Unit 7: Rocks and Minerals

EARTH’S INTERIOR Geology: the study of the physical nature and history of the Earth

we cannot actually see all the way to the Earth’s core holes can only be dug so deep!

look at the inside of the Earth by looking at: • how seismic waves travel through the Earth • types of rocks and

minerals

EARTH’S INTERIOR

like the atmosphere and oceans, there are different layers of the Earth’s interior

layers vary by: • temperature• pressure • size (depth)• composition

Temperature • as you begin digging down, the soil gets cooler• beginning at 20 m down, temperature begins to rise

• for every 40 m, temperature increases by 1C • continues to rise rapidly for about 4,000 km, then increases more

slowly

Pressure • increases with depth

(lots of rocks above!)

EARTH’S INTERIOR - LAYERS

There are three basic layers to the Earth:• crust• mantle• core

(and sub-layers to each one)

Crust: a layer of rock that forms the Earth’s outer surface• located from the tops of mountains to the bottom of

the deepest sea trench • includes both dry land and ocean floor • thickest under mountains, thinnest under a deep

sea trench• between 5-75 km thick

(average ~ 35 km) • average temperature

around 200C

Two types of crust:• oceanic mostly basalt rocks• continental mostly granite-type rocks

Mantle:

a layer of hot, solid material located between the Earth’s crust and core

• begins around 40 km depth• about 2,867 km thick • composed of magma • divided into 2 main layers

• upper mantle• lower mantle

• temperatures are between 500-900C• divided into two layers:

lithosphere: the rigid layer of upper mantle and crust • around 50-120 km thick • broken into 14 big sections that move

asthenosphere: a malleable layer of the upper mantle on which the lithosphere floats

• about 200-240 km thick • solid, but bendable

like plastic

Upper Mantle

• begins around 340 km deep • still solid (not liquid yet!) all the way to the core at

about 2,867 km deep • temperature ranges from 1000-4000C

Lower Mantle

Core• made up of mostly nickel (Ni) and iron (Fe) • about 3,486 km thick• depths are estimates due to blurry boundaries • consists of 2 parts:

• outer core • inner core

Outer Core• liquid, molten metal • about 2,266 km thick • 4000-9000C • spins to form magnetic

field

Inner Core• solid, dense ball of metal • 9000C +• contains mostly Ni and

Fe, but also O, S, and Si

*

Continental Drift: theory stating that continents slowly move across the Earth’s surface

hypothesized in 1915, in Alfred Wegener’sbook: The Origin of Continents and Oceans

Continental Drift

• Wegener was the first to theorize that continents could move • before that time, everyone believed

that land was static and did not move.

• also hypothesized that at one time, millions of years ago, the continents were one large land mass that have since drifted apart• called it Pangaea

• Greek for “All-Earth”

Wegener gathered evidence in support of his hypothesis:

1. Land features • coal bed deposits• mountain ranges lined up

Continental Drift - Evidence2. Fossils

any trace of an ancient organism that has been preserved in rock

• noticed similar fossils (of both plants and animals) were found on separate continents

Continental Drift - Evidence3. Climate change

• as a continent moves farther away from or moves towards the Equator, it also carries the fossils within it

• fossils and glacial traces were found on different continents

• scientific community rejected Wegener’s theory partly because he could not provide a mechanism for how the continents moved

• have evidence today:• mid-ocean ridges

• sea floor spreading• Plate Tectonics• GPS

Mid-Ocean Ridges

• beginning in the 1940’s, scientists began mapping the ocean floor with sonar

• discovered that the ocean floor was not flat !• found a large and

long mountain range over 50,000 miles long

• down the middle of the range was a valley

In 1960, Henry Hess, an American geologist, proposed that the sea floor spreading was a means of moving the continents as proposed by Wegener.

Hess stated that the sea floor spreads apart along both sides of mid-ocean ridges, adding new material • works like a conveyor belt, moving the continents along

the ocean floor• molten magma emerges, pushing older material away • magma cools, forming a new ridge • process repeats continuously • MAKES NEW CRUST

Hess gathered evidence to support his theory:

1. Alvin – the first research vessel to go down 4 km and take photographs

• photographed molten magma

2. magnetic bands• magma contains lots of Fe

(metal) that will align with the magnetic N and S poles when cool

• ridges have different metal orientations due to the Earth’schanging magnetic field

• last reversal was 780,000 years ago

4. drilling samples • samples brought up in order to determine age • older rocks are farther away from the ocean

ridges, newer rocks are next to it

If the sea floor is spreading and making new crust, why is the Earth not getting bigger?

