Rock Formations and Structures

Rock bodies come in all shapes, but the shapes are all variations on a few basic forms. Sedimentary and volcanic rocks all start out as flat layers or beds, which may later be deformed by the continuing motion of the Earth’s surface. If the beds are tilted, the geologist describes the orientation of the bed in terms of its strike and dip.

The strike and dip of an orebody can be imagined as a sheet of plywood sitting on an angle in a tank of water. The line where the sheet and the surface of the water intersect is the line of strike – a horizontal line with a bearing that can be measured with a compass. The angle the wood makes with the surface of the water is its dip. Both strike and dip are usually measured in degrees.

Folds are caused by sideways pressure on the flat-lying bedded rocks which buckle and form a wavy pattern. Bending a book slightly, and viewing the top pages gives an idea of the structure of a fold. Folds with “crest” shapes are called anticlines, and those with “through” shapes are called synclines.

Intrusive rocks generally form large bodies called batholiths or smaller, pipe-like bodies called stocks. A dyke is a sheet-like intrusive body that cuts through the surrounding country rock. A sill is also sheet-like, but it forms along a space between bedded rocks. Like bedded rocks, dykes and sills have a strike and dip.  

Fractures are very common in rock. If the fracture is large enough, and the rocks have been torn apart, the fracture is called a fault. If the fault is not a single fracture, but rather a series of many tiny parallel fractures, it is called a shear zone.

Fractures, faults and shear zones, like beds and dykes, are planar features that can be described by their strike and dip. It is also useful to label the upper surface of an inclined fault the hangingwall, and the lower surface the footwall.

It is important for the geologist to know the sense of movement along the fault – upward, downward, or sideways – because this allows rock units broken apart by the fault to be traced. For example, if an orebody is broken apart by later faulting, knowing whether the hangingwall has moved up or down against footwalls allows the geologist to predict where the rest of the orebody might be. 

Fracturing can also help to form orebodies. Fractures and openings in the rock allow fluids to pass through, and dissolved material in the fluids may be left behind to form a vein.

Some structures have a more linear shape – for example, a pipelike intrusive body, the axis of a fold, or an ore shoot in a vein. A linear structure’s orientation is better described by its trend, which is the direction it heads, and its plunge, which is the angle it makes with the earth’s surface. It is common to speak of structure as “plunging gently to the southwest” or steeply to the north”, which means only that it descends in that general direction; for more precise applications, the trend and plunge are expressed in degrees.