Introduction to Geology Chapter 2 - Basic Geologic Principles The science of geology is founded on basic principles that are useful for making observations about the world around us. This chapter presents a mix of information that is essential fundamental to all following chapters.
This chapter is an introduction to rocks and minerals, and the rock cycle. Basic chemistry is important to all sciences, especially geology! Everything around us is made of chemical compounds that have testable and identifying characteristics, allowing them to be classified, and their age determined. This also applies to rocks, minerals, and derivative materials such as sediments and soil. The chemical composition of Earth's crust has similarities with other stony planets, with silicate-rich rocks being dominant in most locations on the surface.
In addition, basic geologic principles can be applied to resolving the order of events leading to the formation of rocks and landscape features. This section presents many basic concepts that are universal to all physical sciences. Click on thumbnail images for a larger view. Layered rocks in a sea cliff in Encinitas, CA with an angular unconformity.
What are "rocks" and "minerals" - explain the differences. Describe essential concepts of chemistry related to earth materials. What is the chemical and mineral composition of the Earth's crust? List some common silicate and nonsilicate minerals. Describe and illustrate the "rock cycle" as it relates to processes and products. Describe basic geologic principles for interpreting landscape forming processes. How are the ages of rocks determined? Cross Sections - interpretations of vertical views of geologic features below the surface.
Keywords and Essential Concepts 1. Define what rocks and minerals are, and their significance. What is a mineral? A mineral is a naturally occurring, inorganic never living solid with a definite internal arrangement of atoms crystal structure and a chemical formula that only varies over a limited range that does not alter the crystal structure.
On Earth, more than 4, minerals have been identified, however, of those fewer than 2 dozen are common minerals in Earth's physical environment Figure shows common rock-forming minerals.
In contrast, minerals considered "gems" are, mostly, exceedingly rare. What is the difference between a rock and a mineral? A rock is a relatively hard, naturally formed mineral or petrified matter; a naturally formed aggregate of mineral matter constituting a significant part of the earth's crust.
Stone is another common term used to describe rock. Rocks consist of one or more minerals. Figure shows how minerals can be combined to form different kinds of rocks that form under different environmental conditions. The mineral composition of a rock reflects the physical environment and geologic history where a rock formed.
Rock form in a variety of geologic setting ranging from locations on or near the earth surface, deep underground, or even in outer space.
Most of the rocks we see on the surface of the planet formed by processes that happened long ago, but we can see these processes actively taking place in many places. Rapid rock formation can be seen happening such as lava cooling from a volcanic eruption in places like Hawaii or Iceland.
However, most rocks we see around us form very slowly in settings that are not visible on the land surface. Slow processes creating rocks can be inferred by observing reefs growing in the oceans, or sediments being carried by flowing water in streams or moved by waves crashing on beaches. We can see sediments being deposited, but we cannot see them turning into stone because the process may take thousand or even millions of years.
Common rock-forming minerals are the most abundant minerals found on our planet Earth. Combinations of common minerals occur in different kinds of rocks. The kind of rock depends on the geologic setting where they form: Rocks are composed of particles ranging from microscopic grains to full sized crystals and crystal grains of different kinds of minerals, and containing many different identifiable physical characteristics.
It is conceptually important that each rock has an origin in concepts of place, time, and physical and chemical conditions. Once rocks form, they are subject to change. These changes may be rapid such as a volcanic explosion or gradual, taking place over millions or billions of years, and involving movement over great distances, both at the surface or to deep within the Earth's crust below us.
Trying to explain the what, how, and when of a rock's journey is fundamental to explaining why rocks are significant to resolving questions about our Earth's history and conditions within the physical environments where we live. Serpentinite, the "State Rock" of California, is a rock composed of serpentine minerals of which there are many varieties.
Gypsum crystals from a cavern wall in Jewel Cave, South Dakota 2. Essential concepts of chemistry related to earth materials Basic concepts of chemistry are essential to understanding the physical and chemical properties of earth materials minerals, rocks, organic matter, etc.
The chemical characteristics of earth materials are reflect the environments how and where they are formed, they also determine their potential fate when exposed to chemical changes.
For instance, rocks and minerals formed deep underground may not be stable in the surface environment where they are exposed to water, air, temperature changes, and other physical and chemical conditions. All matter is made up of atoms, and atoms are made up of atomic particles electrons, protons, and neutrons - see Figure A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus.
Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, nitrogen, and oxygen. The Periodic Table is a list of known elements arrange by atomic number see Figure Of these, 92 are naturally occurring prior to development of artificial nuclear research and development.
The lightest element, hydrogen, has one proton, whereas the heaviest naturally occurring element, uranium, has 92 protons. Many elements have one or more isotopes. Isotopes are each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties. Some isotopes are not stable and ultimately break down or change in other elements.
I this case, the isotope is considered a radioactive form of an element. Many elements have both stable and radioactive isotopes. For example, the element carbon has 3 isotopes: All there isotopes have 6 protons, but have 6, 7, and 8 neutrons, respectively.
In "nature,"here are at least stable isotopes that have never been observed to decay. Of these 50 are radionuclides unstable isotopes that undergo radioactive decay. They occur among the 80 different elements that have one or more stable nuclides. With the invention of nuclear weapons, and the numerous nuclear bomb test through the s to the present, there are now many more radionuclides loose in the environment. A molecule is a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction chemical compound—a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions.
Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together in a defined spatial arrangement by chemical bonds. All minerals are chemical compounds, but by comparison relatively few compounds are naturally occurring minerals!
Note that earth materials rocks and sediments , magma molten rock , seawater in oceans, and the atmosphere are all mixtures. Structure of an atom: Three types of chemical bonds include ionic bonds, metallic bonds, and covalent bonds. The types of chemical bond influence the physical properties of the molecular compounds they form. Molecular compounds held together by ionic bonds are salts. Salts readily precipitate from and dissolve in water.
Natural salts like halite NaCl and gypsum CaSO4 are soft minerals not suitable for gems because they scratch or fracture easily, and can dissolve in water; see Figures and Metals are held together by metallic bonds. Compounds with metallic bonds transmit electricity.
Metalloids are intermediate between those of metals and solid nonmetals. Although most elements are metals all those on the left and center parts of the Periodic Table , only a few elements occur naturally in metallic form including gold, platinum, copper, iron, and mercury in liquid form. Molecular compounds held together by covalent bonds are non-metallic compounds. These materials can form crystal complexes and do not transmit electricity and tend to be durable compounds.
Most gems are non-metallic compounds. The mineral quartz SiO2 is a non-metallic crystalline compound see Figure More about Minerals is discussed in the next chapter Chapter 3. Salt crystals are held together by ionic bonds. Salt compounds dissolve in and precipitate from water.
This view shows salt crystals precipitating on a dry lakebed in Death Valley, California. Metallic bonds occur in metallic minerals like native copper and gold and metalloid minerals like magnetite and pyrite.
Most minerals are non-metallic crystalline compounds held together by covalent bonds and will not transmit electricity. Rock samples collected from around the world show that the chemical composition of the Earth's crust is not uniform, but certain elements are much more abundant than others.
Silicon and oxygen are the two most abundant elements in the crust. Therefore, silicate minerals compounds that contain some silicon and oxygen are most abundant. Currently there are about 4, known minerals of different composition and mineral arrangement. However, slightly more than a dozen are considered "common minerals. Composition of the crust. General composition of the Earth's crust.
The general terms "felsic, mafic, and ultramafic" are sometimes used to describe masses of silicate rocks.