Subduction:

a process by which the ocean floor sinks beneath a deep ocean trench and back into the mantle

DESTROYS CRUST

• takes millions of years (just like sea floor spreading)

• occurs in trenches• sea floor spreading and subduction work together • changes the size and shape of the oceans • because of these processes, the sea floor is renewed

about every 200 million years

• Pacific Ocean is shrinking • lots of trenches• trenches are swallowing more ocean floor than is

being made by the Pacific Antarctic Ridge • Atlantic Ocean is expanding

• has only a few short trenches • spreading ocean floor

has nowhere to go so it spreads and pushes the continents above it

which leads us to……

Plate Tectonics

In 1965, J. Tuzo Wilson observed that the cracks in the ocean floor were similar to cracks in the continents • cracks made 14 large plates• some involve only the

ocean floor, some involve both the ocean floor and continents

Plate:

a section of the lithosphere that moves over theasthenosphere carrying pieces of crust (oceanic and continental)

Wilson combined continental drift, mid-ocean ridges, sea floor spreading, and the presence of plates into one unifying theory

The Theory of Plate Tectonics

*

Plate Movement • move due to currents in the

asthenosphere • gravity pulls one edge of a plate

down into a trench and into the mantle

• as plates move, collide, pull apart or grind past one another they produce:• earthquakes • volcanoes • mountain ranges • deep sea trenches

• edges of the plates are called boundaries which extend deep into the lithosphere

• 3 types of plate boundaries • divergent boundary• convergent boundary• transform boundary

Divergent boundary• area where two plates are moving away from each

other • most occur along the mid-ocean ridges • on land, they form a deep valley called a

rift valley

Convergent boundary • an area where two plates are

moving towards each other • plate that is more dense

sinks underneath the other • oceanic crust is cooler and

more dense than continental crust

• oceanic crust subducts under the continental crust

• when two continental plates collide, neither sinks very far into mantle so they “bunch up” into mountain ranges

• Transform boundary• an area where two plates slip past each other,

moving in opposite directions • earthquakes often occur • crust is neither created nor destroyed

volcanoa weak spot in the crust where magma comes to the surface

• about 600 active volcanoes on land, and many more are underwater

• volcanic “belts” form along many plate boundaries • diverging and converging,

boundaries causethe crust to fracture and magma to come to the surface

EARTHQUAKES

the shifting plates cause stress in the earth Tension – pulling apart Compression – pushing together Shearing – slipping in opposite directions

Stress in the crust causes faults – breaks in the crust where surfaces slip past eachother

TYPES OF FAULTS

Normal faults – caused by tension Fault is at an angle Hanging wall – part of rock above the fault line,

moves down Foot wall – part of rock below the fault line,

moves up

Reverse fault – caused by compression Fault is at an angle Hanging wall – part of the rock above the fault

line, moves up Footwall - part of the rock below the fault line,

moves down

Strike-slip – caused by shearing Fault is vertical Plates slip past each other

Minerals

mineral:

a naturally occurring inorganic solid that has a crystal structure and a definite chemical composition

for a substance to be classified as a mineral, it must have certain characteristics….

CHARACTERISTICS

1. naturally occurring• not made in a lab • formed by a process

that occurs in nature

2. inorganic • cannot be formed from materials that once were

part of a living thing – plant, animal, bacteria

3. solid • have a definite volume and shape

4. crystal structure • particles (atoms) in a mineral line up in a pattern

that repeats itself over and over

5. definite chemical composition • can be a range of compositions • always contains certain

elements in definite proportions

• a few minerals contain one element (Cu, Ag, Au, Bi, S)• mostly metals• all called native elements

IDENTIFYING MINERALS • over 4,000 minerals

200 are common• 8 physical characteristics

are used to identify most minerals:

1. Color

2. Streak

3. Luster

4. Density/Specific Gravity

5. Hardness

6. Crystal System/Group

7. Cleavage/Fracture

8. Special Features

Color• used to get a ‘idea’ of the mineral• some minerals come in many colors • used to positively identify just a few minerals

• azurite and malachite

Streak• streak is the color of the mineral’s powder • test by rubbing the mineral over an unglazed

porcelain tile • color of a mineral may vary, but streak does not • streak color and mineral color often differ

• pyrite is golden yellow in color, but has a greenish black streak

• work only if the mineral has a hardness less than that of a tile

Luster• how light is reflected off the minerals surface • most tend to be very descriptive words

metallic

waxy

greasy

vitreous (glassy)

adamante (brilliant)

silky (fibrous)

pearly (iridescent)

dull, sub-metallic

earthy

Density or Specific Gravity• recall:

density = mass/volume • no matter what the size of the mineral sample, it will

always have the same density

Hardness• In 1812, Fredrich Mohs invented a test for hardness that he

called the Mohs Scale of Hardness

• hardness is determined by a scratch test • any mineral can scratch (leave a mark) any mineral softer

than itself

Talc Gypsum Calcite Fluorite Apatite Feldspar Quartz Topaz Ruby Diamond

Crystal System/Group• groups based on the number and type of angles

• sometimes hard to tell; incomplete growth, visible only under

a microscope, all ‘smooched’ together, etc.

