Geology or Related Geological Terms +

Externally shell-covered cephalopod: An ammonoid is characterized by strongly curved, angulated, or complex suture patterns and generally by the position of the siphuncle inside the shell differing from that of nautiloids and includes ammonites, ceratites, and goniatites.

Without form, referring to rock or other substance lacking observable texture: In class, Mr. Stone showed the students some minerals, some of which were amorphous that were lacking a real crystalline shape.

Four-legged vertebrate (tetrapod) that reproduces only in an aquatic or very moist environment: An amphibian,breathes by means of gills during at least early life, and develops lungs for air breathing.

Three examples of amphibians are frogs, salamanders, and toads.

Ferromagnesian silicate mineral: Amphibole is a common constituent of igneous and metamorphic rocks.

A class of marine mollusks, generally called chitons: The members of the Amphineura are characterized by large flat feet used in crawling, and their dorsal sides are protected by jointed transverse plates.

Extrusive igneous rock containing many gas-formed vesicles more or less filled by secondary minerals; such as, quartz, calcite, zeolites, etc: Amygdaloids is a basaltic rock with little cavities incorporating nodules of minerals, particularly quartz, calcites, zeolites, and agates.

Individual mineral-bearing vesicle of an amygdaloid: A amygdule is a little gas bubble within an igneous rock that contains zeolite, quartz, or calcite minerals.

Pertaining to organisms; such as, many bacteria, that are able to live in an environment that lacks free oxygen: Such anaerobic one-celled organisms can thrive in an environment without oxygen, like in injured tissues of people.

Branching repeatedly with reunion of the branches to form a netlike pattern: Mushroom gills are considered to be anastomosing because they are interlinked with veins.

Aluminum silicate, a white to pinkish mineral crystallizing as square-based or rhombus-based prisms Andalusite is common in metamorphic rocks formed from clay sediments under low pressure condition.

Andalusite, kyanite, and silimanite are all polymorphs (a compound that has several crystalline forms) of aluminum silicate.

Extrusive igneous rock distinguished by content of silica lower than in rhyolites and especially by the nature of its plagioclase feldspar: Andesite, contains oligoclase or feldspar, and has a dark grayish color.

A surface that separates younger strata from eroded, dipping, older strata and represents a gap in the geologic record: Angular unconformity exists when the older stratum dips at a contrasting angle, normally steeper than the older stratumm.

Angular unconformity can be explained by structural discordance of stratified rocks produced by erosion of the older beds and deposition of younger beds on the edges of the eroded rocks.

Angular unconformities signify deformation of the older strata prior to deposition of the younger ones.

They may be simulated by cross-bedding and by faulting, distinction of which from angular unconformities generally is recognized easily by various criteria.

Rock composed mainly of anhydrite: Anhydristone is also called "anhydrock", a much less suitable name, because "stone" rather than "rock" is used by custom in nomenclature of many lithified sedimentary deposits, as sandstone, limestone, etc.

An anhydrous sulphate of lime that differs from gypsum because it does not contain water (whence the name): Anhydrite is a mineral of a white or a slightly bluish color, usually massive. Anhydrite is a mineral consisting of anhydrous calcium sulfate, commonly formed as massive or laminated evaporites; which may be altered by hydration to gypsum, as in weathering.

A compact, granular sedimentary rock harder than gypsum: Anhydrite evaporite is composed essentially of calcium sulfate, resembles marble, and deposited by evaporation.