Intrusive Rocks

Igneous rocks which form by the crystallization of magma at a depth within the Earth are called intrusive rocks. Intrusive rocks are characterized by large crystal sizes, i.e., their visual appearance shows individual crystals interlocked together to form the rock mass. The cooling of magma deep in the Earth is typically much slower than the cooling process at the surface, so larger crystals can grow. Rocks with visible crystals of roughly the same size are said to have a phaneritic texture.

A body of intrusive igneous rock that crystallizes from cooling magmas beneath the Earth’s surface is called a “pluton”. If the pluton is large, it may be called a batholith or a stock. Minor plutons include dikes and sills. If a penetrating intrusion cuts across the geological layers it is called a dike. If it runs parallel to the layers, it is called a sill. If an intrusion causes the rocks above to rise and form a dome, it is called a laccolith.

Extrusive Rocks

Igneous rocks which form by the crystallization of magma at the surface of the Earth are called extrusive rocks. They are characterized by fine-grained textures because their rapid cooling at or near the surface did not provide enough time for large crystals to grow. Rocks with this fine-grained texture are called aphanitic rocks. The most common extrusive rock is basalt.

Intrusive Igneous Rocks

Intrusive igneous rocks cool underground. Deep in the crust, magma cools slowly. Slow cooling gives crystals a chance to grow. Intrusive igneous rocks have relatively large crystals that are easy to see. Intrusive igneous rocks are also called plutonic. A pluton is an igneous rock body that forms within the crust.

Granite is the most common intrusive igneous rock. Pictured below are four types of intrusive rocks.

Extrusive Igneous Rocks

Extrusive igneous rocks form above the surface. The lava cools quickly as it pours out onto the surface (Figure below). Extrusive igneous rocks cool much more rapidly than intrusive rocks. The rapid cooling time does not allow time for large crystals to form. So igneous extrusive rocks have smaller crystals than igneous intrusive rocks. Extrusive igneous rocks are also called volcanic rocks.

Branches of geology focused on natural resources
Most geology careers involve the extraction of natural resources from the surface. This is where geologists relate rock types and landforms in a specific environment.

For example, petrology uses mineralogy and rock types to understand geological formations from drilling. In addition, they study the chemical properties and how atoms are arranged.

Soils are also considered a natural resource for agriculture production. Agronomy, edaphology and pomology are specific to soil science and how food grows or is cultivated.

PETROLOGY – How types of rocks (igneous, metamorphic, and sedimentary petrology) form in their specific environment.
MINERALOGY – How chemical and crystalline structures in minerals are composed.
GEMOLOGY – How natural and artificial gems are identified and evaluated.
CRYSTALLOGRAPHY – How atoms are arranged and bonded in crystalline solids.
SOIL SCIENCES – How soils relate as a natural resource including their formation factors, classification, physical, chemical and fertility properties.
PEDOLOGY – How soils are classified based on their biological, physical and chemical properties.
EDAPHOLOGY – How soils influence plant growth and living things.
AGRONOMY/AGROLOGY – How the field of agriculture involves science such as crop production, biotechnology and soil science.
HYDROGEOLOGY – How groundwater is transported and is distributed in the soil, rock and Earth’s crust.
POMOLOGY – How fruits grow and are cultivated.

Sedimentology understands weathering, transportation and deposition
Sedimentology looks at the processes of how sediments deposit. For example, sedimentology is concerned with erosion, weathering, transportation, and deposition of sediments.

One of the processes that understands the erosion, movement and deposition of sediments is from glaciers. Specifically, glaciology studies glaciers and how they shape the landforms.

Likewise, surficial geology examines sediments overlying bedrock such as during a glacial retreat. Finally, beneath the regolith is the intact, solid rock that bedrock geology is concerned with.

