Physical Geography Lecture 11 - The Lithosphere 111416

The Lithosphere

Chapter 9

Geologic Time

• Human time is mere seconds when compared to the time frame over which geologic processes have been operating to develop landforms

Geologic Time

How do we date rocks? Relative time

• Superpostion Oldest rocks on the bottom, youngest rocks on the top

Absolute time• Radiometric dating

Atoms with unstable nuclei (isotopes) decay into stable forms

Each isotope has a specific decay rate Half-life--the length of time it takes one half the isotope to

decay Measuring the ratio of unstable to stable material gives a date (1:1 means one half-life has passed)

The oldest rocksEarth first condensed and formed into a planet about 4.6 billion years ago.

Oldest rocks are from the Acasta Gneiss in northwestern Canada: 3.96 billion years old

Oldest grains of rock, from Western Australia: bet. 4.2 and 4.4 billion years old

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Uniformitarianism

Uniformitarianism“The present is the key to the past.”In other words: The same physical processes active in the environment today have been operating throughout geologic time.

Earth’s Interior Structure

Earth’s magnetic field

• Circulations within Earth’s inner and outer cores may be the mechanism for the magnetic field, called the magnetosphere, that protects Earth from solar wind and cosmic radiation.

• The magnetic north and south poles migrate• Geomagnetic reversal also happens in

irregular intervals

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From the crust on down

Which elements make up Earth’s crust?

• Core–Fe, Ni, Si (and/or) O

• Mantle–O, Si, Mg, Fe, Ca, Al

• 99% of the crust is made up of 8 elements:–O, Si (make up 74.3%)–Al, Fe, Ca, Na, K, Mg

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These elements, and trace others, combine to make up Earth’s minerals and rocks...

What is a Mineral? A mineral is…

a naturally-occurring,homogeneous solid

with a definite (but generally not fixed)

chemical compositionand a highly-ordered atomic arrangement.

It is usually formed through inorganic processes.

Okay…quick quiz…

Does ice fit the definition of a mineral?

...naturally-occurring...

Diamonds

...homogeneous solid...

It can’t start out as one mineral and become another mineral halfway through (but it can change color and still be the same mineral)

Tourmaline (Elbaite variety):Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)4

...definite (but generally not fixed) chemical composition...

QuartzSiO2 (silicon dioxide)

Olivine(Mg, Fe)2SiO4

...highly-ordered atomic arrangement.

• Carbon dioxide gas is made up of atoms that are not all bonded together into a structure.

• When frozen, CO2 IS bonded together... forming...what?

The shape of the mineral crystal is the result of its internal atomic structure.

chains

flat sheets

cubes

rhombohedrons

asbestos (crocidolite)

muscovite mica

galena

rhodochrosite

What if a substance doesn’t fit the whole definition?

It’s a mineraloid. Some substances look like minerals--but they LIE.

Glass is a mineraloid. It has an “amorphous” atomic structure.

- O- Si

As light passes through the glass panels, it is distorted by the ripples in the glass. Which panel is the new one?

Biogenic minerals: Exceptions to the “inorganic processes” rule

Formed by living thingsThe pearl and shells of oysters

• Aragonite

The main mineral found in human bones and teeth

• Apatite

Biogenic minerals: Exceptions to the “inorganic processes” rule

Formed by living things

Diatoms and radiolarians in the ocean

• Silicate minerals• Their skeletons are used as polishing

agents...

...(in your toothpaste!)

Is coal a mineral?

Is coal a mineral?

No.Coal, petroleum, and peat are NOT minerals.

–They have no definite chemical composition–They have no ordered atomic arrangement

Petroleum, coal and peat are mineraloids

Is coal a mineral?

But…coal, peat, and petroleum can form minerals under certain conditions

For example: If coal beds are heated to high temperatures and the carbon in them is crystallized, they can form the mineral, graphite

Mineral Formation:Non-organic formation

Minerals can form from:–Magma–Steam (these minerals are called vaporites)–Mineral components left behind when water

evaporates (these are called evaporites)–Mineral components dissolved in water that solidify

again (these are called precipitates)

From cooled magmaThe mineral components come together in the melt and harden as molten rock material solidifies

From steam: vaporitesVaporite

–Water near a magma source heats up, dissolves mineral components in surrounding rocks, and carries them away

–The steam condenses at the surface. The mineral compounds also “condense” and solidify.

Commonly found around volcanic vents, these sulfur crystals are

forming around Kilauea Crater, Hawaii. Also found near oceanic volcanic

vents (at spreading centers).

Harvesting sulfur crystals from a volcano in Indonesia

From steam: vaporites

Harvesting sulfur crystals from a volcano in Indonesia

From evaporating water: evaporites

Evaporite–Water evaporates, but leaves behind any mineral

compounds that were dissolved in the water–Hint: “Please pass the halite!”

Salt crystals like these form in salt beds along the edge of Highway 84, just before the Dumbarton Bridge.

Precipitate–When in too high a concentration to remain

dissolved in a liquid, the mineral components condense and may actually “rain” (precipitate) out of solution

–Often occurs when warmer water, which can hold more dissolved solids, cools slightly

Mineral Formation:Non-organic formation

From dissolved solids: precipitatesOolitic beach sand from the Caribbean, the Bahamas, and Great Salt Lake in Utah form from calcium carbonate precipitating out of the water

From dissolved solids: precipitates

Pyrite “sand dollars” form in sea water and collect on the ocean floor

Experiment

Make a precipitate mineral at home...

