1. Rocks
Chapter 10

2. Rocks & the Rock Cycle Magma is the parent material for all rocks
There are 3 major types
Any type can be changed into any of the other types through a series of changes called the rock cycle

3. 3 Major Rock Types Three major types Igneous = magma cools & hardens
Sedimentary = sediment is deposited and hardens after being compressed & cemented.
Metamorphic = forces such as extreme heat and pressure and chemical processes change the form of existing rock.

4. The Rock Cycle Overview
Hardened magma cools -> igneous rock
Igneous rock breaks down into sediments -> sedimentary rock
Sedimentary rock subjected to extreme heat/pressure -> metamorphic rock
Metamorphic rock under extreme heat/pressure -> melt into magma (start over)

5. The Rock Cycle (cont)
Rock does not always make it through entire cycle
Igneous rock not exposed to surface will never become sediment – may go directly to metamorphic
Igneous and sedimentary may melt directly into magma
All 3 types may form sediments when exposed

6. The Rock Cycle

7. Igneous Rock Two types = classified by where magma cools
intrusive = cool deep below crust
May “intrude” into other rock layers
Cools slowly
extrusive = lava cooling on surface
Cools rapidly

8. Igneous Rock (cont) Texture determined by rate of cooling
Intrusive = cools slowly
Course-grained texture
Ex: granite
Extrusive = cools rapidly
Little time for crystal formation
Fine-grained texture
Ex: basalt

9. Igneous Rock (cont)
Some extrusive rock cools slowly at first then more quickly as it nears the surface
Produces both large and small crystals
Porphyry
Extremely rapid cooling produces rock without crystals
Gases escaping during rapid cooling may form small bubbles in rock

10. Igneous Rock (cont)
Composition of igneous rock is determined by chemical composition of magma.
3 Families – based on composition
Granite
Basalt
Diorite

11. Igneous Rock (cont) Granite Family Felsic magma – high silica
Light color
Main minerals = orthoclase feldspar & quartz
Ex: granite (intrusive)
rhyolite and obsidian (extrusive)

12. Igneous Rock (cont) Basalt Family Mafic lava = low silica, high iron
Dark color
Main minerals = plagioclase feldspar and augite
Ex: basalt & gabbro

13. Igneous Rock (cont) Diorite Family Medium colored
Main minerals = plagioclase feldspar, hornblende, augite, and biotite.
Little or no quartz
Ex: diorite & andesite

14. Igneous Rock Structures Intrusions = underground
Batholith
Largest (cover over 100km²)
Form cores of many mountains
Stock
Similar to batholith but cover less area
Laccolith
Magma pushes overlying rock layer into an arc
May form dome mountains
Sills
Sheet of magma flows between layers of rock (horizontal)
Dikes
Magma flows upward through fractures and cut across rock layers vertically

15. Igneous Intrusions

16. Igneous Rock Structures Extrusions = at surface
Volcanic neck – softer parts of volcanic cones wear away exposing solidified rock in vent.
Many are simply flat lava flows
Lava plateaus – lava flows out long cracks in the surface
Cover vast areas
Fill in valleys and cover hills to form flat plateaus

17. Igneous Extrusions

18. Sedimentary Rock Compaction and cementation form sedimentary rock.
Compaction – weight of overlying sediments causes pressure that squeezes air and water out of sediments
Cementation – water carries dissolved minerals through sediments which provide a cement to hold fragments together.
Sedimentary rocks are classified by the kind and size of their sediments.

19. Sedimentary Rock (cont)
Clastic sedimentary rocks
Rock fragments are carried by wind, water and ice become compacted and cemented together = classified by size
Conglomerate = rounded, gravel-sized pebbles cemented by minerals
Breccia = gravel-sized fragments (angular with sharp edges)
Sandstones are made up of sand-sized grains
Major component = quartz
Many are porous enough to allow liquids (water, crude oil) to move through.
Shale consists of clay-sized particles
Usually pressed into flat, “flakey” layers

20. Sedimentary Rock (cont)
Chemical Sedimentary Rock
Forms from minerals once dissolved in water
Minerals may precipitate out due to temperature changes
Evaporites form when water evaporates and leaves minerals behind

21. Sedimentary Rock (cont)
Organic Sedimentary Rock
Forms from remains of living things
Coal forms from dead plant remains buried and compacted
Limestone forms from the shells of marine organisms that use calcite
Chalk is limestone made of shells of one-celled marine organisms

22. Sedimentary Rock Features
Stratification = layering = occurs when type of sediment changes
May be due to change in river current, sea level, etc.
Thickness of layers (beds) depends on how long the sediment is deposited
Most water-deposited layers are horizontal
Wind-deposited layers may exhibit cross-bedding
Graded bedding = different kinds & sizes of material are deposited in the same layer as they settle out.

23. Sedimentary Rock Features (cont)
Ripple marks
Form from action or wind or water on sand.
Mud cracks
Muddy deposits dry and shrink causing shrinking
River flood plains, dry lake beds
When area floods new deposits fill in and preserve the cracks.

24. Sedimentary Rock Features (cont)
Fossils
The remains or traces of ancient plants and animals
Are preserved in sedimentary rock when remains are buried
Harder parts may remain but most often it’s only the impression

25. Sedimentary Rock Features (cont)
Concretions
Lumps or nodules of rock with a composition different from the main rock body
Precipitated minerals build up around existing rock
Geodes = groundwater deposits dissolved quartz or calcite inside a cavity and crystals form

26. Metamorphic Rock
Changing from one type of rock to another by heat, pressure and chemical processes = metamorphism
Minerals may change into other chemicals
Minerals may change size, shape, or separate into bands (layered look)
Hot fluids in magma may change mineral composition by dissolving some minerals and adding others

27. Metamorphic Rock (cont)
Two types of metamorphism:
Contact metamorphism
Hot magma is near or touching rock
Changes structure and/or mineral composition
May also include hot chemical fluids working through fractures
Regional metamorphism
Occurs over large areas during tectonic activity
Plates moving against each other causes tremendous heat and pressure at the edges – causes chemical changes in rock minerals
Most metamorphic rock is formed this way.

28. Metamorphic Rock (cont)
Classification is according to structure
Foliated
Visible parallel bands
Extreme pressure flattens mineral crystals into bands
Minerals of different densities may separate into bands -> alternating light and dark

29. Metamorphic Rock (cont)
Common foliated rocks
Slate
Pressure on shale produces thin layers that split into sheets
Schist
Pressure on slate – courser grained
Gneiss
Intense heat and pressure on schist -> minerals separate into bands of different densities

30. Metamorphic Rock (cont)
Unfoliated
Rocks do NOT have bands of crystals
Quartzite – sandstone is highly compacted
Marble – compression of limestone