SEDIMENTOLOGY – How sand, silt and clay are deposited and the processes that act on it.
SURFICIAL GEOLOGY – How surface sediment (till, gravel, sand, clay, etc) overlying bedrock was formed such as during glacial retreat or in lakes associated in these periods.
GLACIOLOGY – How ice and glacial deposits have reconstructed landforms as well as how existing (polar) glaciers behave and are distributed.
GEOPHYSICS – How physical processes and properties relate to Earth and its surrounding space.
BEDROCK GEOLOGY – How the intact, solid rock beneath surficial sediments formed including age (stratigraphic sequences), morphology and rock properties (folds, faults, fractures).
LITHOLOGY – How rocks are classified based on their physical and chemical properties.

Topography studies land forms and their processes
Topography also plays an important in geology. Of all the branches of geology, topography examines the physical features that are distributed on the landscape.

For example, orography focuses on topographic relief and how mountains are distributed. Without plate tectonics which is a focal point in geology, mountain building would have not taken place.

Finally, hypsometry measures the height and depth of physical features from the mean sea level. Geologists use hypsometry to understand the profile of Earth and landscape evolution.

OROGRAPHY – How topographic relief in mountains are distributed in nature.
TOPOGRAPHY – How physical features (natural and artificial) are arranged on the landscape.
HYPSOMETRY – How height and depth of physical features are measured land from mean sea level.

Estimates of the tonnage and average grade of ore deposits (ore-reserve estimates) are made for various purposes. They may be made by examining engineers as a basis for placing a value on a mining property in connection with reports on behalf of owners or vendors, on the one hand, or for prospective purchasers or lessees, on the other. They are made for tax purposes and sometimes in connection with mergers of two or more companies or with litigation.

Most operating companies make periodical ore-reserve estimates, usually at least annually, to determine their ore-reserve position as a basis for controlling development and exploration and allocation of funds therefor; for determining deferred, depletion, and depreciation charges per ton; or as a basis for deciding upon operating policy—expansion or contraction of operations, capital expenditures, and the like.

The final estimate in any instance usually is a composite of estimates of different blocks or areas which often differ appreciably from each other in grade or character of ore. A continuous ore-reserve inventory, by blocks, levels, and stopes, in a mine may be required as a basis for controlling stoping operations to maintain a desired grade of output by mixing ores from different blocks.

Some mines produce two or more grades or kinds of ore, which must be mined, milled, and shipped separately; and it becomes necessary, then, to know the grade and type of ore available in each section of the mine.

Thus, iron ores are often mixed to give a desired iron, phosphorus, silica, manganese content. The copper-bearing sulfide ores of Ducktown, Tenn., chiefly valuable for their sulfur, are roasted for the manufacture of sulfuric acid; it is necessary to grade the ore from different stopes carefully, on the basis of sulfur content, to form the right mix for this purpose.

Ore-reserve estimates are based upon the results of exploration and development and analyses of the samples derived therefrom. Unless a deposit is fully developed (and even then to a lesser degree), certain assumptions have to be made regarding the continuity and grade of ore between exposed faces or drill holes that have been sampled. In making these assumptions, the engineer must interpret all available information, and the accuracy of the final results will depend largely on his experience and the soundness of his judgment. Not only must he correctly combine the assay values of the samples, but he must, interpret the geological criteria and consider the influence of structural conditions on continuity and grade of ore, the probable loss of ore in mining, the dilution with waste (which, in turn, may depend on the mining method employed or to be employed), and the cost of mining and milling, which may be an important factor in determining the minimum grade of rock that can be classed as ore.

It is apparent, then, that estimation of tonnage and grade of ore reserves is not a precise science. In some districts where the character, uniformity, and habit of the ore bodies have been learned from long mining experience, more or less arbitrary methods may be applied with considerable confidence in estimating ore extending beyond exposed faces. Thus, in Minnesota and Michigan the mining companies and the State tax commissions accept valuations of ore reserves based upon certain rules as to continuity and grade of the ore.

Ore reserve estimates include the determination of (1) tonnages of ore and (2) average grade or value per ton.