Glossary beginning with A
- Ablation
- Abutment
- Acanthodians
- Albian
- Algae
- Alkalinity
- Allelopathy
- Alligator
- Alluvial Fan
- Alluvial Plain
- Alpine Tundra Ecosystem
- Altiplano
- American Marten
- Ammonite
- Amoeba
- Amphibolite
- Anadromous
- Andesite
- Anemone
- Angle of Repose
- Antelope Jackrabbit
- Anthropogenic
- Anticline
- Aperture
- Aphanitic
- Apical Meristem
- Aplite
- Aptian
- Aquifer
- Arachnid
- Arboreal
- Arch
- Archaean
- Archean Eon
- Areole
- Arete
- Argillaceous
- Argillite
- Arroyo
- Aseismic
- Ash
- Ash Cloud
- Ash Flow
- Aspen Tree
- Assemblage
- Aster
- Asthenosphere
- Autotroph
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