Advanced Topic in Earth Science

Intrusive igneous rocks are formed from magma that cools and solidifies within the earth. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are coarse grained. The mineral grains in such rocks can generally be identified with the naked eye. Intrusive rocks can also be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, stocks, laccoliths, sills and dikes.
The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite. When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth's surface.

Coarse grained intrusive igneous rocks which form at depth within the earth are termed as abyssal; intrusive igneous rocks which form near the surface are termed hypabyssal.


Extrusive igneous rocks are formed at the Earth's surface as a result of the partial melting of rocks within the mantle and crust.
The melt, with or without suspended crystals and gas bubbles, is called magma. Magma rises because it is less dense than the rock from which it was created. When it reaches the surface, magma extruded onto the surface either beneath water or air, is called lava. Eruptions of volcanoes into air are termed subaerial whereas those occurring underneath the ocean are termed submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.

The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions:
%u2022 divergent boundary: 73%
%u2022 convergent boundary (subduction zone): 15%
%u2022 hotspot: 12%

Petroleum and natural gas are called fossil fuels because most scientists believed that it was formed from the remains of ancient sea plants and animals. When the plants and tiny sea animals died, they sank to the bottom of the oceans where they were buried by sediment and sand, which turned into sedimentary rocks. The layers of plant and animal matter and sedimentary rock continued to build until the pressure and heat from the earth turned the remains into petroleum and natural gas.


most believe that petroleum products, and natural gas come from ancient plants and animals that have died and their bodies have decomposed.diagram Note The erosion process carried these biological remains down rivers and streams onto shorelines, where they were deposited along with mud and silt. Over time, they were covered by increasing amounts of this sediment, and gradually they were compressed by the weight of the sedimentary layers. With time, the material that originally contained the biological remains became sedimentary rock. Today, these sedimentary rocks, sandstone, shale and dolomite are often where deposits of petroleum are found.


Eventually, the organic materials transform into petroleum products due to the intense pressure and heat present in the rock formations. The oil and gas migrate through the pores in the sedimentary rock, upwards to the earth's surface. If the gas reaches the surface, it is dispersed into the atmosphere. Light oils eventually evaporate also. However, most often, the petroleum products never make it as far as the surface. Many times they are trapped beneath the surface by layers of rock that have formed above the sedimentary rock layer that produced them.


The layers of rock that trap the deposits are impermeable layers that are usually shaped into domes by folding or faults. The layer that traps the gas and oil is called a cap rock, and the resulting formation is called a trap. As the gas and oil move upwards in through the permeable layers of rock, the oil and gas displace sea water that was also trapped in layers of sedimentary rock. When the oil and gas reach the trap and cease upward movement, they separate from one another. If you placed equal amounts of gas, oil and water into a glass, you would see that they naturally separate themselves according to their varying densities. It's easy to see the same sort of thing by putting vegetable oil and water in a glass. Not all of the water is separated from the petroleum, however. Between ten and fifty percent of the oil and gas accumulation contains salt water, which must be removed before the gas or oil can be used by people.

The rock cycle is a fundamental concept in geology that describes the dynamic transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. An igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments. The rock cycle is an illustration that explains how the 3 rock types are related to each other and how processes change from one type to another over time.


Transition to igneous
When rocks are pushed deep under the Earth's surface, they may melt into magma. If the conditions no longer exist for the magma to stay in its liquid state, it will cool and solidify into an igneous rock. A rock that cools within the Earth is called intrusive or plutonic and will cool very slowly, producing a coarse-grained texture. As a result of volcanic activity, magma (which is called lava when it reaches Earth's surface) may cool very rapidly while being on Earth's surface exposed to the atmosphere and are called extrusive or volcanic rocks. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian. Any of the three main types of rocks (igneous, sedimentary, and metamorphic rocks) can melt into magma and cool into igneous rocks.


