rocks

Rock

Aggregates of one or more minerals or biochemical components.

Mineral

Naturally occurring, inorganic crystalline solid with a fixed chemical formula.

Rock Cycle

Summary of rock forming processes that occur within the Earth and on the Earth's surface.

Igneous Rocks

Rocks formed from magma that has cooled either underground (Granite) or quickly at the surface (Basalt).

Sedimentary Rocks

Rocks formed from the accumulation and lithification of sediments.

Metamorphic Rocks

Rocks formed when rocks are heated and squeezed, causing the formation of new minerals.

Magma

Molten rock underground.

Lava

Molten rock that erupts at the surface.

Viscosity

Resistance to flow of magma or lava.

Phaneritic Texture

Coarse-grained texture indicating slow cooling underground.

Aphanitic Texture

Fine-grained texture indicating fast cooling on the Earth's surface.

Porphyritic Texture

Texture composed of two different crystal sizes, indicating two stages of cooling.

Vesicular Texture

Texture with holes formed by gas bubbles in volcanic rock.

Glassy Texture

Texture of volcanic glass, lacking visible crystals.

Pyroclastic Texture

Texture formed from erupted material that solidifies before hitting the ground.

Ultra-Mafic Composition

Igneous composition with high amounts of olivine and possibly pyroxene.

Mafic Composition

Igneous composition with pyroxene and calcium feldspar.

Intermediate Composition

Igneous composition with feldspars, amphibole, and biotite mica.

Felsic Composition

Igneous composition with quartz, feldspars, muscovite mica, and amphibole.

Weathering

Process of breaking down rocks into smaller pieces.

Mechanical Weathering

Physical breaking of rocks into smaller pieces.

Chemical Weathering

Breaking down and altering rocks at the atomic level.

Mechanical Weathering

The collision, breaking, and grinding of rock by the movement of gravity, water, ice, or air.

Frost Wedging

The process of water seeping into cracks in a rock and freezing, causing the rock to break.

Thermal Expansion Weathering

The process of rocks breaking due to repeated expansion and contraction as they heat and cool.

Unloading or Exfoliation

Breakage that occurs within rocks when they cool quickly or experience extreme reduction in pressure.

Biological Mechanical Weathering

Weathering caused by plants, animals, and humans, where small roots grow and expand into fractures, splitting rocks apart.

Chemical Weathering

The process of rocks being chemically broken down by dissolution, hydrolysis, or oxidation.

Dissolution

The breaking apart of a mineral or rock into individual atoms or molecules by water.

Hydrolysis

The process in which a hydrogen atom from a water molecule replaces the cation in a mineral, altering the mineral into softer clay minerals.

Oxidation

The alteration of the valence state of a cation by oxygen atoms, typically occurring in iron-rich minerals.

Lithic Fragments

Broken pieces of parent rock, producing sediments such as silt, sand, and pebble-sized particles.

Resistant Mineral Grains

Minerals that are relatively stable at the Earth's surface and resistant to weathering processes, such as quartz.

Clay

Sediment formed by the chemical weathering of feldspar minerals, consisting of particles smaller than 1/256 mm.

Ions in Solution

Ions released during chemical weathering of non-resistant minerals, present in lakes, rivers, groundwater, and the ocean where chemical sedimentary rocks form.

Biological and Chemical Material

Weathered sediments that contain biological organisms, such as fossils and shell fragments, which can contribute to the formation of sedimentary rocks.

Transportation and Deposition of Sediment

The movement of weathering products by water, ice, or wind to a new location and the subsequent accumulation of sediment.

Grain Size

The size of carried sediment, indicating the energy or distance from the source.

Rounding

The removal of sharp edges of rock fragments and mineral grains through abrasion, indicating the amount of weathering and transportation.

Sorting

The process through which sediment grains are selected and separated according to grain size, shape, or density, indicating the constancy or variability of energy over time.

Indicators of Energy and Deposition Environment

Features such as sediment size, rounding, and sorting that provide information about the energy level and stability of the environment during deposition.

Deposition

The process of sediment being deposited when transporting agents lose energy and can no longer carry the sediment load.

Property of Rock

Characteristics of rocks, including color, texture, and fossils, that provide information about their composition and formation environment.

Terrestrial (Continental) Environment of Deposition

Environments such as alluvial fans, glacial areas, dunes, rivers, lakes, and swamps where sediment is deposited on land.

Transitional (Marine Coastlines) Environment of Deposition

Environments such as deltas, lagoons, beaches, and tidal flats where sediment is deposited at the interface between land and sea.

Marine Environment of Deposition

Environments such as shallow marine areas and deep marine abyssal plains where sediment is deposited in the ocean.

Conglomerate

Rock fragments larger than 2 mm and rounded in shape.

Breccia

Rock fragments larger than 2 mm and usually very jagged or broken looking.

Sandstone

Rock fragments ranging in size from 1/16-2 mm. Found in glacial areas, rivers, dunes, and beaches.

Silt/Mudstone or Shale

Rock fragments between 1/56-1/16 mm. Found in rivers (floodplains), lake beds, tidal flats, and deep marine. May have imprints of fossils.

Limestone

Calcite crystalline material. Found in shallow marine areas, lagoons, and some very large freshwater lakes. May contain fossils.

Chalk

Very fine ground carbonate deposits. Found in shallow marine areas and lagoons.

Chert

Microcrystalline silica-based organics. Found in deep marine areas.

