothesis explains the formation of the solar system. Our solar system formed approximately 5 billion years ago from a big, huge cloud of dust and

gasses called a nebula. Under its own gravitational influence, the large mass of gasses and dust began to contract and

rotate toward a central point. More than 90% of the matter moved toward the center of gravity forming a protosun (primitive

sun). The remaining gasses and dust continued to revolve around the protosun, smacking into one

another and ‘accreting’ into larger objects forming the primordial planets. Planets continued to accumulate more debris and the space between them cleared.

EvidenceAll planets revolve around the sun in the same direction.All planets revolve around the sun in the same plane (except Pluto).Rocks collected from the moon, meteor and the oldest minerals found on earth all date back to

4.6 billion years ago.All planets rotate about their axis in the same direction

Earth StructureThis section will focus on learning the layering of the earth. The earth layers can be divided by

chemical composition (the percentage of iron, magnesium versus silicon and oxygen) or by physical properties (solid, liquid, plastic, elastic).

The 8 most abundant elements in the earth are iron (Fe), oxygen (O), silicon (Si), magnesium (Mg), sodium (Na), calcium (Ca), aluminum (Al) and potassium (K).

The percent Si, O, Fe and Mg changes with these layers. The ‘lighter’ lower density elements are Si and O (the denser are Fe and Mg).

REMEMBER: Heavier stuff tends to ‘sink’ and lighter stuff tends to ‘float’.

Earth’s Core The earth’s core is composed of two parts, the inner core and the outer core. The inner core is SOLID iron (Fe) and nickel (Ni) and the outer core is LIQUID Fe and Ni.

Mantle A very thick layer of the earth that is composed of minerals primarily made of Fe, Mg, Si and O. Mantle rocks contain minerals with a higher percentage of Fe and Mg making it denser than the

crust above it. The mantle extends from roughly 50-2900km below the earth’s surface.

Crust The crust is a very thin layer of the earth (similar to the shell of an egg or the skin of an apple in

comparison to volume size). There are two types of crust: Oceanic crust is denser than continental crust (so it has more Fe and Mg than continental crust,

BUT less Fe and Mg than the mantle). This crust is usually located below the oceans and it is thinner than continental crust (7-10 km thick).

Continental Crust which is 20-70km thick is made of minerals with the highest percentage of Si and O and the lowest percentage of Fe and Mg. This layer is the least dense.

Both continental and oceanic crust are attached to one another.

So, the layers of the earth from lowest density to highest density are as follows:Continental Crust (High high Si &O, Low low Fe & Mg.Oceanic Crust (High Si&O, Low Fe&Mg).Mantle (Low low Si&O, High high Fe&Mg).Outer Core liquid Fe and NiInner Core solid Fe and Ni

The mantle contains MORE Fe and Mg than the oceanic crust and the oceanic crust contains MORE Fe and Mg than the continental crust. Because Fe and Mg are DENSER elements, the mantle is denser than the oceanic crust and the oceanic crust is denser than the continental crust.

Important things to remember!Density increases with depth in the earth. % Fe and Mg increases with depth in the earth

(except the core).Temperature increases with depth in the earth.Pressure increases with depth in the earth.

Lithosphere and Asthenosphere The earth is layered with respect to physical properties. The crust and the upper part of the mantle is called the lithosphere and it is ELASTIC and

BRITTLE. Just below the lithosphere is the asthenosphere which contains part of the upper mantle

directly below the lithosphere. The asthenosphere behaves like PLASTIC (rocks are under extremely high pressures and temperatures here).

Energy Sources for A Dynamic Planet1. Gravity2. Earth’s Internal Heat3. Solar Energy

1. GravityThe planet Earth generates a pull on every other object in the entire universe! It is this pull that

creates weight. It is this pull that causes an object to fall. Such a pull is called a force. The force that causes an object to fall is the force due to gravity.

A force applied to an object causes an acceleration in that object. The force of gravity is what causes the acceleration due to gravity. (9.8m/s)

2. Earth’s Internal HeatThree Sources of the Earth’s Internal Energy (Heat) Originating from Planet Formation

1. Impacts of asteroids2. Decay of radioactive elements3. Gravitational contraction

3. Solar Energy

Geologic Time Any geology student should be familiar with the geologic time scale Although I do not require you

to memorize the time periods, or years, there will be a few things you need to know about the scale (that will come up later).

First and foremost, the earth’s age is 4.6 billion years old (according to radiometric dating of minerals and rocks on earth). Not million years, BILLION.

Eons are the largest divisions with the oldest being Hadean, then Archean, Proterozoic and Phanerozoic. Everything excluding phanerozoic is collectively termed PRECAMBRIAN.

The Phanerozoic means ‘visible life’. Rocks of this age contain an abundance of fossils that document major evolutionary trends.

Life DID exist in the Precambrian, however, they were single celled organisms or those that lack ‘hard parts’ and were not preserved well in the rocks.

Eras are smaller subdivisions of eons. The eras of the phanerozoic (paleozoic—old life, mesozoic—middle life and cenozoic—recent life).

Eras are bound by profound worldwide changes (mass extinctions and radiations) in life forms. Paleozoic began with the appearance of abundant, multicellular organisms containing shells. Reptiles and dinosaurs were predominant terrestrial (land) animals during the Mesozoic.

Eras can be subdivided into periods, epochs, ages. These time periods are divided up based on less profound changes in fossil assemblages.

The Rock Cycle For our purposes right now, you need to know that there are three types of rocks Igneous,

Sedimentary and Metamorphic.Rocks are grouped into these three categories based on the WAY THAT THEY FORM.

Igneous rocks form when liquid rock cools and solidifies. Sedimentary rocks form when sediments are ‘glued’ or compacted together. Metamorphic rocks form by high heat and pressure (changes the minerals without actually

melting them).

A Note on Scientific Theory in Science• As an expression (english language), a theory is speculative… ‘its just a theory’.• SCIENTIFIC THEORY IS DIFFERENT!• A scientific theory is a hypothesis that underwent rigorous, RIGOROUS testing and has not been disproved. • Knowledge obtained from this hypothesis can be used to answer other hypotheses and explain other data AND answer other questions.• A theory is not proven 100% but there is a umpteen tons of supportive data available from

many sources and different experiments. ONLY scientific LAW is proven 100%.• In science, ‘its just a theory’ is big stuff that all scientists take as fact but cannot prove by 100% (but they get close)!