Primary Structures in Igneous Rocks

Engr. Sultan Ahmed Khoso Primary Structures in Igneous Rocks

. Primary Structures developed in the igneous rock during the time of cooling, crystallization and solidification of magma or lava. . Since, there are two main classifications of igneous rocks : intrusive igneous rocks and extrusive igneous rocks , therefore, Primary structures in the igneous rock may also be classified into two main divisions:

1. Intrusive or Plutonic Igneous Rock Structures

2. Extrusive Igneous Rock Structures. Intrusive Igneous Rock Structures

. Intrusive igneous rock structures formed bellow the Earth’s surface where magma is subjected to cooling and solidification above the outer core (super-heated liquid molten layer).

. Intrusive igneous rock structures are classified in two ways, based on the relationship of intrusive igneous structures to country rock (pre-existing rocks in the area).

• Some intrusion flow parallel to the bedding plane, i.e. they don’t disturb the individual layers of country and deposited with the bed or layer.

• Some intrusion move perpendicular to the bedding plane and they disturb the individual layers of country . Intrusive Igneous Rock Structures Intrusive Igneous Rock Structures

Concordant Intrusion: Concordant Intrusion are those which are parallel to the bedding plane, i.e. they don’t disturb the individual layers of country rocks but however they deposited with the bed or layer. I. Laccoliths II. Lopoliths III. Sills Discordant Intrusion: Discordant Intrusion are those which are Perpendicular to the bedding plane, i.e. they cuts across the bedding layers of country rocks. I. Dike II. Stock III. Batholith Intrusive Igneous Rock Structures

1. Laccolith Intrusion:

. Laccoliths are the intrusions that result in uplift folding of the pre existing rocks.

. They are concordant types of intrusions, occur between parallel layers of rock at relatively shallow depths.

. When the pressure of the injecting magma is high enough to deform the overlying strata.

. The low pressure of overlying strata allows the magma to dome up the overlying rock.

. The roof of the intrusion is convex upwards and the floor of the intrusion is nearly flat. Intrusive Igneous Rock Structures Intrusive Igneous Rock Structures

2. Lopolith Intrusion:

. Lopoliths are the intrusions that result in down lift folding of the pre existing rocks.

. They are concordant types of intrusions, occur between parallel layers of rock at relatively deeper depths.

. When the pressure of the injecting magma is very low and pressure of overlying strata is high.

. The floor of the intrusion is convex downwards and the roof of the intrusion is nearly flat. Intrusive Igneous Rock Structures Intrusive Igneous Rock Structures

3. Sill Intrusion:

. A sill is a horizontal or near-horizontal sheet like intrusion that follows through the bedding planes and does not cut across planes.

. Sills are similar to dykes, but are concordant , that is, they intrude parallel to the layering of the country rock. Intrusive Igneous Rock Structures Intrusive Igneous Rock Structures

4. Batholith:

. The batholith is the largest igneous intrusion, usually so large that their bottom is rarely exposed.

. Batholiths are formed deep below the surface when large masses of magma cool and solidify.

. Batholiths are believed to extend to unknown depths. 4. Stocks:

. Stocks are just the top of a largest body of batholith, that has a relatively small part of it exposed at the surface. Intrusive Igneous Rock Structures Intrusive Igneous Rock Structures

4. Dike:

. Dike is the thin sheet of the igneous rock that formed when magma moves vertically through fracture in a pre-existing rock body.

. It is normally inclined or nearly vertical tabular sheet of igneous rock body. Extrusive Igneous Rock Structures

1. Pillow Structure:

. It’s a pillow-shaped structures formed when:

lava is comparatively viscous and less mobile,

extrusion of the lava is under water, or subaqueous extrusion.

. Pillows forms when hot viscous lava is suddenly exposed to cold water. Extrusive Igneous Rock Structures Extrusive Igneous Rock Structures Extrusive Igneous Rock Structures

2. Flow Structure: . Flow structures are formed by continuous and smoothly moving lava on the sloping surface of Earth. . These structures formed when magma is less viscous and more mobile. Extrusive Igneous Rock Structures

3. Orbicular Strucutres:

. Orbicular structures are Identified by the presence of “Ball, spherical or rounded shape” in the body of an igneous rock. Extrusive Igneous Rock Structures

4. Spherulitic Structures:

. Spherulitic Structure are Identified by the presence of the Needle like Mineral Grains in igneous rock masses. Extrusive Igneous Rock Structures

5. Vesicular Structures:

. Vesicular Structure are Identified by the presence of the empty cavities in the igneous rocks. These cavities are produced due to the escape of gaseous mater from the Lava during cooling and solifification. Extrusive Igneous Rock Structures

6. Miarolitic Structures:

. In Miarolitic Structure the cavities (formed during the cooling of Lava) are filled with some (one or more) different minerals. Extrusive Igneous Rock Structures

7. Columnar Structures:

. This structure consists of columns (mostly commonly hexagonal in shape) that are separated by joints or fractures in the rock, formed when the rock contracted, most often during cooling.