• basic shapes are:

1.isometric or cubic

2.monoclinic

3.triclinic

4.orthorhombic

5.tetragonal

6.hexagonal

Cleavage/Fracture• the way a crystal breaks • depends on how the atoms are arranged • arrangement may allow the mineral to break easily

in one direction but not another • cleavage is indicated by breakage along a smooth,

flat surface • fracture describes how a

mineral looks when it breaks apart in an irregular manner

Special Featuressome minerals have unique features:

• radioactivity• fluorescence• magnetic • optical • taste • reaction to acid (carbonates)• odor • feel

andersonite

Habit• each mineral has one or more characteristic forms • determined by mineral growth and aggregation • refers to the most common mode of growth

Rocks

rock: • a naturally forming aggregate or mass of mineral

matter, forming the Earth’s crust • why are rocks important?

the ultimate ingredient in soil

• rocks are classified by:• mineral composition • color

• texture

MINERAL COMPOSITION

• rocks are mixtures of minerals and other material • mixture of minerals will determine rock type

Ex: quartz + mica + feldspar + = granite!

• 20+ rock forming minerals• make up most of the rocks

=++

COLOR

provides clues to mineral composition

TEXTURE the look and feel of a rock’s surface • grains (particles of minerals) give rocks their texture• use terms based on size, shape, and pattern of grains

• size:

coarse grained – can see with naked eye fine grained – need a microscope to see grain glassy – no grain formation

• shape: sandy, rounded, jagged, seedy, crystalline, etc.

• pattern:

foliated - layered, wavynon-foliated - interlocking grains,

massive

ROCK TYPE - IGNEOUS

rocks that have formed from magma or lava classified 3 ways:• origin • texture • mineral composition

IGNEOUS ROCKS - ORIGIN intrusive:

magma hardened beneath the Earth’s surface Ex: Granite

extrusive:

lava erupted onto the Earth’s surface and then hardened Ex. Basalt

IGNEOUS ROCKS - TEXTURE coarse grained

slowly cooled lava, allowing for crystal formation and growth (generally intrusive rocks)

fine grained

quickly cooling lava, not enough time for much crystal growth (generally extrusive rocks)

glassy

cooled very rapidly (generally extrusive rocks)

IGNEOUS ROCKS - COMPOSITION

composition determines rock color

ROCK TYPE - SEDIMENTARY

rocks that have been formed by WTDL • weathering • transportation• deposition• lithification

W = Weathering erosion (breaking up of the rock) due to water (rain, glaciers/ice, waves), wind and temperature changes

T = Transportationmovement of weathered rocks due to wind, gravity, water or combination of the three

D = Deposition occurs when the weathered rock finally stops moving and settles out of the water into a low-lying region

L = Lithification rock formation due to compaction and cementation• compaction

particles squeezed together due to pressure • cementation• particles are held together

by mineral solutions • calcite, silica, and

hematite are the most common cementing minerals

Three (3) types of sedimentary rock:• clastic:

rock fragments/particles make up the rock • classified by size of particle

• boulders to microscopic in sizeex. conglomerate, sandstone, shale

• organic:

rocks formed from the remains of plants and animals, deposited in thick layers

ex. coal, limestone, coquina, chalk

• chemical:

rocks formed from minerals that were dissolved in water

ex. halite, chert

ROCK TYPE - METAMORPHICrocks that have changed form due to high heat and pressure• changed in appearance, texture, crystal structure

and mineral content • two (2) types of

metamorphic rock:• foliated • non-foliated

Foliated • grains are arranged in layers

or bands • tend to split apart along the

grains/bands ex. gneiss (made from granite)

slate (made from shale)

schist (made from high mica granite, gabbro)

Non-foliated • grains are arranged randomly• do not split into layers

• ex. marble (made from limestone or dolomite)

HOW DOES THIS ALL COME TOGETHER?

The Rock Cycle

Igneous

Sedimentary

Metamorphic

The Rock Cycle

Where do sedimentary and metamorphic rocks generally become igneous?

• deep sea trenches

Where does WTDL begin occurring?

• mountains, due to plate collision