{"id":464,"date":"2017-01-23T16:36:49","date_gmt":"2017-01-23T16:36:49","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/introduction-to-oceanography\/chapter\/13-3-landforms-of-coastal-erosion\/"},"modified":"2021-10-27T15:49:51","modified_gmt":"2021-10-27T15:49:51","slug":"13-3-landforms-of-coastal-erosion","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/introduction-to-oceanography\/chapter\/13-3-landforms-of-coastal-erosion\/","title":{"raw":"13.3 Landforms of Coastal Erosion","rendered":"13.3 Landforms of Coastal Erosion"},"content":{"raw":"
\r\n\r\nLarge waves crashing onto a shore bring a tremendous amount of energy that has a significant eroding effect, and several unique erosion features commonly form on rocky shores with strong waves.\r\n\r\nWhen waves approach an irregular shore, they are slowed down to varying degrees, depending on differences in the water depth, and as they slow, they are bent or refracted (section 10.3<\/a>). In Figure 13.3.1, wave energy is represented by the blue arrows. That energy is evenly spaced out in the deep water, but because of refraction, the energy of the waves is being focused on the [pb_glossary id=\"824\"]headlands[\/pb_glossary]<\/strong>. On irregular coasts, the headlands receive much more wave energy than the intervening bays, and thus they are more strongly eroded. The result of this is [pb_glossary id=\"632\"]coastal straightening[\/pb_glossary]<\/strong>, where an irregular coast will eventually become straightened, although that process may take millions of years.\r\n\r\n \r\n\r\n[caption id=\"attachment_371\" align=\"aligncenter\" width=\"1024\"]\"Illustration<\/a> Figure 13.3.1<\/strong> The approach of waves (blue lines) towards a coastal headland. The blue arrows represent wave energy; most of that energy is focused on the headlands, causing greatest erosion in this area (PW).[\/caption]\r\n\r\nWave erosion is greatest in the surf zone, where the [pb_glossary id=\"1248\"]wave base[\/pb_glossary] is impinging strongly on the seafloor and where the waves are breaking. The result is that the substrate in the surf zone is typically eroded to a flat surface known as a [pb_glossary id=\"1250\"]wave-cut platform[\/pb_glossary]<\/strong> (or wave-cut terrace<\/strong>) (Figure 13.3.2). A wave-cut platform extends across the [pb_glossary id=\"870\"]intertidal zone[\/pb_glossary].\r\n\r\n \r\n
\r\n\r\n[caption id=\"attachment_460\" align=\"aligncenter\" width=\"600\"]\"Image<\/a> Figure 13.3.2<\/strong> A wave-cut platform in bedded sedimentary rock on Gabriola Island, B.C. The wave-eroded surface is submerged at high tide (Steven Earle, \"Physical Geology\").[\/caption]\r\n\r\n<\/div>\r\n[pb_glossary id=\"550\"]Arches [\/pb_glossary] <\/strong>and [pb_glossary id=\"1110\"]sea caves[\/pb_glossary]<\/strong> form as a result of the erosion of relatively non-resistant rock. Wave action and strong [pb_glossary id=\"912\"]longshore currents[\/pb_glossary] can carve a cave into a [pb_glossary id=\"824\"]headland[\/pb_glossary], and if the erosion extends all the way through, it becomes an arch. If a hole develops in the ceiling of a cave, a [pb_glossary id=\"594\"]blowhole [\/pb_glossary] <\/strong>can be created, shooting water into the air when waves crash in the cave. An arch in the Barachois River area of western Newfoundland, Canada, is shown in Figure 13.3.3. This feature started out as a sea cave, and then, after being eroded from both sides, became an arch. During the winter of 2012-2013, the arch collapsed, leaving a small [pb_glossary id=\"1120\"]stack [\/pb_glossary] at the end of the point.\r\n\r\n \r\n\r\n[caption id=\"attachment_461\" align=\"aligncenter\" width=\"600\"]\"Two<\/a> Figure 13.3.3<\/strong> Top: An arch in tilted sedimentary rock at the mouth of the Barachois River, Newfoundland, July 2012. Bottom: The same location in June 2013. The arch has collapsed and a small stack remains (Photo: Dr. David Murphy, used with permission in Steven Earle, \"Physical Geology\").[\/caption]\r\n\r\n
<\/div>\r\nThe tower of rock left behind from a collapsed arch is called a [pb_glossary id=\"1120\"]sea stack[\/pb_glossary]<\/strong> (Figure 13.3.4). But sea stacks can also form during the formation of [pb_glossary id=\"1250\"]wave-cut platforms[\/pb_glossary] or other features, when relatively resistant rock that does not get completely eroded remains behind to form the stack.\r\n\r\n \r\n\r\n[caption id=\"attachment_462\" align=\"aligncenter\" width=\"640\"]\"Image<\/a> Figure 13.3.4<\/strong> A sea stack, likely created from the collapse of a sea arch (Doug Lee [CC BY-SA 2.0], via Wikimedia Commons).[\/caption]\r\n
<\/div>\r\n
Figure 13.3.5 summarizes the process of transformation of an irregular coast into a straightened coast with [pb_glossary id=\"1112\"]sea cliffs [\/pb_glossary]<\/strong> (wave-eroded escarpments) and the remnants of [pb_glossary id=\"1120\"]stacks[\/pb_glossary], [pb_glossary id=\"550\"]arches[\/pb_glossary], and [pb_glossary id=\"1250\"]wave-cut platforms[\/pb_glossary]. The next stages of this process would be the continued landward erosion of the sea cliffs and the complete erosion of the stacks and wave-cut platforms in favor of a continuous and nearly straight sandy beach.<\/div>\r\n
<\/div>\r\n
<\/div>\r\n<\/div>\r\n
\r\n\r\n[caption id=\"attachment_463\" align=\"aligncenter\" width=\"650\"]\"Diagram.<\/a> Figure 13.3.5<\/strong> Evolution of a straightened coast through the erosion to stacks and arches, sea cliffs, and wave-cut platforms (Steven Earle, \"Physical Geology\").[\/caption]\r\n\r\n
\r\n\r\n
\r\n\r\n*\"Physical Geology\" by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http:\/\/open.bccampus.ca\r\n\r\n<\/div>\r\n<\/div>","rendered":"
\n

