Rocks

 Chapter 6, Section 1 - Rocks and the Rock Cycle                                                                                                                                  

Rock - The material that makes up the solid parts of the Earth.
            Can be made of minerals, solid organic matter, or noncrystalline solids (i.e. - glass).
            Classified by how it forms.

Three Major Rock Types
  1. Igneous - "Coming from Fire"
    • Formed as magma/lava cools and hardens.
      • Magma - molten rock below the Earth's surface.
      • Lava - molten rock on the Earth's surface.
  2. Sedimentary - "Made of Sediments"
    • Formed as sediments are compressed and cemented together.
    • Sediments - broken up bits of rock, minerals, and organic matter that has been deposited by wind/water/ice.
  3. Metamorphic - "Changed in Form"
    • Formed from igneous/sedimentary rock that has been changed by heat/pressure/chemical processes.

The Rock Cycle
  • A series of processes in which rock forms, changes from one type to another, is destroyed, and forms again by geological processes.
    • Igneous rock forms from magma/lava.
    • Rock can be broken into sediments, which form sedimentary rock.
    • Rock can be changed by heat/pressure into metamorphic rock.
    • Rock can melt into magma, then cool into igneous rock.
  • Any type of rock can form into any other type - they do not always follow the same path on the cycle.

Properties of Rocks
  • Physical and chemical properties are determined by how/where the rock formed.
  • Physical properties reflect the chemical composition of both the whole rock and the individual mineral components.
  • The chemical stability of the minerals determine the rate of weathering of the rock.
    • Weathering - natural processes which break down rock physically and chemically.
Bowen's Reaction Series
  • Shows the order in which  minerals crystallize from cooling magma according to their chemical composition and melting point.
    • Note: Melting point and freezing point are the same point - you just approach it from different directions!
  • As magma cools, certain elements crystallize first, which removes certain elements from the magma.
    • This changes the magma's composition.
  • Changes in the composition of the magma allow different minerals to form.
  • Different minerals form at different times in the process, but generally in the same order!




Two graphics of Bowman's reaction series to aid in understanding.  Please look at them both!

These show the order that minerals form as magma cools.  All of these are forming from the same batch of magma, to make a rock that contains all of these minerals.






Chemical Stability of Minerals
  • The tendency of a chemical compound to maintain its composition (to resist breaking down into another substance).
    • Minerals formed at lowest temperatures tend to be most stable.                 (Formed in conditions closest to those on Earth's surface.)
    • Minerals formed at highest temperatures tend to be least stable.
Physical Stability of Rocks
  • All rocks have zones of weakness, determined by how/where they form.
  • Examples...
    • Sedimentary rocks may break between layers of their component sediments.
    • Metamorphic rocks may break in layers formed from aligned minerals.
    • Igneous rocks tend to have evenly spaced zones of weakness (joints) that form as the rock cools and contracts.
    • Inside the Earth, rock is under great pressure.  When lifted to the surface, decreased pressure allows joints/fractures to open.
      • Once weaknesses are exposed to air/water, physical and chemical weathering can begin to break down the rock.

 Chapter 6, Section 2 - Igneous Rock                                                                                                                                                      


Igneous Rock forms when magma cools and hardens.  Most is crystalline, as crystals usually form as magma solidifies.

Magma forms when rock melts...

  • When temperatures rise above the melting point of the minerals in the rock.
  • When there is less pressure on the rock, it tends to melt at lower temperatures.  (Higher pressure = higher melting points.)
  • Adding water to hot rocks can lower the melting point.
Partial melting occurs when different minerals melt at different temperatures.  This changes the composition of the magma over the melting process.
  • As more minerals melt, more elements are added to the magma.
Fractional crystallization occurs when different minerals form at different temperatures as the magma cools and solidifies.  (Opposite of fractional cooling.)                            Video
  • Minerals with highest freezing points form first.  This removes certain chemicals from the magma, changing its composition.
  • As magma's composition changes, new minerals form.  (See Bowen's reaction series.)
    • As minerals form, they tend to...
      • settle out to the bottom
      • stick to ceiling or sides of magma chamber
    • Earliest crystals to form tend to be largest; they have the most time to grow.
    • Sometimes, the center of the crystal has different composition from outside (because the magma's composition changes).

