Sedimentary rocks form in a variety of environments on Earths surface.
Geologist study modern environments to understand how rocks form today and to use these data to better interpret ancient environments as preserved in the rock record
We can read these rocks to decipher past climates, reconstruct former landscapes, and predict where to search for oil and gas resources
This lesson has two learning objectives, we will describe the three major classes of sedimentary rocks, discuss their formation and explain where they are forming today on or near the Earths surface.
There are three classes of sedimentary rocks, clastic, chemical and biochemical sedimentary rocks
We can differentiate them on the basis of the materials that compose the rock and the processes by which they form.
Lets look at the first category, the clastic sedimentary rocks.
There are three steps involved in the formation of clastic sedimentary rocks
clastic sediments are composed of fragment of rock. In fact a clast is a technical word geologist use to describe a rock fragment.
These fragments weathered from pre-existing rocks to form sediment of various sizes
this sediment is then transported away from its source area and finally is deposited and converted to form a new rock through lithification
Rocks break down on Earths surface to form sediment
this sediment may be composed of the rocks constituent minerals, which may then be physically or chemically broken up further to form even smaller grains or other minerals.
We can see the individual feldspar minerals weathering out of this igneous rock.
and notice the grains of sediment at the base.
now we need to erode it or transport it away from its source.
sediment is eroded by transportation agents such as streams, for example,
this river in Alaska is carrying a high sediment load as reflected by the muddy color of the water
and the sand and gravel bars in the stream channel
sediment is also eroded by the winds. Much of the topsoil of western Oklahoma and northern Texas was blown away during the dust bowl era of the 1930s/
And we see similar dust storms known as haboobs today in drought stricken western states.
Elsewhere, dust and sediment from the Saharan desert may be transported across the Atlantic Ocean to Central and North America
Finally, sediment of all sizes can be transported by glaciers. For example, the dark surface on the Reid Glacier in Alaska
reflects the sediment that is exposed as the ice melts.
if we look closely at the glacier we can see layers of sediment encased in the ice
So sediment is first weathered from rocks during the breakdown phase
next it is eroded and transported and finally it is dumped or deposited in some location.
sediment is deposited when rivers enter the ocean or when wind velocity decreases or when glaciers melt.
As more and more sediment builds up, the material at the base of the pile is compacted.
Compaction squeezes out spaces between grains and pushes the grains closer together.
Finally chemicals precipitate out of water, to cement the grains together and form a rock.
Each size of clastic sediment produces a different type of sedimentary rock.
Sometimes the length between sediment and rock are obvious. Such as the lithification of sand to form sandstone.
Other times the names of the sediments and the resulting sedimentary rock are quite different.
For example, conglomerate forms from rounded gravels and clay sized particles are consolidated to form shale.
the second major category of sedimentary rocks are the chemical sedimentary rocks.
Imagine that you left a pan of salted water on the stove for long enough
the water would evaporate away leaving the salt behind
essentially the same things happens in nature as water evaporates, leaving any dissolved mineral behind.
For example, ancient Lake Bonneville covered much of Utah 10,000 years ago
Most of the lake dried up, leaving the flat lake bed covered in a layer of salt.
Most chemical sedimentary rocks, form as a result of precipitation from seawater.
But minerals are also dissolved in lesser concentrations in groundwater, freshwater lakes and rivers
and are also concentrated in hot waters associated with thermal features such as hot springs and gueysers.
Travertine a type of limestone, forms when minerals precipitate in places where warm chemical rich waters come to the surface
Biochemical sedimentary rocks also result from precipitation, but this precipitation occurs due to the actions of organisms.
Typically, marine organisms, think shellfish for example. They form their shells by causing minerals to precipitate out of seawater.
The same process can happen for microscopic marine organisms like coccolithophores.
Coccolithophores are composed of tiny plates known as coccoliths
Even though you couldn't see one of these with your naked eye if you get enough of them together you can create spectacular chalk cliffs.
such as those along the southeast coast of England
Other marine organisms, such as corals, clams and oysters also build shells
that eventually form biochemical sedimentary rocks.
Plants can also end up as rocks. For example, the vegetation in tropical swamps can be compacted and compressed to form coal.
Finally, we can summarize what we have learned today to recognize that different types of sedimentary rocks leave clues to past environments on Earth.
Clastic sedimentary rocks such as conglomerates, shale and sandstone can form in environments such as mountain streams, swamps, deltas, deserts and beaches.
Chemical sedimentary rocks are formed in environments such as lagoons and groundwater systems.
Biochemical sedimentary rocks such as limestone or chalk form in marine environments, while coal indicates the former presence of tropical wetland conditions.
We had two learning objectives for todays lesson
How confident are you that you could complete both of these tasks?
Okay, that is it for us...
we're off to use some dead coccolithophores to write some stuff on chalk boards.