What is a Rock?• A solid, cohesive aggregate of one or

more minerals or mineral materials–A few rock varieties are made up almost

entirely of one mineral—this is called a pegmatite• Example: massive accumulations of halite

The Rock Cycle

Rocks have been continuously forming and reforming over millions and millions of years

The rock cycle is a closed flow system

The Rock Cycle

What is the power source that drives the rock cycle?

• Earth’s internal heat–Radioactive decay from its initial formation

• the power source for the flow system

• The sun–Secondary power source–Drives weather systems that erode material

Convection Currents• Slow-moving convection

currents within the mantle transfer heat from the outer core to the upper mantle

• The convergence and divergence of these currents near the surface is believed to be one of the driving forces behind the movement of Earth’s crustal plates

The 3 Classes of Rock

Classified based on the processes which form them

Igneous rocksSedimentary rocksMetamorphic rocks

Igneous Rocksfrom cooling magmas

• The kinds of rocks you end up with (and ultimately the kinds of structures built by the formation of those rocks) are determined by the chemistry of the magmas you start with

Felsic Magmas• The “fel” is for feldspar (made from Si, Al, O), “si” is for

silicates (made from Si and O)• Lighter in color, lower in density than mafic minerals • Continental crust is predominantly felsic material

Felsic Magmas• Cooler magmas, containing lots of silica (SiO2)• Highly viscous (resistant to flow)• High concentration of gases under high pressure• Gases can’t rise easily, so they stay trapped until near

the surface• As pressure is released at the surface, these

pressurized gases tend to explode– (Example: Mount St. Helens)

Mount Saint Helens• The explosion blew off 1,300 feet of the mountain's top and

sent ash and debris more than 12 miles into the sky covering three states - Washington, Oregon, and Idaho. Sixty two people were dead, beautiful forests and lakes were destroyed resulting in $3 billion worth of damage.

• ‘[At about 10:00 a.m.] in the city of Yakima, Washington…a black cloud covered the city and "snowed" ash. [Not] a street light nor a neighbor's porch light could be seen. The ash was so heavy it sank swimming pool covers and caved in old roofs. Businesses and schools were closed down and all normal activity… ceased to exist.”

From Disasters: Blowing Your Top http://www.boisestate.edu/history/ncasner/hy210/volcano.htm

Felsic MagmasExplosive Mount St. Helens

(Pre-eruption)

Felsic MagmasExplosive Mount St. Helens

(May 18, 1980)

Felsic MagmasExplosive Mount St. Helens

Mt. St. Helens Today

• The “ma” is for magnesium, the “f” for iron (Fe--ferris); contains more metallic, heavy elements (Al, Mg, Fe, K, Ca)

• Darker in color, higher in density• Oceanic crust is predominantly mafic• Low viscosity (flow easily for long distances)• Very little gas content, relatively little SiO2

• Hotter magmas, so gasses stay dissolved• Rarely explosive

– (Example: Hawaiian volcanoes)

Mafic Minerals and Rocks

Mafic MagmasHawaii

2002-10-11 view northwest across coastal plain of Kilauea from West Highcastle lava delta to Pu`u `O`o in upper right skyline

Intrusive vs. ExtrusiveIgneous Rocks

• Intrusive igneous rocks—magma cools beneath the crust; crystals have more time to form; harder, more erosion-resistant rocks

• Extrusive igneous rocks—magma cools on the surface (lava); crystals don’t have time to form good crystal faces, if it cools fast enough, no crystals will form (obsidian--“volcanic glass”)

Extrusive Igneous RocksBasalt and Obsidian

Intrusive Igneous RockGranite

…vs. extrusive rhyolite

Sedimentary RocksFrom hardened sediment

Sedimentary Rocks

• The result of erosion, transportation, deposition, and lithification

• Sediment—Weathered rock fragments that have been transported and deposited, usually by water (or by air or by glacial ice movements)

ErosionMechanical or Chemical

• Mechanical weathering—The physical force of a particular process acting on rocks, such as hydraulic action pounding rocks in a riverbed

- OR -• Chemical weathering—Chemical reactions, usually in

the presence of water, which operate to change the structure of the minerals, and break them apart–(Example: iron in the presence of water and oxygen,

incorporating those elements into its structure, expanding as it becomes rust, breaking rock apart)

Transportation

• Distance of transport and speed of the fluid medium determine size, roundness and sorting of transported material

Deposition• Deposition generally occurs in flat layers called

strata (or “beds”)• Principle of Original Horizontality

states that material is originally deposited in horizontal layers andlater is shifted as it is affected bycrustal movements

Sedimentary RocksBadlands National Park, SD

Horizontal layersafter tectonic movement

Lithification

Compaction and cementation• Lithification is the process of turning

sediment into rock–As sediment is deposited, the addition of more

layers causes compaction• (“trash compactor”)

–Dissolved minerals such as silica recrystallize in pore spaces

–Sediment grains are cemented together

Three Main Types ofSedimentary Rocks

• Clastic (or detrital)–Formed from pieces of

other rocks• Chemical

–Dissolved mineral compounds that solidify again (precipitates/evaporites)

• Organic–Formed from the tissues of

living things

Metamorphic RocksAny kind of rock subjected to heat and/or pressure

Two types of metamorphism

• Regional metamorphism–Occurs over 100s or 1000s of sq. miles–Common in subduction zones

• Contact metamorphism–Localized–Heat and pressure of rising, intruding magmas

“bakes” surrounding rocks

Contact Metamorphism

Parent Rock

quartzite gneiss

• Parent rock—Name given to the original rock before the addition of heat and/or pressure–what you start with will determine what you end up with

Foliation

• The arrangement of mineral crystals into parallel or nearly parallel bands–The classification of metamorphic rocks is

generally based on the amount of foliation in the rock

Metamorphic Rock Foliation