Textures of Igneous Rocks

    Intrusive Igneous Rock - Forms from magma beneath the surface.
    Extrusive Igneous Rock - Forms from lava at the surface.

    Igneous Rock Textures

Coarse-Grained
  • Typically intrusive rock with large mineral crystals.
  • Magma cools slowly, crystals have more growing time.
  • ex. - granite
Fine-Grained
  • Typically extrusive rock with small mineral grains (cannot be seen by unaided eye).
  • Magma cools rapidly, crystals don't have time to grow large.
  • ex. - basalt, rhyolite
Porphyritic 
  • Mixture of large and small crystals (large crystals embedded in a matrix of small grains).
  • Formed as magma rises towards the surface - cools slow initially, faster as it approaches surface.
Glassy
  • Formed when viscous magma cools quickly, so no crystals can grow.
  • ex. - obsidian
Vesicular
  • Formed when quickly cooling magma contains a lot of dissolved gas, which creates bubbles (vesicles).
  • ex. - pumice

Composition of Igneous Rock

    There are three families of igneous rock, based on their composition:

1. Felsic Rock
  • rich in feldspar (potassium type) and silica
  • usually light in color
  • common mineral components:
    • potassium feldspar
    • quartz
    • plagioclase feldspar
    • biotite mica
    • muscovite mica
  • examples:
    • granite
    • rhyolite
    • obsidian
    • pumice
2. Mafic Rock
  • low in silica, rich in iron and magnesium
  • usually dark in color
  • common mineral components:
    • plagioclase feldspar
    • pyroxene minerals
    • ferromagnesian minerals (i.e. - hornblende)
    • olivine
  • examples:
    • basalt
    • gabbro
3. Intermediate Rocks
  • less silica than felsic rocks, more than mafic rocks
  • common mineral components:
    • plagioclase feldspar
    • hornblende
    • pyroxene
    • biotite mica
  • examples:
    • diorite
    • andesite

Intrusive Igneous Rocks
Igneous rock masses that form underground (intrusions) by entering other rock masses and cooling.
  • Batholiths - large intrusive formations spreading over 100 kmat the surface.
  • Stock - similar to batholiths, but cover less than 100 kmat the surface.
  • Laccolith - formed when magma pushes between two layers, pushing the overlying rock up into a dome shape.
  • Sill - formed when magma flows between rock layers and hardens.
  • Dike - formed when magma cuts across rock layers and hardens.
Extrusive Igneous Rocks
Igneous rock masses that form on the surface (extrusions) by volcanic activity.
  • Volcano - a vent through which magma, gases or volcanic ash is expelled.
  • Volcanic Neck - magma in the volcano's vent hardens, then the softer outer rock erodes away; this leaves the hardened vent.
  • Lava Flows - flat masses of rock formed when lava cools and hardens.
  • Lava Plateau - a series of lava flows that cover a large area with thick rock.
  • Tuff - formed from volcanic ash and particles; can form deposits hundreds of meters thick and hundreds of kilometers across.

Chapter 6, Section 3 - Sedimentary Rock                                                                                                                           

Sediment
  • Loose fragments of rock, minerals and organic material.
  • Generally transported by wind, water or ice.
    • Can be physically changed or chemically altered during transport.
      • sharp edges rounded off
      • minerals added or dissolved out
  • Converted to sedimentary rock by...
    • Compaction - sediment is squeezed by weight of overlying layers, reducing volume and porosity.
    • Cementation - sediments are glued together by minerals deposited by water.
Types of Sedimentary Rock
  1. Chemical Sedimentary Rock
    • Forms when dissolved or suspended minerals precipitate out of solution.
      • Often happens when water evaporates.
    • Evaporites - rocks formed from minerals left behind by evaporating water.
    • examples: gypsum, halite (rock salt)
  2. Organic Sedimentary Rock
    • Formed from the remains of living things.
      • Coal - plant remains buried before they decay, compressed into carbon-rich minerals.
      • Limestone - shells from sea creatures build up on the ocean floor, get compressed into limestone and chalk.  (Different from chemical limestone.)
  3. Clastic Sedimentary Rock
    • Formed from rock fragments that are deposited, cemented and compacted.
    • Classified by sediment size...
      • Conglomerate & Breccia
        • Large fragments (>2mm) held together by a cement
        • Conglomerate - rounded fragments
        • Breccia - angular fragments
      • Sandstone
        • Sand-sized fragments
        • Often porous, allowing groundwater, natural gas, and crude oil to flow through.
      • Shale
        • Clay-sized particles
        • Usually compressed into flat layers that can be split apart.