Post-volcanic changes
Rock masses of igneous origin have no sooner cooled than they begin to change. The gases with which the magma is charged are slowly dissipated, lava flows often remain hot and steaming for many years. These gases attack the components of the rock and deposit new minerals in cavities and fissures. The zeolites are largely of this origin. Even before these "post-volcanic" processes have ceased, atmospheric decomposition or weathering begins as the mineral components of volcanic and igneous rocks are not stable under surface atmospheric conditions. Rain, frost, carbonic acid, oxygen and other agents operate continuously, and do not cease until the whole mass has crumbled down and most of its ingredients have been resolved into new products or carried away in aqueous solution. In the classification of rocks these secondary changes are generally considered unessential: rocks are classified and described as if they were ideally fresh, though this is rarely the case in nature.


Secondary changes
Epigenetic change (secondary processes) may be arranged under a number of headings, each of which is typical of a group of rocks or rock-forming minerals, though usually more than one of these alterations will be found in progress in the same rock. Silicification, the replacement of the minerals by crystalline or crypto-crystalline silica, is most common in felsic rocks, such as rhyolite, but is also found in serpentine, etc. Kaolinization is the decomposition of the feldspars, which are the most common minerals in igneous rocks, into kaolin (along with quartz and other clay minerals); it is best shown by granites and syenites. Serpentinization is the alteration of olivine to serpentine (with magnetite); it is typical of peridotites, but occurs in most of the mafic rocks. In uralitization secondary hornblende replaces augite; this occurs very generally in diabases; chloritization is the alteration of augite (biotite or hornblende) to chlorite, and is seen in many diabases, diorites and greenstones. Epidotization occurs also in rocks of this group, and consists in the development of epidote from biotite, hornblende, augite or plagioclase feldspar.


Transition to metamorphic
This diamond is a mineral from within an igneous or metamorphic rock that formed at high temperature and pressure.
Rocks exposed to high temperatures and/or pressures can be changed physically or chemically to form a different rock, called metamorphic. Regional metamorphism refers to the effects on large masses of rocks over a wide area, typically associated with mountain building events within orogenic belts. These rocks commonly exhibit distinct bands of differing mineralogy and colors, called foliation. Another main type of metamorphism is caused when a body of rock comes into contact with an igneous intrusion that heats up this surrounding country rock. This contact metamorphism results in a rock that is altered and re-crystallized by the extreme heat of the magma and/or by the addition of fluids from the magma that add chemicals to the surrounding rock (metasomatism). Any pre-existing type of rock can be modified by the processes of metamorphism.


Transition to Sedimentary
Rocks exposed to the atmosphere are variably unstable and subject to the processes of weathering and erosion. Weathering and erosion breaks the original rock down into smaller fragments and carries away dissolved material. This fragmented material accumulates and is buried by additional material. While an individual grain of sand is still a member of the class of rock it was formed from, a rock made up of such grains fused together is sedimentary. Sedimentary rocks can be formed from the lithification of these buried smaller fragments (clastic sedimentary rock), the accumulation and lithification of material generated by living organisms (biogenic sedimentary rock - fossils), or lithification of chemically precipitated material from a mineral bearing solution due to evaporation (precipitate sedimentary rock). Clastic rocks can be formed from fragments broken apart from larger rocks of any type, due to processes such as erosion or from organic material, like plant remains. Biogenic and precipitate rocks form from the deposition of minerals from chemicals dissolved from all other rock types.

Gravity is an agent of erosion. Rocks break apart only when a force makes them do so. Gravity is one force that helps break down rocks to smaller pieces. This never ending force works 24 hours a day, 7 days a week, 365 days a year. It has done so for millions and millions of years.

Water running down all the mountains and valleys is pulled down by this force of gravity. Wind is ultimately a result of air molecules being held to Earth's surface due to gravity. Water, wind, and ice working under the influence of gravity are the greatest sources of erosion on the surface of Earth. The effects of gravity combined with wind, water and ice create many types of geologic features.