Sedimentary rocks are formed at or close to the earth’s surface. They are deposited in layers or beds, often horizontally. Bed: A bed is defined as a layer of rock separated from the layer above and below by a bedding plane and represents a single unbroken episode of sediment accumulation. Beds vary in thickness from 1cm to many tens of metres 1. On the picture above label a bed. 2. What is the purpose of the coin in the picture? Most sedimentary rocks are formed from clasts produced as a result of weathering and erosion. Clasts can range in size from boulders to clay and include solid fragments called pyroclasts that have been ejected from volcanoes. Sedimentary Rocks Sedimentary Rocks
Transcript
Sedimentary rocks are formed at or close to the earth’s surface.
They are deposited in layers or beds, often horizontally.
Bed: A bed is defined as a layer of rock separated from the layer above and below by a bedding plane and represents a single unbroken episode of sediment accumulation. Beds vary in thickness from 1cm to many tens of metres
1. On the picture above label a bed.2. What is the purpose of the coin in the picture?
Most sedimentary rocks are formed from clasts produced as a result of weathering and erosion. Clasts can range in size from boulders to clay and include solid fragments called pyroclasts that have been ejected from volcanoes.
Sedimentary rocks can be grouped in three ways according to the mode of origin. They can be: mechanically formed, chemically formed or biologically formed.
Clast: A clast is a rock fragment or grain resulting from the breakdown of larger rocks.
Sedimentary RocksSedimentary Rocks
3. As we discuss the three groups of sedimentary rocks, make notes below: Mechanically formed:
Chemically formed:
Biologically formed:
Classification
Classification of sedimentary rocks involves handling them in the field, in hand specimens, through photos and thin sections.
The three groups can be classified more precisely by using the following:
Grain size Grain shape Mineral composition Degree of sorting Fossil content
Texture: refers to the interrelationship of grains in a rock. It includes grain size, sorting, roundness and shape of sedimentary grains.
Grain size
Accepted accuracy:
Coarse >2mmMedium 0.0625mm – 2mmFine <0.0625mm
4, Match the grain size to the sediment name
>2mm Sand – coarse
0.125mm Silt
0.039mm Gravel, pebbles, cobbles, boulders
1mm Sand – very coarse
0.25mm Sand – very fine
0.0625mm Clay
0.25mm Sand – medium
2mm Sand - fine
Sorting
Sorting provides information about how sediments were transported.
5. Label the below diagrams with the following labelsa. Moderately sortedb. Very well sortedc. Poorly sortedd. Well sortede. Very poorly sorted
Roundness and Sphericity
Individual clasts can be assigned to one of six classes based on visual observation of the clasts in the rock.
This can be subjective as one person’s subangular could be another person’s subrounded.
Roundness: defines the relationship of the outline of a grain to a circle. A grain with few sharp corners has a high degree of roundness.
Shape: defines the relationship of a grain to a sphere, rod, disc or blade.
Identifying Sedimentary Rocks
You are going to be investigating and attempting to name a selection of sedimentary rocks.
For each rock around the lab you need to record the:
Colour Grain size Grain shape Degree of sorting Minerals present (if you are able to identify any) Reaction to acid
You also need to sketch each rock. There is a key to help you with some of the identification.
Plasticity: the ability of a material to permanently change shape without fracturing.
