In class we said that rocks are like history books.“Sedimentary rocks preserve a record of the environments that existed when they formed. By looking at sedimentary rocks of different ages, scientists can figure out how climate and environments have changed through Earth’s history.  Fossils of ancient living things are preserved in sedimentary rocks too.” (Windows to the Universe)  If we look at the rocks here in the Berkshires, we can tell what the ancient environment used to be.

 

 

We discussed that weathering was a process that breaks existing rocks apart.  Then we learned that erosion is a process that carries weathered rock (sediment) to new places.  When conditions are right, sediment of the same size will collect (deposit).  Quiet conditions are suited for small particles like silt and clay.  Moderate conditions are suitable for medium sized particles like sand, and faster conditions are right for larger particles (pebbles). 

   After these particles are deposited, they are often buried.  Over thousands or millions of years, the buried sediment eventually turns into rock.  Rocks that are made from sediment are called sedimentary rock.

 

Common sedimentary rocks:

 

Conglomerate

Sandstone

Shale

Limestone

  • Sandstone is made of sand. It can form from a beach or desert sand dunes.
  • Shale is made of mud and clay. It can form in moderately deep ocean water, a swamp, the bottom of a lake or some other muddy environment.
  • Conglomerate is made of gravel and sand. It may form from sediments at the bottom of a stream.
  • Limestone is often made up of the shells of marine organisms.  It can form in deeper ocean water.

Environment

Type of Sediment

Biological remains you might find there

Rock type produced

 

River

Pebbles

fish, plants

conglomerate rock

 

Beach

Sandy

shells, seaweed

sandstone

 

Bottom of the shallow ocean

Silt/mud

shells fish

shale

 

Bottom of the deep ocean

Crushed white chalk

few shells, fish

limestone

 

How to make a sedimentary rock!

 

Step 1.  Weathering.  This is basically breaking apart existing rock.  It can be mechanical or chemical. Broken pieces of  weathered rock is called regolith.

 

Step 2.  Erosion.  This is basically transporting the regolith to a new location.  This is called erosion.  Erosion is caused by running water, waves, wind, glaciers, and gravity.  Regolith transported by erosion is called sediment.

 

Step 3.  Deposit the sediment.  When running water or wind slows down, it looses energy.  The less energy there is, the easier it is for a particle to settle out.  For example, water with a lot of energy, like a fast flowing stream, will sweep away most small and medium particles like silt and sand.  Only the larger boulders and pebbles will be left behind.  A quiet location, like a lake or pond, will allow the small particles to settle out.

 

Step 4.  Burial.  New sediment gets deposited all of the time.  The sediment that gets deposited will have new sediment laid on top of it in the future.  As more and more sediment piles on top of the old sediment, the sediment on the bottom gets compacted and cemented together to make a rock.  This can take thousands to millions of years to happen.

 

 

Fossils are found in sedimentary rock.  An index fossil is a fossil that can be used to find the relative age of a rock.  Suppose that you find some shale in England and California that have the same index fossil in them.  You know that both shale layers are about the same age. 

 

To be an index fossil, a fossil must :

  • Have existed for a short time
  • Have existed in many places on the planet (wide range)
  • Be unique
  • Must be plentiful (a lot of these fossils can be found)

 

 

 

A trace fossil is evidence in a rock that a living thing had once been there.  Footprints, trails, burrows and even fossilized dino droppings would be examples of trace fossils. 

 

 

To find the actual date of a rock, you need to use radioactive dating.  Radioactive dating involves the decay of radioactive elements.  Some atoms, like uranium and carbon, have isotopes that are unstable and decay into a new type of element.  Scientist use the half- life of the radioactive element to determine its age.  A half-life is the amount of time it takes for one half of the radioactive elements to decay into the new element. 

Carbon 14, for example, has a half life of about 5800 years.  So if you start with 100 atoms of Carbon 14, you will have 50 atoms of Carbon 14 in 5800 years. 

 

To find the age of a rock or fossil, you need to figure out how many half lives have gone by.

 

Suppose you found a bone that has twelve Carbon 14 atoms in it.  You know that it originally had 100 atoms of Carbon 14.  How old is the rock?

 

Well, after one half-life (5800 years) you would have 50 atoms. After another half-life (another 5800 years) you would go down to 25.  The third half life would take you to 12.  So three half-lives have gone by, each one taking 5800 years.

 

So 5800 + 5800 + 5800 =  17,400.  The rock is 17,400 years old.