Mechanisms

Sea-level change has been a feature on Earth for billions of years, and it has important implications for coastal processes and both erosional and depositional features. There are three primary mechanisms of sea-level change, as described below.

Eustatic

Eustatic sea-level changes are global sea-level changes related to changes in the volume of glacial ice on land or changes in the shape of the seafloor caused by plate tectonic processes. For example, changes in the rate of mid-ocean spreading will change the seafloor’s shape near the ridges, which affects sea level. Over the past 20,000 years, there have been approximately 125 meters (410 feet) of eustatic sea-level rise due to glacial melting. Most of that took place between 15,000 and 7,500 years ago during the significant melting phase of the North American and Eurasian Ice Sheets. At around 7,500 years ago, the rate of glacial melting and sea-level rise decreased dramatically, and since that time, the average rate has been in the order of 0.7 mm/year. Anthropogenic climate change led to an accelerating sea-level rise starting around 1870. Since that time, the average rate has been 1.1 mm/year, but it has been gradually increasing. Since 1992, the average rate has been 3.2 mm/year. (4)

Isostatic

Isostatic sea-level changes are local changes caused by subsidence or uplift of the crust related either to changes in the amount of ice on the land or to growth or erosion of mountains.  Almost all of Canada and parts of the northern United States were covered in thick ice sheets at the peak of the last glaciation. Following the melting of this ice, there has been an isostatic rebound of continental crust in many areas. This ranges from several hundred meters of rebound in the central part of the Laurentide Ice Sheet (around Hudson Bay) to 100 m to 200 m in the peripheral parts of the Laurentide and Cordilleran Ice Sheets – in places such as Vancouver Island and the mainland coast of BC. Although the global sea level was about 130 m lower during the last glaciation, the glaciated regions were depressed at least that much in most places, and more than that in places where the ice was thickest. (7)

Tectonic

Tectonic sea-level changes are local changes caused by tectonic processes. The subduction of the Juan de Fuca Plate beneath British Columbia creates tectonic uplift (about 1 mm/year) along the western edge of Vancouver Island, although much of this uplift is likely to be reversed when the next sizeable subduction-zone earthquake strikes. (4)

Emergent and Submergent Coasts

Coastlines that have a relative fall in sea level, either caused by tectonics or sea-level change, are called emergent. Where the shoreline is rocky, with a sea cliff, waves refracting around headlands attack the rocks behind the point of the headland.

They may cut out the rock at the base forming a sea arch that may collapse to isolate the point as a stack. Rocks behind the stack may be eroded, and sand eroded from the point collects behind it, forming a tombolo, a sand strip that connects the stack to the shoreline. Where sand supply is low, wave energy may erode a wave-cut platform across the surf zone, exposed as bare rock with tidal pools at low tide.  Sea cliffs tend to be persistent features as the waves cut away at their base, and higher rocks calve off by mass wasting. If the coast is emergent, these erosional features may be elevated compared to the wave zone. Wave-cut platforms become marine terraces, with remnant sea cliffs inland from them. (4)

Tectonic subsidence or sea-level rise produces a submergent coast. Features associated with submergence coasts include estuaries, bays, and river mouths flooded by the higher water. Fjords are ancient glacial valleys now flooded by post-Ice Age sea level rise. Barrier islands form parallel to the shoreline from the old beach sands, often isolated from the mainland by lagoons behind them. Some scientists hypothesize that barrier islands formed by rising sea levels as the ice sheets melted after the last ice age. Accumulation of spits and far offshore bar formations are also mentioned as formation hypotheses for barrier islands.

Estuaries and fiords commonly characterize coastlines in areas where there has been a net sea-level rise in the geologically recent past. This valley was filled with ice during the last glaciation, and there has been a net rise in sea level here since that time. Uplifted wave-cut platforms or stream valleys characterize coastlines in areas where there has been a net sea-level drop in the geologically recent past. Uplifted beach lines are another product of relative sea-level drop, although these are difficult to recognize in areas with vigorous vegetation.

Backyard Geology:  Sea-level changes recorded in rocks

While it seems fairly obvious that there are no real coastlines in Arizona currently, their existence is preserved in rocks throughout the state.  These ancient deposits imply that they were formed in a similar environment as the rocks of present day, so there must have been oceans and shorelines throughout Arizona in the past.

Figure 8.5.2 shows the three uppermost layers of the Grand Canyon.  Though these layers were deposited millions of years ago, they preserve the environments that were present at their time of deposition.  The Kaibab Formation, the youngest set of rocks found in the Grand Canyon, are a 270 million-year-old limestone that was deposited in a shallow marine (ocean) environment.  The Toroweap Formation was formed in an intertidal zone, as sea-level changed several times.  The older Coconino Sandstone is a 275 million-year-old wind-blown sand which forms a dramatic cliff in the present day.  Together, these rock show that sea-level changed dramatically in just 5 million years, and much of Arizona was entirely under water!