Paleobotany

Types of Rocks

Rocks are the major units of the earth’s crust. A rock is a compositional unit which is an aggregate of minerals. Geologists study these rocks under the branch of science known as Petrology.

A thorough knowledge of the various types of rocks formed due to various geological phenomena is helpful in paleobotanical exploration. All types of rocks are naturally occurring materials made of inorganic compounds with a definite chemical composition. Depending on the mode of their origin they can be classified into three main categories,

Igneous
Metamorphic
Sedimentary

Igneous Rocks

Igneous rocks owe their origin to volcanic activity. They are formed by the cooling and solidification of material that was originally hot and molten. The hot fluid material is known as magma which ranges in temperature from 1000-2000 degrees Fahrenheit, when still within the earth’s crust. It is more commonly known as lava after it erupts on the earth’s surface.

Igneous rocks, also termed primary rocks, are classified based on their texture and mineral composition. The texture of the rocks depends on the rate at which the magma cools down. Slow cooling of the lava results in the formation of rocks with a granite-like texture, whereas rapid cooling results in fine-grained rocks as in seen in basalts.

Igneous rocks are least important for Paleobotany and are referred to as non-fossilised rocks as fossils are never preserved within them. However, if these rocks are subjected to denudation and erosion, they may promote the formation of sedimentary rocks.

Sedimentary Types of Rocks

The ultimate result of the geological process of sedimentation which involves the settling of rock material, consolidation, and compaction, is the formation of sedimentary rocks. In terms of volume, sedimentary rocks are 1/10 as abundant as igneous rocks in the earth’s crust. However, in terms of surface exposed rocks, sedimentary rocks cover about ¾ of the land surface.

The process of sedimentation which results in the formation of sedimentary rocks involves two important stages- lithification and cementation.

In lithification, rock-forming sedimentation is altered from a loose or unconsolidated into a consolidated state. On the other hand in cementation, the individual sedimentary particles are cemented together by materials such as fine clay particles, calcite particles, iron oxides, etc.

Based on their mode of origin, sedimentary rocks, also termed secondary rocks, are classified into three types of rocks.

Clastic sedimentary rocks
Chemical precipitate sedimentary rocks
Organic sedimentary rocks

Clastic Sedimentary Rocks

These are composed of broken fragments of pre-existing rocks usually deposited after being transported from one locality to another. These rocks are further classified according to the size of individual rock particles. The categories of such rocks in the descending order of their size of constituents are,

CATEGORY	CHIEF COMPONENT	SIZE OF INDIVIDUAL ROCK PARTICLE
Conglomerate	Rounded pebbles	> 2mm
Breccia	Angular pebbles	> 2mm
Sandstone	Sand grains	1/16 – 2 mm
Shale	Silt or clay	< 1/16 – 2mm

Classification of Clastic sedimentary rocks

Chemical Precipitate Sedimentary Rocks

These rocks are formed due to the accumulation of solid chemical particles precipitated from pre-existing rock chemicals. Chemical sedimentary rocks are formed in various ways.

Certain chemicals present in the rocks dissolve in water and form solutions. These chemicals precipitate from the solution giving rise to the chemical precipitate sedimentary rocks.
These rocks are also formed from seawater as a result of evaporation.
Another way of chemical precipitate sedimentary rock formation is the deposition of chemicals such as calcium carbonate and calcium magnesium carbonates derived from plants and animals.
Limestone is a chemical precipitate sedimentary rock which is highly fossiliferous.

Organic Sedimentary Rocks

These rocks are composed largely or completely of the remains of once-living plants or animals. They are highly fossiliferous and also economically important due to the various chemicals preserved in them. Three types are usually recognized in these types of rocks.

Peat- composed of slightly altered plant fragments, bound by clay particles. Geologically speaking, peat is of recent origin.
Lignite- also known as brown coal, is geologically older than peat. Here, the plant fragments are greatly altered in lignite.
Coal- is the oldest sedimentary rock composed and above the other two types of rocks. It consists of highly altered plant material mixed with clay.

Metamorphic Rocks

The metamorphic type of rocks are formed due to metamorphism which means a change in form. Both igneous and sedimentary rocks undergo metamorphism. In this process alteration of the original rock takes place under the extreme conditions of temperature, pressure, and chemically active fluids associated with depths of several thousand meters inside the earth.

The classification of metamorphic rocks is based on their composition and texture. Based on the texture, there are two types of rocks- foliated such as slate and schists, and non-foliated such as quartzites.

Metamorphic rocks are essentially non-fossiliferous. However, if sedimentary rocks rich in fossils undergo metamorphosis, fossils remain very rarely, in a highly distorted condition. The metamorphic equivalents of some common sedimentary rocks are listed below.