Sedimentary RocksUnit 10

Some  rocks  form  from  rock  particles   Most  sedimentary  rocks  are  made  from  other  rock  particles  called  sediments.  Sediments  are  any  material  that  settles  out  of  water  or  air  to  form  a  sedimentary  rock.  Examples  of  sediments  include  rocks,  minerals,  and  the  remains  of  dead  plants  and  animals  (Figure  1).  A  sedimentary  rock  is  a  rock  made  out  of  sediments.  Sedimentary  rocks  mostly  form  in  layers.  You  can  see  this  in  Bigure  2

Fig.  1:Examples  of  sedimentary  rocks  made  out  of  other  rocks.  (Left)  Conglomerate  and  (right)  breccia.

Fig.  2:Sedimentary  rocks  often  form  in  layers.  These  can  be  easily  seen  in  canyons  and  cliffs.  Below  is  a  picture  from  Canyonlands  National  Park  in  Utah.

Unit 10 | Sedimentary Rocks Reading! Page 1

Most  sedimentary  rocks  are  formed  in  a  4ive-­‐step  process   In  order  for  a  sediments  to  become  a  sedimentary  rock,  they  must  go  through  a  process.  This  process  helps  the  particles  become  glued  together  to  form  a  sedimentary  rock.  There  are  Bive  steps  to  this  process.  Figure  3  illustrates  this  process.

Step  One:  WeatheringWeathering  is  the  process  that  breaks  down  rocks  and  minerals.  This  creates  sediments  needed  to  form  a  sedimentary  rock.

Step  Two:  ErosionErosion  is  the  process  that  transports  the  sediments  formed  during  weathering.  Erosion  is  caused  by  the  water  and  wind.  These  carry  the  particles  away.  Erosion  does  not  break  down  rocks  into  particles.  However,  erosion  can  cause  weathering.

Step  Three:  DepositingDeposition  is  when  the  sediments  are  deposited  out  of  the  water  or  wind  carrying  the  sediments.  When  deposition  occurs,  bigger  and  heavier  particles  settle  Birst.  These  particles  will  settle  into  layers.

Step  Four:  CompactingCompaction  is  the  process  that  presses  the  layers  together.  This  causes  the  spaces  between  the  sediments  to  become  much  smaller  and  disappear.  The  layers  are  pushed  together  by  the  weight  of  the  layers  above.

Step  Five:  CementingCementation  is  the  process  that  glues  the  sediments  into  a  rock.  This  is  caused  by  minerals  crystallizing  and  gluing  the  sediments  together.  This  creates  a  strong  sedimentary  rock.  If  this  step  doesn’t  happen,  the  sedimentary  rock  is  brittle  and  is  easy  to  break  apart.

Fig.  3:  This  diagram  shows  steps  2-­‐5  of  the  Give-­‐step  process.  (A)  Sediments  are  eroded  (transported)  and  deposited.  (B)  Sediments  are  compacted  together.  (C)  Sediments  are    cemented  together  and  form  a  sedimentary  rock.

Unit 10 | Sedimentary Rocks Reading! Page 2

Sedimentary  rocks  are  classi4ied  by  the  sediment  they  are  made  of   With  so  many  sedimentary  rocks,  scientists  classify  the  rocks  into  groups  based  on  what  sediments  make  up  the  rock.  There  are  three  main  groups  of  sedimentary  rock:  clastic,  organic,  and  chemical.

Sedimentary  rock  formed  from  other  rocks  is  clasticClastic  rock  is  the  most  common  type  of  sedimentary  rock.  It  is  made  of  broken  pieces  of  other  rocks.  Common  examples  include:•Shale  made  of  small  grains  of  clay.•Sandstone  (Figure  4)  made  of  grains  of  sand.•Conglomerate  made  of  rounded  rock  grains.•Breccia  made  of  jagged  rock  grains.

Sedimentary  rock  formed  from  the  remains  of  plants  and  animals  is  organic   Organic  rock  contains  a  high  amount  of  organic  materials.  Organic  material  comes  from  once  living  things,  like  plants  and  animals.  Examples  include  shells  and  leaves.  The  two  most  common  organic  rocks  are  coal  and  limestone.   Coal  (Figure  5)is  an  organic  rock  made  of  the  remains  of  plants:  wood,  bark,  leaves,  stems,  etc.  Coal  forms  through  the  Bive  step  process.  Dead  plant  matter  is  layered,  compacted,  and  glued  together.  The  coal  we  have  today  is  made  of  the  remains  of  organisms  from  millions  of  years  ago!   Limestone  is  an  organic  rock  made  up  of  carbonate  minerals,  such  as  calcite.  These  minerals  come  from  the  shells  and  skeletons  of  animals.  Limestone  forms  just  like  coal:  the  skeletons  and  shells  of  dead  animals  form  layers,  get  compacted,  and  cemented  together.  A  very  common  use  of  limestone  is  chalk!  This  type  of  limestone  is  called  fossilitic  limestone  (Figure  6,  bottom)    because  it  is  made  of  once-­‐living  things.

Sedimentary  rock  formed  from  dissolved  minerals  is  chemical   Chemical  rocks  form  when  minerals  dissolved  in  a  solution  crystallize  together.  This  is  considered  a  rock,  and  not  a  mineral,  because  it  is  not  chemically  consistent  and  lacks  a  crystal  structure.

Unit 10 | Sedimentary Rocks Reading! Page 3

Fig.  5:  Example  of  an  organic  rock  showing  the  imprints  of  the  plants  that  helped  make  it.

Fig.  4:  Picture  of  banded  sandstone.  It  is  made  of  particles  of  sand.  This  makes  it  a  clastic  sedimentary  rock.

  Chemical  rocks  form  in  a  different  process  than  the  Bive-­‐step  process.  To  form  a  chemical  rock,  minerals  must  Birst  dissolve  in  water.  This  is  done  by  water  that  has  carbon  dioxide  dissolved  in  it.  This  is  normally  called  “acid  rain.”  The  second  step  is  when  the  water  evaporates.  When  this  happens,  the  minerals  begin  to  recrystallize-­‐or  form  again.   Many  rocks  form  through  this  process.  The  most  common  chemical  rocks  are  gypsum,  rock  salt  (Figure  7),  and  limestone.  This  type  of  limestone  is  called  oolitic  limestone.  Oolitic  limestone  (Figure  6,  top)  is  made  of  little  circles  of  calcite  called  oolites.

Sedimentary  rocks  have  many  uses   Sedimentary  rocks  are  used  a  lot  in  construction.  For  example,  limestone  and  sandstone  are  used  in  buildings  because  they  are  easy  to  cut.  Rock  salt  is  used  to  help  melt  snow  and  ice  on  roads.  Coal  is  used  as  a  source  of  energy  and  as  a  way  to  punish  bad  children.

Unit 10 | Sedimentary Rocks Reading! Page 4

Fig.  6:  These  are  two  pictures  of  limestone.  The  top  picture  is  oolitic  limestone.  Notice  how  it  is  made  of  small  circles  of  calcite  called  oolites.  The  bottom  picture  is  fossilitic  limestone.  Notice  how  it  is  made  of  shells.

Fig.  7:  This  an  example  of  a  chemical  rock:  rock  salt.  It  was  formed  when  halite  (salt)  was  dissolved  bywater  and  later  re-­‐crystallized  to  form  rocksalt.  It  is  commonly  used  to  de-­‐ice  roads  and  melt  snow.