An unconformity is a surface representing a significant break or gap in the geological record. This boundary indicates a time when rock layers were either not deposited or were actively eroded away before new layers began to form above them. Geologists examine these contacts to piece together the sequence of events that shaped a region of the Earth’s crust, determining periods of uplift, erosion, and subsequent deposition.
Defining the Angular Unconformity
An angular unconformity is characterized by an erosional surface that separates two sets of sedimentary layers that are not parallel to each other. Below the boundary, the older sedimentary strata are visibly tilted, folded, or otherwise deformed. These lower layers were deposited horizontally, then subjected to powerful tectonic forces that caused them to uplift and become angled from their original position.
The next step involved a long period of erosion, which wore down the tilted layers to a nearly flat surface. Younger sedimentary rock layers were then deposited horizontally on top of this eroded plane. The striking visual difference between the angled layers below and the flat layers above gives this unconformity its name and makes it relatively easy to identify.
Defining the Nonconformity
A nonconformity is an erosional surface that separates younger, typically flat-lying sedimentary rock layers from much older, underlying igneous or metamorphic rock. The layers above the boundary formed from sediments deposited in water or on land. The layers immediately below the surface are composed of crystalline rock types that formed deep within the Earth’s crust.
This underlying crystalline rock is non-sedimentary, formed either by the cooling and solidification of magma (igneous rock) or by the alteration of existing rock under intense heat and pressure (metamorphic rock). For the sedimentary layers to be deposited on top of this material, the deep-seated rock must have been brought to the surface and extensively eroded before the younger sediments were laid down.
The Distinguishing Characteristic: Rock Type Below the Surface
The characteristic that most directly separates an angular unconformity from a nonconformity is the specific type of rock found immediately beneath the erosional surface. Both unconformity types involve an eroded surface covered by younger, typically horizontal sedimentary rock. However, the composition of the older rock below the boundary is fundamentally different, reflecting distinct geological histories.
In an angular unconformity, the rock below the gap is older sedimentary rock that has been tilted or folded, maintaining its stratified character. This structure indicates that the original layers were subjected to a major compressional event, such as mountain building, before being leveled by erosion. The underlying material remains stratified, even if its bedding planes are no longer horizontal.
In contrast, the rock immediately below a nonconformity is a non-stratified, crystalline material, such as granite or schist, which is either igneous or metamorphic. The presence of these crystalline rocks signals an immense amount of uplift and erosion. These materials form at significant depths beneath the surface, requiring the complete removal of overlying rock to expose them.
The Geological Story Told by Unconformities
These boundaries represent immense spans of time where the rock record is missing, often encompassing tens to hundreds of millions of years. The angular unconformity implies a history of major tectonic activity, where crustal forces folded and tilted existing layers before the land was exposed to surface erosion. This sequence requires significant vertical movement and crustal shortening.
The nonconformity tells a story of even more profound uplift and erosion, as it signifies that deep-seated, basement-type rocks were exhumed and exposed at the surface. This requires the complete removal of all overlying rock layers, which could have been many kilometers thick, to expose the crystalline core.