Cycle 16: Rocks and the Rock Cycle

Relative Dating

• Done in areas with lots of sedimentary rocks• Don’t need a geochronology lab• Geo Time Scale developed like this before we had

radiometric dating to give absolute dates.• Relative Dating=sequencing of rock record, life

forms and events in the evolution of life

Not just a nice view…

Correlation

“Absolute” Dating• Carbon-14 dating• Dating with tree rings• Dating with “varves” or sediment layers in a stream• Radiometric/Radioisotope dating– Measure the ratio between the radioactive parent isotope

and the stable daughter product in sample. The more daughter, the older the rock.

– If you know the half-life of that radioactive isotope you can get an absolute date of when that rock cooled (igneous) or was metamorphosed (metamorphic)

Class Opener:

• Look at the volcano diagram and igneous rocks placed on diagram:– What do all the rocks that form deep in the root of

the volcanoes have in common?– How are they different than the rocks that are

formed near the top/in the air above the volcano?

Crystallization Experiment

• Which do you think will have larger mineral crystals form?

• Why?

What are Igneous Rocks?

• All igneous rocks for from– Cooling & solidifying of magma/lava (hot liquid

rock)

Two types of igneous rocks: focus on INTRUSIVE

• Intrusive and extrusive• Igneous rocks that form from magma,

underground, are called _____________ igneous rocks and they have __________ mineral xtals

• Large crystals form during slow cooling as atoms have a chance to move around and get organized into large atomic structures

Intrusive Igneous RocksRock Name # Description

Granite Large mineral crystals.Speckled with more than 50% light colored minerals (white, pink, grey, blue)

Diorite Large mineral crystals Speckled with 50% light 50% dark minerals (green, black)

Gabbro Large mineral crystalsSpeckled 100% dark minerals

Two types of igneous rocks: focus on EXTRUSIVE

• Intrusive and extrusive• Igneous rocks that form from magma, near/at

surface, are called _____________ igneous rocks and they have __________ mineral xtals

• Snall crystals form during fast cooling as atoms DO NOT have a chance to move around and get organized into large atomic structures

EXtrusive Igneous RocksRock Name # Description

Rhyolite small mineral crystals.Speckled with more than 50% light colored minerals (white, pink, grey, blue)

Andesite small mineral crystals Speckled with 50% light 50% dark minerals (green, black)

Basalt small mineral crystalsSpeckled 100% dark minerals

Super-Extrusive Igneous rocks

• Lava shot into the air cools soooo quickly that there are no mineral crystals…these rocks are made of randomly arranged atoms.

Rock Name # Description

Obsidian Glassy, black (although can vary) few to no mineral crystals, some gas holes, circular fracture pattern.

Pumice Lots of holes, color varies from light grey to red to black. So lightweight it floats!

Why does granite sometimes form and gabbro forms other times…both form from magma cooling slowly?

• Depending on what part of Earth’s crust is melted into magma, it will have a different composition (set of atoms to make minerals)

Type of Crust Rocks Formed Color/comp

Ocean Floor dark

Continental Crust light

Coastal (both) 50%-50%

Check on your experiments

• Which one had the larger mineral crystals?• Why?• The faster the magma/lava cools, the

_________ the mineral crystals• The trial on ice represents…• The trial on the hot plate represents…

Class Opener:Review Igneous Rocks

• How do igenous rocks form?• What does the size of the mineral xtal of an

igneous rock tell you?• What does the overall “color” (percent light to

dark minerals) of an igneous rock tell you?

Absolute Dating

• While early geologists like William Smith could only determine the relative ages of rock layers (sequencing them), today geologists can determine the ______________ age of a rock using ______________________________ of radioactive elements in minerals from ____________________ rocks

• What minerals are used and why?– Zircons– Because they are common in igneous

rocks, resistant to weathering, contain some amount of radioactive uranium

Radioactive Decay• What causes an element to be radioactive?

– Happens if the nucleus of an atom (protons and neutrons) is “unstable” meaning the force that binds it together is not strong enough so particles are given off

– Called radioactive decay• What happens during radioactive decay?

– Energy and particles released,– produces a different element– Same amount of mass left– Stops when a “stable” nucleus is formed– Rate at which it occurs is logrithmic and

measureable

• Half-life:– The amount of time it

takes for half of the radioactive substance to react and become a different, more stable element.

How much radioactive strontium is there at time 0? How much after 27 years?How much after 54? 108?What is the pattern in decorease of mass?What happens to the mass of strontium as it decays?

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Metamorphic Rocks

• All metamorphic rocks form from…(one or both of the following)…– HEAT that does NOT melt the rock into magma but

allows the atoms that make up minerals to re-arrange into new minerals or grow together

– PRESSURE that causes the minerals to re-arrange themselves into new patterns

HEAT & Metamorphism

• Chemical changes do not happen to minerals below __________. Above ________ the minerals don’t just change, they MELT!

• Rocks can get these hot temperatures from being near or touching ______________

PRESSURE & Metamorphism• Pressure can come from

_______________________ or from ________________________and can cause minerals to align into bands or stripes

Foliated Metamorphic RocksRock Name Rock # Characteristics

Gneiss Wavy ands of alternating light and dark minerals 

Schist Silvery bands 

Slate Looks like a very smooth shale, breaks into very flat thin pieces

NONFoliated Metamorphic RocksRock Name Rock # Characteristics

Anthracite Coal Black, lightweight, somewhat soft compared to other rocks

Quartzite Comes in lots of colors like pink, white, & grey. Glassy looking but pretty smooth feeling. DOES NOT react to acid

Marble Comes in lots of colors like pink, white, & grey. Can contain fossils. DOES react to acid.