Characteristics of Clastic Sediments
    Physical characteristics of sediments are mainly determined by how they are transported.
    Sediments can be transported by...
  • Water
  • Ice
  • Wind
  • Gravity
    The speed of transport determines the size of the particles moved and the distance they are moved.

    Sorting - The tendency of water or wind to separate sediments according to their size.
        Faster current can carry larger particles.  As the current slows, larger sediments are dropped off first.

    Angularity refers to the shape/texture of the sediments.
        Newly broken sediments tend to be angular.  The longer/farther they travel, the smoother they tend to become.

Sedimentary Rock Features
    Depositional Environment - The place/setting in which sediment is deposited.  (ex. - rivers, deltas, beaches, oceans)
        Different environments have different characteristics and create different structures.
    Stratification - Layering of sedimentary rock.
        Occurs when the conditions of deposition change (creating differing layers).
        Beds - stratified layers of rock.
            Vary in thickness due to the rate and duration of deposit.
        Massive Beds - beds with no internal structure (no layers in them).  Usually more than 10cm thick.
            Formed when similar sediments deposited under similar conditions over long periods of time OR large amounts deposited at one time.

    Cross-Beds - Beds that contain slanting layers.  Usually form in sand dunes and river beds.
    Graded Bedding - Beds with different sizes and shapes of materials, sorted by size.  (Largest usually on the bottom.)
        Reverse Grading - When the largest sediments are on the top, smallest on the bottom.  
            Can be formed under specific conditions, such as mudflows.

    Ripple Marks - Caused by the action of wind/water on sand.  Ripple marks in rock indicate a beach or river bed.

    Mud Cracks - Formed when muddy deposits shrink as they dry.  Flooding may fill in the cracks with other minerals.

    Fossils - The remains or traces of ancient organisms.  (When buried, the hard parts may get preserved or leave an impression.)

    Concretions - A lump of rock within the bed that has a different composition.  
        Formed when minerals precipitate out of groundwater around a nucleus.

    Geode - Formed when water deposits minerals inside cavities in sedimentary rock.


Chapter 6, Section 4 - Metamorphic Rock                                                                                                                                    

Metamorphism - The process by which one type of rock changes into metamorphic rock due to heat, pressure, and/or chemical processes.

Formation of metamorphic rocks...
  • heat/pressure/hot fluids can change some minerals into others
  • minerals can change size/shape
  • minerals can separate into parallel bands (layered appearance)
  • hot fluids can dissolve some minerals and add others
The type of metamorphic rock indicates the conditions present at the time the rock was changed.

Two types of metamorphism...
  1. Contact Metamorphism
    • occurs when existing rocks come into contact with magma.
    • only a small area, surrounding the magma, is changed.
      • hot chemical fluids, moving through fractures, can also change the surrounding rock
  2. Regional Metamorphism
    • occurs as a result of tectonic activity (such as when mountains are formed).
    • occurs over large areas (thousands of km2)
      • since volcanoes may occur during tectonic activity, contact metamorphism can occur in areas of regional metamorphism.

Classification of Metamorphic Rocks
  1. Foliated Rocks
    • Foliation - metamorphic rock texture in which mineral grains are arranged in planes/bands.
    • Formed when...
      • extreme pressure causes mineral crystals to realign/regrow in parallel bands.
      • minerals of different compositions separate to form alternating light and dark bands.
    • Examples...
      • Slate - Comes from shale, layers of clay grains pressed into thin layers.
      • Schist - Formed when large amounts of heat and pressure are applied to slate.
      • Gneiss - Formed when the minerals in schist separate to form light and dark bands.
  2. Nonfoliated Rocks
    • Nonfoliated metamorphic rock texture in which mineral grains are not arranged in planes/bands.
    • Formed because...
      • original rock may contain only one mineral (or very little of other minerals).
      • original rock may contain grains that are round/square, and do not change position when compressed.
    • Examples
      • Quartzite - Formed when quartz sandstone is compressed.
      • Marble - Formed when limestone is compressed.



Resources
Online Activities

Chapter Assignment:
  • Read Chapter 6.
  • Pg. 160-161 #6-24, 34-35.