GEOLOGICAL STRUCTURES

1. IGNEOUS STRUCTURES

Any intrusive igneous body is a PLUTON. Intrusive occupying a small area, having an irregular to cylindrical shape, and cutting across the intruded rock is called a STOCK. If such an intrusion occupies an extremely large area, it is referred to as a BATHOLITH. Some cross-cutting plutons gradually lose their penetrating force during the latter stages of intrusion, and merely push the overlying rocks upward rather than cutting through them, thus form DOMES or LACCOLITHS. DIKES are tabular intrusions which cut across enclosing rock, while intrusions that penetrate parallel to bedding or foliation are called SILLS.  (Igneous structures are shown in Figure 3a.)

Extrusive structures result from volcanic activity. During such activity the extruded material (lava, ash or cinders) may spread outward in gently-dipping layers, or may flow from a fissure or vent. Often, however, this material accumulates adjacent to the volcanic crater in the form of a CONE. CALDERAS are enlarged craters, formed through partial collapse of underlying rock, or through explosive activity. The subterranean conduit for a volcano is called a PIPE, PLUG, or NECK.

Igneous intrusions often brecciate the rock along their margins and the path in advance of their path forming STOCKWORKS and BRECCIA PIPES, and a complex of mineralized fractures.

A VEIN is a relatively narrow tabular mineralized structure. A LODE is either a single vein or a system of related roughly parallel vein. A STOCKWORK is a mass of intersecting veins. All are related to late magmatic hydrothermal fluid deposition, or deposition from circulating ground waters.

2. SEDIMENTARY STRUCTURES

Horizontal and/or vertical movements of the earth's crust and the effects of igneous intrusions may crumple layers of sedimentary rocks into folds. Down warped beds form SYNCLINES and up warped beds form ANTICLINES. Long, relatively narrow, and very large warps of the earth's crust are designated as GEOANTICLINES or GEOSYNCLINES. Where the down warping is large, significant portions of the continental crust may become flooded and become the site of widespread deposition of sedimentary rocks, often attaining great thicknesses -- the environment of great thicknesses of limestone. Smaller rounded down warps are designated as BASINS.  (Sedimentary structures are shown in Figure 3a.)

Formations of sedimentary rocks are seldom uniform in thickness. The deeper portions of sedimentary troughs or basins receive thicker sediments, and the layers of sediments thin and pinch out at the margins of depositional areas. Shifting currents in the water or air or obstructions may result in beds deposited at angles to each other (CROSS-BEDDED).

Silt, sand and gravel deposited in and on the margin of streams or rivers (FLOODPLAIN DEPOSITS) form discontinuous, serpentine-shaped deposits. Melt-waters from melting glaciers deposit a variety of OUTWASH GRAVELS, ESKERS, MORAINES, TILL, etc., that form distinctive land forms.

Sedimentary beds deposited on an older erosion surface are said to unconformably overlie the older rock. The older and younger beds may be parallel (DISCONFORMITY) or the younger beds may rest on tilted older beds (ANGULAR UNCONFORMITY).

3. TECTONIC STRUCTURES

Where forces exerted by the earth movements exceed the strength of the rock or its ability to bend into folds, the rock is broken and segments on opposite sides of the break may be moved relative to each other. The rock is designated as FAULTED. Relative movement along FAULTS may be vertical, lateral or diagonal (or all three at various times) with relative displacement from less than an inch to hundreds of miles laterally, and tens of miles in depth. Different faults range in attitude from nearly horizontal to vertical.

Rock caught between opposing walls of a fault may be ground completely to rock flour (GOUGE) or crushed into angular fragments (FAULT BRECCIA).

4. LANDFORMS

Erosional processes (air, water, ice) etch the land surface revealing underlying rock structures and rock character. In addition, surface processes (fluvial, glacial, volcanic, etc.) leave deposits having characteristic forms, and mass-wasting leaves evidence of landslides, mud flows, etc. This evidence facilitates geological analysis using aerial photographs and satellite imagery.