A marine terrace is a distinctive geological feature representing an ancient coastal platform, characterized by its elevated position above the present-day ocean. These landforms are preserved remnants of former shorelines that have been lifted out of the reach of modern wave action. Often referred to as raised beaches, marine terraces appear as flat, step-like surfaces along coastlines. This elevated, relatively level terrain indicates a period when the sea level was stable enough for waves to erode the bedrock. These raised platforms provide geologists with a unique window into the Earth’s history, documenting the dynamic interplay between ocean levels and crustal movement over thousands of years.
Defining the Structure of a Marine Terrace
The physical anatomy of a marine terrace consists of two primary components: a gently sloping surface and a steep, landward cliff. The flat area, known as the wave-cut platform or abrasion platform, is the surface planed off by wave erosion when it was at sea level. This platform often retains a veneer of marine deposits, such as rounded beach cobbles and sand, left behind as the sea retreated.
The second defining feature is the paleocliff, the steep cliff face that backs the platform on the landward side. This fossilized sea cliff marks the original inland limit of wave erosion during the terrace’s formation. The sharp inflection point where the platform meets the paleocliff, termed the shoreline angle, approximates the elevation of the ancient high tide line.
Along many actively uplifting coastlines, multiple marine terraces are stacked one above the other, resembling a giant staircase. Each step in this sequence represents a distinct period of sea-level stability and subsequent uplift. The highest terraces are typically the oldest, while the lowest terraces are the most recently formed. This tiered structure provides a chronological sequence of coastal evolution.
The Dual Process of Formation
The creation of a preserved marine terrace requires two distinct geological processes: marine abrasion and relative sea-level change. The first stage involves the erosive power of ocean waves, which use sediment and gravel to cut a broad, flat platform into the bedrock. This process of abrasion is most effective when sea level remains stable for an extended period, allowing a wave-cut platform to widen at the base of a retreating sea cliff.
The second stage is the process that removes the newly formed platform from the destructive influence of the sea, ensuring its preservation. This removal is primarily achieved through tectonic uplift, the steady, upward movement of the Earth’s crust along active faults. If the rate of crustal uplift is sufficient, the platform is raised before the next high sea-level stand can re-erode or destroy it.
The formation of extensive terrace sequences is linked to the global sea-level fluctuations associated with glacial and interglacial cycles. During an interglacial period, when global sea level is high, a new platform is cut into the landmass. As the next glacial period begins, sea level falls, and the platform is exposed. If the coastline is simultaneously being tectonically uplifted, the exposed platform is permanently elevated above the subsequent high-water marks, allowing a new, lower terrace to form during the next interglacial highstand.
Geological Significance and Research Value
Marine terraces serve as precise geological markers for studying Earth’s history. One of their primary applications is accurately measuring the rate of regional tectonic uplift. Since the elevation of global sea level during certain past interglacial periods, such as the Marine Isotope Stage 5e (about 125,000 years ago), is known, the current height of the corresponding terrace allows scientists to calculate the average rate of crustal movement since that time.
Terrace sequences also offer paleoclimate data, as their formation is linked to the cyclic changes in global sea level driven by ice ages. By dating the sediments and fossils found on a terrace, scientists can correlate the feature to a specific sea-level highstand. This provides a chronological record of past oceanic volumes and climate conditions, helping in understanding the Earth’s long-term response to climate forcing.
Furthermore, the study of uplift rates derived from these features is directly applicable to seismic hazard assessment. In tectonically active regions, the calculated rate of crustal deformation provides insight into the potential for future large earthquakes. For example, the vertical offset of a single terrace across a fault line can be used to quantify the cumulative displacement caused by seismic events over thousands of years.
Global Examples and Notable Locations
Sequences of marine terraces are found along tectonically active coastlines bordering the Pacific Ocean. The coast of California, particularly around Santa Cruz and the Palos Verdes Peninsula, is famous for its numerous, well-preserved terraces. The Palos Verdes Peninsula exhibits a sequence of 13 distinct terrace levels, documenting a long history of continuous coastal uplift.
Other significant locations include the coasts of Chile and New Zealand, both situated along active plate boundaries. The Chilean coast is noted for sequences that can contain dozens of terraces. On New Zealand’s North Island, multiple terraces have been studied, and the uplifted surfaces near the Cook Strait offer evidence of vertical displacement from historic seismic events. Marine terraces are also prominent features along the coastlines of Japan and Oregon.