The Geology of Stone An excerpt from the Dimension Stone Design Manual, Version VIII (May 2016) Produced and Published by the Marble Institute of America 380 East Lorain St. Oberlin, Ohio 44074 Telephone: 440-250-9222 Fax: 440-774-9222 www.naturalstoneinstitute.org © 2016 All rights reserved. No part of this document may be reproduced or transmitted in any form or by means electronic or mechanical, including photocopy, recording, or by an information storage and retrieval system, without written permission from the Natural Stone Institute. The geology of 2.0 STONE CATEGORIES Stone 2.1 Three general rock or stone categories are recognized according to their mode of 1.0 INTRODUCTION origin. This is a genetic classification, and it not only states how and under what general 1.1 Earth is geologically classified as a “stony conditions a stone was formed, but also implies planet,” as it is entirely stone (rock) of various a general compositional range. The basic stone mineral compositions and forms, excluding its groups are: water and atmosphere. 2.1.1 Igneous rock is formed by 1.2 Earth scientists prefer the term “rock,”1 solidification (cooling) or, in some cases, by while the commercial stone industry, prefers solid-state transformation3 of molten or semi- the term “stone”.2 Both words are correct in molten material in the Earth’s upper mantle or their respective frame of reference, and for crust into crystalline rock generally consisting practical purposes, interchangeable. Every of silicates (compounds with SiO4) and some type of rock or stone is composed of one or dark-colored accessory minerals such as iron more minerals. For those who work with oxides or other iron- and magnesium-bearing dimension stone, it is important to know these silicate minerals. Example: granite. minerals—there are about 25 common minerals—that make up the bulk of all 2.1.2 Sedimentary rock can be: dimension stone commonly used. These 25 minerals are easy to learn to recognize, and this 2.1.2.1 Detrital sedimentary stone, basic knowledge in turn helps to identify the which is the naturally cemented accumulation many varieties of natural stone. of solid granular materials or particles derived from both mechanical and chemical weathering 1.3 Identifying the minerals in a particular of any existing rock. Examples include type of stone is important because the limestone, shale, sandstone, and properties and behavior of stone are the sum conglomerate. total of the properties and behavior of the minerals found in the stone. Knowing 2.1.2.2 Chemical sedimentary stones, something about the mode of formation or the precipitates of chemicals like salt that are “genesis” of a stone further aids predicting the the dissolved weathering products of any behavior of a stone. Thus, performance existing rocks. Chemical weathering yields questions can be knowledgeably approached, some soluble salts, and examples of the such as how a particular stone might behave in resulting stones include onyx (CaCO3), a given application. Stone industry limestone (CaCO3), dolomite (CaMg(CO3) 2), professionals need this information to reduce alabaster (CaSO4), some types of travertine waste and avoid costly mistakes. (CaCO3 + SiO2) and common table salt (NaCl). Onyx is actually precipitated in caverns and travertine is a precipitate deposited 1 The Glossary of Geology, 4th edition, 1997, Robert L. 3 A solid-state transformation is an atomic or molecular- Bates and Julia A. Jackson, eds., American Geological level process by which a compound changes from one Institute (AGI), defines rock as “a mixture of one or crystalline solid to another new crystalline solid– more minerals.” without going through an intervening liquid and/or 2 The Glossary of Geology (ibid) defines stone as: “A gaseous state. This process occurs under conditions of general term for rock that is used in construction, very high pressure, temperature, and chemistry either crushed for use as aggregate or cut into shaped through time at depths in the crust of several to tens of blocks as dimension stone.” kilometers. 2016 Marble Institute of America The Geology of Stone • Page 1-1 around freshwater springs in shallow marine rigid outermost rock layer, which varies from (salty) water. Alabaster is usually a slightly about 3 to 25 miles thick. metamorphosed anhydrite or calcium sulfate. 