Earth and Life Science

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Igneous Environments and Igneous Rocks

The environment in which magma completely solidifies to form a rock determines:

- The type of rock

- The appearance of the rock as seen in its texture

- The type of rock body.

In general there are two environments to consider:

The intrusive or plutonic environment is below the surface of the earth. This environment is characterized by higher temperatures which result in slow cooling of the magma. Intrusive or plutonic igneous rocks form here.

Where magma erupts on the surface of the earth, temperatures are lower and cooling of the magma takes place much more rapidly. This is the extrusive or volcanic environment and results in extrusive or volcanic igneous rocks.

Extrusive Environments

When magmas reach the surface of the Earth they erupt from a vent called a volcano. They may erupt explosively or non-explosively.

- Non-explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas and sometimes rhyolitic magma).

- Usually begin with fire fountains due to release of dissolved gases

- Produce lava flows on surface

- Produce Pillow lavas if erupted beneath water

- Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas).

- Expansion of gas bubbles is resisted by high viscosity of magma - results in building of pressure

- High pressure in gas bubbles causes the bubbles to burst when reaching the low pressure at the Earth's surface.

- Bursting of bubbles fragments the magma into pyroclasts and tephra (ash).

- Cloud of gas and tephra rises above volcano to produce an eruption column that can rise up to 45 km into the atmosphere.

Tephra that falls from the eruption column produces a tephra fall deposit.

If eruption column collapses a pyroclastic flow may occur, wherein gas and tephra rush down the flanks of the volcano at high speed. This is the most dangerous type of volcanic eruption. The deposits that are produced are called ignimbrites.

Intrusive Environments

Magma that cools at depth form bodies of rocks called intrusive bodies or plutonic bodies called plutons, from Greek god of the underworld - Pluto. When magma intrudes it usually affects the surrounding rock and is also affected by the surrounding rock. It may metamorphose the surrounding rocks or cause hydrothermal alteration. The magma itself may also cool rapidly near the contact with the surrounding rock and thus show a chilled margin next to the contact. [pic 8]

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It may also incorporate pieces of the surrounding rocks without melting them. These incorporated pieces are called xenoliths (foreign rocks).

Magma intrudes by injection into fractures in the rock and expanding the fractures. The may also move by a process called stoping, wherein bocks are loosened by magma at the top of the magma body with these blocks then sinking through the magma to accumulate on the floor of the magma body.

Classification of Igneous Rocks

Igneous rocks are classified on the basis of texture and chemical composition, usually as reflected in the minerals that from due to crystallization. You will explore the classification of igneous rocks in the laboratory portion of this course.

Extrusive/Volcanic Rocks

Basalts, Andesites, and Rhyolites are all types of volcanic rock distinguished on the basis of their mineral assemblage and chemical compostion. These rocks tend to be fine grained to glassy or porphyritic. Depending on conditions present during eruption and cooling, any of these rock types may form one of the following types of volcanic rocks.

- Obsidian - dark colored volcanic glass showing concoidal fracture and few to no crystals. Usually rhyolitic .

- Pumice - light colored and light weight rock consisting of mostly holes (vesicles) that were once occupied by gas, Usually rhyolitic or andesitic.

- Vesicular rock - rock filled with holes (like Swiss cheese) or vesicles that were once occupied by gas. Usually basaltic and andesitic.

- If vesicles in a vesicular basalt are later filled by precipitation of calcite or quartz, the fillings are termed amygdules and the basalt is termed an amygdularl basalt.

- Pyroclasts = hot, broken fragments. Result from explosively ripping apart of magma. Loose assemblages of pyroclasts called tephra. Depending on size, tephra can be classified as bombs. lapilli, or ash.

- Rock formed by accumulation and cementation of tephra called a pyroclastic rock or tuff. Welding, compaction cause tephra (loose material) to be converted in pyroclastic rock.

Intrusive/Plutonic Igneous Rocks

Shallow intrusions like dikes and sills are usually fine grained and sometimes porphritic because cooling rates are similar to those of extrusive rocks. Classification is similar to the classification for volcanic/extrusive rocks. Coarse grained rocks, formed at deeper levels in the earth include gabbros, diorites, and granites. Note that these are chemically equivalent to basalts, andesites, and rhyolites, but may have different minerals or different proportions of mineral because their crystallization history is not interrupted as it might be for extrusive rocks .

Pegmatites are very coarse grained igneous rocks consisting mostly of quartz and feldspar as well as some more exotic minerals like tourmaline, lepidolite, muscovite. These usually form dikes related to granitic plutons.

Sedimentary Rocks

Sedimentary rocks are derive from sediments which is a loose accumulation of unconsolidated fragment. After the fragment accumulate the material above compacts the sediments, expelling water and reducing pore space. Cementation if silica, calcium,