Large waves crashing onto a shore bring a tremendous amount of energy that has a significant eroding effect, and several unique erosion features commonly form on rocky shores with strong waves.<\/p>\n

When waves approach an irregular shore, they are slowed down to varying degrees, depending on differences in the water depth, and as they slow, they are bent or refracted (section 10.3<\/a>). In Figure 13.3.1, wave energy is represented by the blue arrows. That energy is evenly spaced out in the deep water, but because of refraction, the energy of the waves is being focused on the headlands<\/a><\/strong>. On irregular coasts, the headlands receive much more wave energy than the intervening bays, and thus they are more strongly eroded. The result of this is coastal straightening<\/a><\/strong>, where an irregular coast will eventually become straightened, although that process may take millions of years.<\/p>\n

 <\/p>\n

\"Illustration<\/a>
Figure 13.3.1<\/strong> The approach of waves (blue lines) towards a coastal headland. The blue arrows represent wave energy; most of that energy is focused on the headlands, causing greatest erosion in this area (PW).<\/figcaption><\/figure>\n

Wave erosion is greatest in the surf zone, where the wave base<\/a> is impinging strongly on the seafloor and where the waves are breaking. The result is that the substrate in the surf zone is typically eroded to a flat surface known as a wave-cut platform<\/a><\/strong> (or wave-cut terrace<\/strong>) (Figure 13.3.2). A wave-cut platform extends across the intertidal zone<\/a>.<\/p>\n

 <\/p>\n

\n
\"Image<\/a>
Figure 13.3.2<\/strong> A wave-cut platform in bedded sedimentary rock on Gabriola Island, B.C. The wave-eroded surface is submerged at high tide (Steven Earle, “Physical Geology”).<\/figcaption><\/figure>\n<\/div>\n

Arches <\/a> <\/strong>and sea caves<\/a><\/strong> form as a result of the erosion of relatively non-resistant rock. Wave action and strong longshore currents<\/a> can carve a cave into a headland<\/a>, and if the erosion extends all the way through, it becomes an arch. If a hole develops in the ceiling of a cave, a blowhole <\/a> <\/strong>can be created, shooting water into the air when waves crash in the cave. An arch in the Barachois River area of western Newfoundland, Canada, is shown in Figure 13.3.3. This feature started out as a sea cave, and then, after being eroded from both sides, became an arch. During the winter of 2012-2013, the arch collapsed, leaving a small stack <\/a> at the end of the point.<\/p>\n

 <\/p>\n

\"Two<\/a>
Figure 13.3.3<\/strong> Top: An arch in tilted sedimentary rock at the mouth of the Barachois River, Newfoundland, July 2012. Bottom: The same location in June 2013. The arch has collapsed and a small stack remains (Photo: Dr. David Murphy, used with permission in Steven Earle, “Physical Geology”).<\/figcaption><\/figure>\n