100 common geological terms with clear, concise definitions

A–C

1. Ablation – Loss of ice or snow from a glacier due to melting or evaporation.
2. Absolute Dating – Determining the actual age of a rock or fossil using radiometric methods.
3. Alluvium – Loose sediment deposited by rivers and streams.
4. Andesite – A fine-grained volcanic rock typically found in subduction zones.
5. Anticline – An upward arching fold in rock layers.
6. Aquifer – A rock layer that stores and transmits groundwater.
7. Asthenosphere – The ductile, semi-fluid layer beneath the lithosphere.
8. Atoll – A ring-shaped coral reef, island, or series of islets.
9. Basalt – A dark, fine-grained volcanic rock from rapid lava cooling.
10. Batholith – A large mass of intrusive igneous rock formed deep underground.
11. Bedding Plane – The boundary between sedimentary rock layers.
12. Biotite – A dark, iron-rich mica mineral found in many igneous and metamorphic rocks.
13. Breccia – A rock composed of angular fragments cemented together.
14. Caldera – A large crater formed by a volcanic collapse.
15. Carbonate Rock – Sedimentary rock made mostly of carbonate minerals (e.g., limestone).
16. Cataclasite – A fine-grained, fractured rock produced by faulting.
17. Cementation – The process by which sediment grains are bound together by minerals.
18. Chert – A hard, fine-grained sedimentary rock made of silica.
19. Cleavage – The tendency of minerals to break along flat surfaces.
20. Clastic – Rock made from fragments of pre-existing rocks.

D–F

21. Dendritic Drainage – A tree-like river pattern common in horizontal rock layers.
22. Dike – A sheet of intrusive igneous rock cutting across rock layers.
23. Dip – The angle at which a rock layer or fault is inclined from the horizontal.
24. Dolomite – A carbonate mineral and rock similar to limestone but containing magnesium.
25. Drumlin – An elongated hill formed by glacial ice.
26. Earthquake – A sudden ground shaking due to fault movement or volcanic activity.
27. Eclogite – A high-pressure metamorphic rock formed deep within subduction zones.
28. Erosion – The removal of rock or soil by wind, water, or ice.
29. Evaporite – A sedimentary rock formed by the evaporation of water.
30. Extrusive Rock – Igneous rock formed from lava cooling at the Earth’s surface.
31. Facies – A body of rock with specific characteristics indicating a particular environment.
32. Fault – A fracture in rock with movement on either side.
33. Feldspar – A group of common silicate minerals in igneous and metamorphic rocks.
34. Felsic – Silica-rich rocks or minerals, usually light in color.
35. Foliation – Planar arrangement of minerals in metamorphic rocks.
36. Fold – Bends or curves in layered rock.
37. Formation – A mappable body of rock with distinct characteristics.
38. Fossil – Preserved remains or impressions of ancient organisms.
39. Fracture – A break in rock without movement along the break.
40. Fumarole – A volcanic vent releasing steam and gases.

G–I

41. Gabbro – A coarse-grained mafic intrusive igneous rock.
42. Garnet – A common metamorphic mineral often used as a geothermometer.
43. Geomorphology – The study of landforms and their processes.
44. Geosyncline – A large-scale depression filled with sediment and later uplifted.
45. Glacier – A large, slow-moving mass of ice.
46. Granite – A coarse-grained, light-colored igneous rock rich in quartz and feldspar.
47. Gravel – Rock fragments greater than 2 mm in diameter.
48. Greenstone – Metamorphosed basaltic rock, typically Archean in age.
49. Groundwater – Water stored underground in pore spaces of rocks.
50. Gneiss – A banded, high-grade metamorphic rock.
51. Hanging Wall – The block of rock above a dipping fault.
52. Hornfels – A hard, fine-grained metamorphic rock formed by contact metamorphism.
53. Hot Spot – A volcanic region fed by a mantle plume, independent of plate boundaries.
54. Hydrothermal – Related to hot, mineral-rich fluids, often responsible for ore deposits.
55. Igneous Rock – Rock formed from solidified magma or lava.
56. Index Fossil – A fossil used to define and identify geologic periods.
57. Inlier – Older rock surrounded by younger formations.
58. Isostasy – The equilibrium between Earth’s crust and mantle.

J–L

59. Joint – A crack in rock without movement.
60. Kaolinite – A clay mineral formed from weathered feldspar.
61. Karst – A landscape formed from dissolution of limestone, featuring sinkholes and caves.
62. Laccolith – A dome-shaped intrusive body with flat base and arched top.
63. Lahar – A volcanic mudflow of ash and debris.
64. Laminae – Thin sedimentary layers less than 1 cm thick.
65. Laterite – A red, iron-rich soil formed in tropical areas.
66. Lava – Molten rock erupted at the surface.
67. Leaching – Removal of dissolved ions from soil or rock by water.
68. Lenticular – Lens-shaped body of sediment or rock.
69. Lithification – The process turning sediment into rock.
70. Lithology – Description of rock based on texture, color, and composition.

M–O

71. Magma – Molten rock beneath Earth’s surface.
72. Mantle – The thick layer between Earth’s crust and core.
73. Marble – Metamorphosed limestone, composed mainly of calcite.
74. Marker Horizon – A distinctive layer used to correlate rock units.
75. Matrix – The fine-grained background in which larger grains are embedded.
76. Metamorphic Rock – Rock altered by heat, pressure, or fluids.
77. Mid-Ocean Ridge – A divergent plate boundary in the ocean floor.
78. Mohs Scale – A scale ranking mineral hardness from 1 to 10.
79. Monocline – A single bend or step in otherwise horizontal layers.
80. Normal Fault – A fault where the hanging wall moves down relative to the footwall.
81. Nodule – A rounded mineral mass, often of silica or phosphate.
82. Obsidian – A natural volcanic glass.
83. Oolitic Limestone – Limestone made of tiny spherical grains.
84. Ore – Rock containing valuable minerals for extraction.
85. Outcrop – Exposed bedrock at the Earth’s surface.
86. Overburden – Material lying above a resource like coal or ore.

P–S

87. Paleontology – Study of ancient life through fossils.
88. Pangaea – A supercontinent that existed ~300 million years ago.
89. Peat – Partially decayed plant matter found in bogs.
90. Permeability – Ability of rock to transmit fluids.
91. Phyllite – A fine-grained, low-grade metamorphic rock.
92. Pluton – A body of intrusive igneous rock.
93. Porosity – The percentage of void spaces in a rock.
94. Pyroclastic – Rock fragments from explosive volcanic eruptions.
95. Quartz – A hard, common mineral made of silica.
96. Recumbent Fold – A fold lying on its side.
97. Rift Valley – A valley formed by the pulling apart of Earth’s crust.
98. Sandstone – A sedimentary rock composed of sand-sized grains.
99. Schist – A medium- to high-grade metamorphic rock with platy minerals.
100. Stratigraphy – The study of layered sedimentary rocks.