3.3. 5. Compile sentences using the following phrases.
EXAMPLE: Conglomerate is formed from gravel.
Sandstone |
through the sedimentation of mineral substances |
Rock salt |
from sand |
Mineral oil |
from clay minerals |
Shale |
from organic remains |
Peat and coal |
from particles of pre-existing rocks |
Igneous rocks |
by the accumulation of plant remains |
Sedimentary rocks |
from magma |
Metamorphic rocks |
from either igneous or sedimentary rocks |
3.3.6. State whether the following statements are true or false.
The rocks of the Earth’s crust are divided into two main groups.
Igneous rocks are composed of particles of pre-existing rocks.
Sedimentary rocks are stratified.
Sediments are formed by the action of glaciers.
Igneous rocks make up 75% of exposed rocks.
Conglomerates are formed as a result of the accumulation of materials caused by the destructive mechanical action of water.
Sandstones are consolidated rocks.
Clays are unconsolidated mechanical sediments.
Chemical sediments are formed by the destructive chemical action of water.
Peat and coal are the organic sediments, which are of great practical value.
Claystone was formed at the beginning of the sedimentation period and clay was formed later.
3.3.7. Answer the following questions.
What main groups of rocks do you know?
Do sedimentary rocks consist of particles of pre-existing rocks?
How sedimentary rocks are originated?
What is the most important characteristic feature of sediments?
What are cementing substances?
What are chemical sediments?
How are organic sediments formed?
TEXT 2 Occurrence of sedimentary rocks
Read the text and fulfill the after reading exercises (R.P – 6.2)
Estimates of the volumes for the major rock types include in the order of about 10% sedimentary strata, which occur as a nearly sphere relatively thin surficial covering of both continental and ocean floors. Sedimentary units are of great economic importance, for in them is gathered much of the world’s readily extractable mineral ores, including coal, petroleum, natural gas, nuclear fuels.
Sediments are accumulations of materials that have been formed by surficial processes from preexisting rocks of any type distinguishing between clastic sediments or detritus, the products of mechanical accumulation of individual grains; and chemical sediments –materials that are precipitated from inorganic solutions. As sedimentary rocks result from the following processes:
weathering of source materials;
transportation (commonly in running water);
deposition by organic or inorganic means;
lithification, that is, compaction and cementation whereby the original unconsolidated sediment is converted into a more cohesive, less porous rock.
Fig. 36. Sedimentary stages in the rock cycle
The nature of sediment depends in part on the source from which it is derived, and on the attendant weathering occurring by the altering of the rock surface; involves mechanical and chemical breakdown of the original materials, usually by interaction with the atmosphere or with water. High temperatures, elevated temperature weathering accelerates chemical sediments. The nature of sediment depends chiefly on the mode and length of transportation from the source area to the depositional site.
The manner of deposition strongly influences the nature of the resultant sediment. Rapid accumulation of ill-sorted clastic debris of several size ranges and high rainfall characteristic of the tropics promote chemical weathering, generally, result in weakly laminated or massive deposit – coarse sand; on the other hand, slow building of fine-grained particulate matter, or chemical precipitation alternating with the settling of fine clastic debris, tends to produce fine-bedded sediments such as laminated muds. The amount and rate of deposition from the transporting medium is a function of the flow velocity and the nature of the transported sediment.
Two principal types of sediment are distinguished based on the site of deposition: continental (or terrestrial) and marine. Terrestrial sediments include the products of mass movements, stream gravels, lakebeds, sand dunes and glacial deposits. Marine sediments tend to be more widespread and continuous. To this group belong shallow-water deposits of deltas, beaches, continental shelves and slopes, as well as deep-sea sediments of the ocean basin. Both chemical precipitates and clastic materials are abundant in such sediments.
The process of lithfication, whereby unconsolidated sediment is converted into a coherent rock, involves several distinct but intergradational phenomena. Compaction of a sediment due to the weight of the overlying rocks decreases pore space and results in deformation of soft mineral grains, clast rotation, flattening and squeezing out of water from the pore spaces and also produces interlocking grain boundaries. Iron oxide, silica and calcium carbonate minerals precipitate from pore solutions and cement the particles together. Reaction among the original, chemically dissimilar fragments produces new minerals that transect former grain boundaries and increase rock coherence: this recrystallization is known as diagenesis.