Coal

Plant fragments and carbonized organic material. Found in swamps.

Shale

Rock fragments as small as or smaller than 1/256 mm.

Mudstone/Siltstone

Rock fragments between 1/56-1/16 mm.

Sandstone

Rock fragments ranging in size from 1/16-2 mm.

Arkose Sandstone

Rock fragments ranging in size from 1/16 to small pebbles.

Conglomerate

Rock fragments larger than 2 mm and rounded in shape.

Breccia

Rock fragments larger than 2 mm and usually very jagged or broken looking.

Fossil Limestone

Calcite crystalline material. May be organic or inorganic. May contain fossils. Rocks may fizz when HCl acid is applied.

Coquina

Calcareous often broken bits of shells or corals. Frequently tan and beige in color.

Chalk

Very fine ground carbonate deposits.

Chert/Flint

Microcrystalline silica-based organics.

Coal - Lignite/Bituminous

Plant fragments and carbonized organic material.

Rock Salt

Halite (sodium chloride - salt) deposits from evaporated sea water.

Rock Gypsum

Gypsum mineral (calcium sulfate) evaporated sea water.

Metamorphism

Changes in the solid state of a pre-existing rock due to increased temperature, pressure, and chemically active fluids or gases.

Agents of Change

Conditions that can change one rock into another type of rock, including increased temperature, increased pressure, and chemically active fluids or gases.

Thermal or Contact Metamorphism

Metamorphism that occurs when rocks are heated by an intrusive igneous magma source. Results in changes similar to cooking, making the rock drier, more brittle, and darker in color.

Hornfels

Rock surrounding a magma intrusion where contact occurs during thermal or contact metamorphism. Produces a unique rock with a dull matte surface look.

Lithostatic or Confining Pressure

Pressure equal in all directions experienced by rocks beneath the surface of the Earth due to the weight of overlying sediment and rock layers.

Differential Pressure

Unequal pressure on a rock, causing it to become squeezed in one direction more than another. Can result in a significant change in the appearance of a rock.

Hydrothermal Metamorphism

Metamorphism due to chemically active fluids, often hot water, that react with minerals in a rock. Can result in a change in the chemical composition of the rock.

Metasomatic or Skarns

Rocks altered by hydrothermal metamorphism, characterized by unique mineralogy due to alteration/interaction with heated water.

Protolith

The pre-existing rock from which a metamorphic rock is formed.

Index Minerals

Minerals that form under specific temperature and pressure ranges within a specific chemistry of a protolith. Used to determine the temperature/pressure of metamorphism.

Foliated

Metamorphic rocks with parallel or linear alignment of grains in an interlocking crystalline form. Most commonly aligned minerals are mica and amphiboles.

Non-Foliated

Metamorphic rocks without foliation, typically composed of one type of mineral.

Slaty

Foliation type seen in the metamorphic rock called slate, formed by low temperature and low-pressure alteration of a shale protolith.

Phyllitic

Foliation resulting from increased level of metamorphism of slates, with increased pressure and temperature. Phyllites have shiny surfaces and break more easily than slates.

Schistose

Foliation defined by the presence of flat or platy minerals, such as muscovite or biotite micas. Metamorphic rocks with this foliation pattern are called schist.

Gneissic

Foliation characterized by alternating dark and light-colored mineral bands throughout the rock. Metamorphic rocks with this foliation are called gneiss.

Quartzite

Metamorphosed sandstone composed primarily of quartz. Non-foliated texture.

Marble

Metamorphosed limestone or dolostone. Non-foliated texture. Can react with acid and make bubbles.

Metamorphic Grade

The intensity of metamorphism, determined by the amount of pressure and heat applied, which affects the size of crystals in rocks.

Quartzite

A metamorphic rock with equigranular grains of quartz and a hardness of 7, commonly used for countertops and dimension stones.

Marble

A metamorphic rock with equigranular grains of carbonates, such as calcite, commonly used for countertops, dimension stones, statues, and grave markers.

Migmatite

A rock formed when part of a high-grade metamorphic rock melts, with the liquid part being magma and the solid part still retaining its metamorphic characteristics.

Blueschist

A type of metamorphic rock that forms uniquely as the oceanic crust is subducted, characterized by its blue color due to the presence of a mineral called glaucophane.

Shock Metamorphism

A type of metamorphism that occurs when a meteor strikes the Earth, resulting in unique rock formations.

Fault Zones

Unique high-stress, low-temperature conditions along fault lines that can cause metamorphism.

Mylonites

Rocks formed when rocks become plastic due to high heat and pressure, resulting in a fine-grained texture.

Slaty

A low-grade foliated rock with a fine-grained texture that tends to split in parallel fragments, commonly found in shale and slate.

Phyllitic

A medium-grade foliated rock with a shiny appearance and abundance of very small mica grains, often found in slate and phyllite.

Schistose

A medium-grade foliated rock containing abundant large mica grains, sometimes with garnet minerals, commonly found in shale or igneous rocks.

Gneissic

A high-grade foliated rock characterized by alternating bands of light and dark minerals, formed from shale or igneous rocks.

Sandstone

A non-foliated rock with equigranular grains of quartz and a hardness of 7, commonly used for construction purposes.

Limestone or Dolostone

A non-foliated rock with equigranular grains of carbonates, such as calcite, commonly used for construction purposes.

Bituminous or Lignite Coal

A non-foliated rock containing mostly carbon, with a shiny black appearance and light weight, commonly used as a fuel source.