Chapter 2

Rock

•      A rock is a mixture of minerals, volcanic glass, organic matter, or other material

Rock Cycle

•             The rock cycle is a model showing processes that create and change rock

•             Any rock can become another type of rock

–         Any rock can be changed by heat and pressure into metamorphic rock

–         Any rock can melt and cool to form igneous rock

–         Any rock can be broken into fragments that may later form sedimentary rock

 

Rock Cycle

•      Law of Conservation of Matter – matter cannot be created or destroyed

–   The rock cycle never destroys  elements of rocks but merely redistributes them

•      James Hutton recognized the rock cycle in 1788

–   He saw that rocks had undergone changes into different rocks at Siccar Point, Scotland

Igneous Rock

•      Igneous rocks form from magma found deep under Earth’s surface

•      Magma reaching the surface flows from a volcano as lava.

–   When lava cools, igneous rock forms

•      Magma cools quickly at or near Earth’s surface

–   Forms small-grained extrusive igneous rock

 

Igneous Rock

•      Magma trapped below the surface cools slowly

–   Forms large-grained intrusive igneous rock

Basaltic Igneous Rock

•      Basaltic igneous rocks are dark-colored and dense

•      Contain iron and magnesium, but very little silica

•      Basaltic lava flows freely from a volcano

Granitic Igneous Rock

•      Granitic igneous rocks are lower density and lighter color

•      Contain more silica and less iron and magnesium

•      Granitic magma is thick and stiff

Andesitic Igneous Rocks

•      Andesitic rocks have a more balanced composition of minerals and density than basaltic or granitic rocks

Identifying Igneous Rock

•      Crystal size, large or small, can help identify and igneous rock

–   Small crystals – extrusive

•    Some so quickly, it seems smooth - obsidian

–   Large crystals – intrusive

•      Some rocks have holes formed around once-trapped air and other gases – pumice  (less dense)

Identifying Igneous Rock

•             Igneous rocks are classified in two ways

•             Where they are formed

•           Under the Earth’s surface – intrusive

•           At or near the Earth’s surface - extrusive

•             Magma type

•           Basaltic (dark)

•           Granitic (light)

•           Andesitic (kind of in the middle)

 

Metamorphic Rock

•      Metamorphic rocks form when existing rock is changed by temperature, pressure, and hot fluids

•      Temperature and pressure are directly related.

–   As temperature rises, pressure increases

•      Below the Earth’s surface, there is                 a lot of pressure

–   As a result, things get hot and rock                    can melt

Metamorphic Rock

•      Sometimes temperature and pressure are great enough to melt rock, forming magma

•      Sometimes pressure flattens mineral grains in rock without melting them

–   Ex:  granite -> gneiss

•      As pressure and temperature continue to increase over time, one type of rock can change into several different metamorphic rocks

•      Hot water-rich fluids can move through rock, chemically changing it

Classification of Metamorphic Rock

•      Metamorphic rocks are classified by composition and texture

•      In foliated texture, mineral grains flatten and line up in parallel layers or bands

•      In nonfoliated texture, mineral grains grow and rearrange but do not form layers

Sedimentary Rock

•      Sedimentary rocks are mostly found on the exposed surface of Earth

•      Rock fragments, mineral grains, and bits of plants and animal remains moved by wind, water, ice or gravity are called sediments.

•      Sedimentary rocks form in layers as sediments come together

Classifying Sedimentary Rock

•      Sedimentary rocks are classified by what they were made of and how they were formed

•      Detrital

•      Chemical

•      Organic

Detrital Sedimentary Rock

•      Detrital sedimentary rocks are made from broken fragments of other rocks

•      When layers of small sediments stick together because of pressure, compaction occurs

•      When water and other minerals move through open spaces between large sediments, gluing them together, cementation occurs

Detrital Sedimentary Rock

•      Detrital rocks often have a granular texture

•      Rocks are named according to size and shape of sediments

–   Sediment size can be large like gravel, or small like clay

–   Sediments can be well-rounded or have sharp angles

Chemical Sedimentary Rock

•      Chemical sedimentary rocks are formed when dissolved minerals came out of solution

•      Limestone forms calcite, which was calcium carbonate in solution

•      Rock salt forms from halite, which was salt in solution

Organic Sedimentary Rock

•      Organic sedimentary rocks are made from remains of once-living plants or animals

•      Chalk is made of microscopic calcite-shell remains of animals

•      Coal is made of plant remains, chemically changed by microorganisms and compacted over millions of years.