3.2 The underlying mantle is about 1,620 2.1.2.3 Many other chemical precipitates are miles thick and can be is envisioned as two of no use as dimension stone due to solubility parts: a weak upper mantle of about 190 to and softness, but may be much more valuable 440 miles of rock, mostly uniform in as chemical feedstock, table salt, or fertilizer. composition and capable of flow, i.e., not quite molten. Below it, the lower mantle is a 2.1.3 Metamorphic4 rock is formed denser, “mushy” or partially molten material from any pre-existing rock type in the Earth’s somewhat different in composition from the crust under variable conditions of high upper mantle. pressures, high temperature, chemistry, and time. The process produces mechanically 3.3 Deep in its center, Earth has a solid, deformed stone and chemically alters the iron-rich inner core about 760 miles thick, mineral assemblages of the parent stone. The and a molten metallic outer core about 1,420 new mineral suite may be a different or the miles thick.5 As the inner core is under same chemical composition as the parent, but tremendous pressure, it is hard to conceive as newly formed crystals. Examples of what its “molten” physical state is really like. metamorphic stones include marble, slate, schist, and gneiss. 3.4 Ocean basins cover approximately 70% of Earth’s surface and are underlaid by 2.1.3.1 Earth scientists have assigned oceanic-type crust, while a thicker continental- hundreds of names for rocks in each of the type crust, being lighter in weight, floats on genetic groups, and for minerals or stones older, denser, oceanic crust. Continents resulting from different processes of formation comprise the remaining 30% of Earth’s and slightly different chemical makeup. The surface. many technical names are confusing; the few names given in this chapter will suffice for most 3.5 The chart on the following page commercial purposes. When legal questions contrasts characteristics of the two major kinds arise, it may be appropriate to consult a of rock in the lithosphere (see next page): professional earth scientist about stone types and nomenclature of a particular stone. 3.0 STONE FORMATION 3.1 Igneous Stone (Oceanic and Continental Stone). The Earth’s crust is composed of two general types of stone: oceanic and continental. Each has distinct characteristics, mineral and chemical compositions, colors, specific gravities, and behaviors. Most, but not all, commercial stone is quarried from continental rock. Earth’s lithosphere, or rocky crust, is the cool and 4 Metamorphic is a word derived from Greek that 5 Since deep Earth materials cannot be physically literally means to change in form. examined, the above information is known through geophysical studies of earthquakes and vibrations from atomic bomb tests. Page 1-2 • The Geology of Stone 2016 Marble Institute of America Oceanic Stone Continental Si, K, Na, etc., to low Fe, Mg, Ca and high Si, Stone K and Na. Dark in color Light in color Comp: High Iron High silica (SiO2, 3.8 Andesite, named after the Andes (Fe), Magnesium ≈70%), Potassium Mountains of South America, is a lava stone (Mg), Calcium (Ca) (K), Sodium (Na) with an intermediate composition at about the and Aluminum (Al) middle of the element spectrum. It is gray and Low: SiO2 Low: Fe, Mg, Ca has a medium specific gravity. Andesite flows (<50%), K, Na & well or poorly depending on its silica content, Al and may contain all of the following elements Heavy: Specific Not heavy: Specific in its variable composition: Si, Fe, Mg, Na, Ca, gravity ≈2.9 Gravity ≈2.6+ K and Al. It is known simply as an Melting “intermediate” or “Andean-type” stone. temperatures: High Low: ≈800°+ C. (1000-1500° C.) 3.9 Plate Tectonics. According to the Viscosity of Lava: theory of plate tectonics, the lithosphere of the Very runny – flows Very stiff – Earth is divided into large crustal plates that like water like honey in winter have continuously separated or collided with Kind of eruption: each other over millions of years, always Quiet like Hawaii Explosive like Mt. moving and forming new and destroying old St. Helens crustal material in the process. Almost all of Scientific name of the ocean floor is basaltic or volcanic rock lava type: Basaltic Granitic created by plate movement called “sea-floor Scientific code spreading,” initiated from mid-oceanic ridges. word: Mafic6 Felsic7 These linear ridges or rifts are the spreading centers between undersea crustal plates that 3.6 In each case, it is the elemental content allow molten rock to emerge from the mantle of the major minerals that make the sharply through volcanic action. Conversely, when different characteristics shown in the chart. plates collide, the far edge of one oceanic floor The generalizations of composition in the chart plate is thrust under the edge of the adjacent hold for most cases. Although the differences plate in a process called subduction. Subducted in specific gravity or density of the two material is reincorporated into the Earth’s classifications of stone appear to vary little, that upper mantle. apparently small difference is enough for oceanic rock to float on denser layers 3.9.1 The life-span of oceanic floor is typically underneath. Like blocks of wood in ponds, the no more than 180 million years from formation world’s continents are floating on heavier, at spreading centers to disappearance of darker crust below.