Rocks and the Rock Cycle
Three Major Types of Rocks
• Igneous rock- forms when magma, or molten rock, cools
and hardens. Magma is called lava when it is exposed to
the surface.
• Ex. Granite
• Sedimentary rock – forms when sediment deposits are
compressed, cemented together, and harden.
• Ex. Sandstone
• Metamorphic rock – forms when tremendous heat, extreme
pressure, and chemical processes CHANGE existing rock.
• Ex. Gneiss
• Is it possible for igneous and sedimentary rock to become metamorphic
rock?
The Rock Cycle
• All rock has physical and chemical properties that are
determined by how and where the rock formed.
• The physical characteristics of rock reflect the
chemical composition of the rock as a whole and the
individual minerals that make up the rock.
Bowen’s Reaction Series
• Bowen studied how minerals crystallize from magma. As magma
cools, certain minerals tend to crystallize first.
• Bowen reaction series – the simplified pattern that illustrates
the order in which minerals crystallize from cooling magma
according to their chemical composition and melting point.
• According to Bowen’s hypothesis the rocks form in two ways
• 1 – the gradual and continuous formation of minerals that
have similar chemical compositions.
• 2 – sudden changes in mineral types.
• The pattern of mineral formation depends on the chemical
composition of the magma.
Bowen’s Reaction Series
Stability of Minerals
• Chemical stability – a measure of the tendency of a chemical
compound to maintain its original chemical composition rather than
break down to form a different chemical.
• The rate at which a mineral breaks down is dependent on the
chemical stability of minerals.
• Rocks have natural zones of weakness that are determined by how
and where the rocks form.
• Sedimentary tends to break between layers
• Igneous rocks have joints that form as the rock cools and contracts
• Metamorphic rocks develop zones of weakness due to the intense
pressure inside Earth.
Quiz
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How do we use the Mohs Hardness Scale to identify minerals?
Compare and contrast chatoyancy and asterism.
Describe three properties you could identify calcite.
What are the three rock types?
What are the possible pathways of the rock cycle?
How can we use sedimentary rock to prove the rock cycle exists?
Igneous Rocks
• When magma cools and hardens it forms igneous rock.
Also, since minerals crystallize as igneous rock forms,
most igneous rocks can be identified as crystalline, or
made of crystals.
• Magma forms when rock MELTS! Three factors that
determine if a rock melts are:
• Temperature
• Pressure
• Presence of fluid in rock
Igneous rock
• Different minerals have different melting points, and
minerals that have lower melting points are the first to
melt. As the temperature rises, different minerals
melt and change the composition of the magma.
• Partial melting – the process by which different
minerals in rock melt at different temperatures.
Partial Melting
Fractional Crystallization
• When magma cools, the cooling process is the reverse
of the process of partial melting. As temperature
decreases, the first minerals to crystallize from the
magma are minerals that have the highest freezing
point. As these minerals crystalize, they change the
chemical composition of the magma.
Fractional Crystallization
Textures of Igneous Rock
• Intrusive (Plutonic) – cools slowly with very large crystals.
• Course-grained igneous rock
• Ex. Granite
• Extrusive (Volcanic) – cools rapidly with small crystals.
• Fine-grained igneous rock
• Ex. Rhyolite
• Glassy – Obsidian
• Vesicular – Full of holes called vesicles. Ex. Pumice
Composition of Igneous Rock
• The mineral composition of igneous rock is determined by the
chemical composition of the magma from which the rock formed.
• Felsic – Light color, high silica content. Main components are potassium
feldspar and quartz.
• Mafic – dark color, low silica content. Main components are plagioclase
feldspar, pyroxene minerals, and may include ferromagnesian minerals.
• Intermediate – made up of plagioclase feldspar, hornblende, pyroxene, and
biotite mica.
Intrusive Igneous Rock Structures
• Intrusions – igneous rock masses that form underground
• Intrusions form when magma intrudes, or enters, into other rock masses and
the cools deep inside Earth’s crust.
• Batholith – intrusions that cover at least 100 km2 when they are
exposed on Earth’s surface. These masses form the cores of many
mountain ranges.
• Stocks – intrusions that cover less than 100 km2
• Laccoliths – an intrusion that forms when magma flows BETWEEN
rock and pushes the overlying rock layers into a dome.
Intrusive Igneous Rock Structures
• Sill – an intrusion of magma that flows between layers of rock and
hardens creating a parallel layer. (Like a windowsill)
• Dike – an intrusion that forms when magma follows existing
vertical fractures in rock or creates new ones. These are common
in volcanic areas.
Extrusive Igneous Rock
• Extrusive igneous rock – rock masses the form on Earth’s surface.
• Ex. Volcano, lava flow, and tuff, or volcanic ash deposits.
Sedimentary Rock
• Sediment – loose fragments of rock, minerals, and organic
material that result from natural processes, including the physical
breakdown of rocks.
• After sediments are formed, they are generally transported by
wind, water, or ice to a new location. The source of the sediment
determines the sediment’s composition.
• Compaction – the process in which the volume and porosity of a
sediment is decreased by the weight of the overlying sediments.
• Put “compaction” into your own words.
Sedimentary Rocks
• Cementation – the process in which minerals precipitate into the
pore spaces between sediment grains and bind sediments together
to form rock.
• Compare and contrast compaction and cementation
• There are 3 main classes of sedimentary rock
• Chemical
• Organic
• Clastic
Sedimentary rock
• Chemical sedimentary rock – forms from minerals that were once
dissolved in water. These minerals precipitate because of evaporation.
• Ex. Evaporites like Gypsum and Halite.
• Organic Sedimentary Rock – forms from the remains of plants or animals.
• Ex. Coal and limestone
• Clastic sedimentary rock – forms when fragments of pre-existing rock are
compacted or cemented together.
• Ex. Sandstone
• Kentucky has all of these rocks. What do you think Kentucky must have
looked like millions of years ago?
Sedimentary Rock Features
• Depositional setting – the setting in which sediment is deposited.
Ex. Rivers, deltas, beaches, oceans.
• Each setting creates rocks with different characteristics, which
helps scientists identify the setting of the deposits.
• Stratification – layering of sedimentary rock. This type of feature
occurs when the conditions of sediment deposition changes.
• Cross-beds – slanted layers in sedimentary rock. Associated with
sand dunes or river beds.
Sedimentary Rock Features
• Graded bedding – different sizes and shapes of sediment settle to
different levels. Most of the time the transition is from largest
grains on the bottom to smallest on the top, but this can be
reversed by events like mudflows.
• Ripple marks – these are caused by the action of wind or water on
sand. These ripples become preserved in the rock.
• Mud cracks – form when muddy deposits dry and shrink.
Associated with flood plains or dry lake beds.
• Fossils – as sediment piles up, plants and animals are buried and
preserved in the rock.
• Concretions form when lumps of rock have a different composition
than the surrounding rock. Ex. Geodes
Identify the rock features!
• What can we learn from this
rock?
• What does this rock tell us
about its depositional
environment?
• What do you think the
landscape was like when this
rock formed? What could have
happened?
• Where do you think this rock
formed?
• What does this rock tell us
about its environment?
Quiz
1. Draw or define the following depositional environments:
• Stratification
• Cross beds
• Ripple Marks
• What does rock in each environment tell us?
1. Compare and Contrast Compaction and Cementation
2. What are the three classes of sedimentary rocks? Provide
examples for each class.
3. Which of the three classes of sedimentary rock would you be?
Why?
Create a story about this rock. What can it
tell us?