Zach Schonfield's Geo Postcard on Mud Pools and Marine Terraces

This mud pool near Wai-o-tapu is the result of nearby volcanic activity. Over time, volcanic ash and glass accumulate in a basin. These sediments are then converted to smectite by hot water. The hot water also is the result of volcanic activity; to the east in the Hikurangi Trough the Pacific plate is subducted under the Australian plate. This subduction creates a heat source for ground water. As the smectite is heated, it becomes less dense than the smectite and water around it causing it to rise to the surface, creating a bubbling mud pool. The water is so hot that it readily evaporates, causing steam to form on the surface. The hot water trapped in the rising smectite also can result in the release of steam at the surface.


Marine terraces typically form when the ocean sea level drops, the shoreline is raised, or a combination of the two. The last interglacial sea level should be about 20 meters above the present sea level; however you can see in this photograph at Turakirae Head that the marine terrace from this period is over 100 meters above the current sea level, indicating that substantial uplift has been occurring. In 1855 the coast was uplifted in some places to 7 meters, creating a new marine terrace. These terraces can be used to determine recurrence intervals of earthquakes in the surrounding area as well, which is very useful for insurance companies to determine premiums for natural disaster claims. Similar uplifts have been used in Papua New Guinea to determine sea level changes over time. Marine terraces are vital to the understanding of geological processes in the past and for predicting the future.

Brandon Boldt's Geology Post Cards

Pumice Terrace

On day three of the New Zealand trip, the group visited an outcrop near Lake Taupo that presented an interesting dynamic to an otherwise traditional depositional environment. The picture (above) shows strata alternating between layers of clast supported, well sorted, small, sub-rounded pumice clasts and layers of larger, rounded, pumice clasts in a fine grained matrix. This outcrop also happens to be one of many river terraces in the area. River terraces are formed when debris fills in valleys during glacial stadials due to ice driven forces and low river transport rates, followed by high river transport rates during interstadials that erode material, creating "V" shaped incisions in the land. The alternation between "filling in" during stadials and "down-cutting" during interstadials produces the recognizable terrace topography. Fluvial processes, like rivers, are the reason for the alternating layers in the outcrop. In normal circumstances larger clasts are evidence of high flow (more power needed to move the clast) while smaller clasts and layers of silt show a decrease in velocity and can even allude to the evolution of rivers to streams and lacustrine environments. The interesting dynamic to this outcrop is that the clasts are pumice stones. Pumice is a highly vesicular (filled with gas bubbles from a "gas blown" origin) form of rhyolite that has a lower density than water. While inspecting this outcrop one must question whether the larger clasts actually mean higher water velocity. The argument can be made that because the layers with larger pumice clasts are more rounded and have some fine grained matrix (while most fluvial deposits have little matrix material and are clast supported due to water removing the materials between clasts), and that larger pumice stones may float better than fine pumice with few vesicles, that the more fine grained layers are evidence of high flow rates and the larger pumice layers of more quiescent periods in the history of this system. In other words, how these layers appear and what they represent is opposite in respect to most outcrops - would you expect anything different from the antipodes?

Volcanic Blocks

Volcanic blocks are pieces of solid country rock or rocks formed from previous eruptions, ejected from the volcano. Unlike volcanic bombs, which are ejected in a molten state as spurts of lava that may solidify in air or on the ground, volcanic blocks are nearly always angular and may be gigantic. The volcanic block above , found near the summit of Mt. Ruapehu, is slightly larger than a backpack (in the picture for scale) and has made a clear impact in the snow. This ejected material is more likely a volcanic block than a volcanic bomb because it is angular, does not have flow textures (common in volcanic bombs due to their molten history) and most interestingly the bottom border is covered in yellow sulphur crystals. Sulphur is commonly precipitated and crystallized in volcanically active areas, and due to this volcanic block's short life span (snow has not yet covered it and the last eruptive event for Ruapehu was within a year) it is more than likely that the sulphur had grown on the country rock before being displaced. Volcanic blocks and volcanic bombs are only a couple geologic hazards associated with volcanoes, along with others like lava flows, lahars, pyroclasitc flows, and corrosive ash falls that can collapse structures and cause airplane jets to clog and malfunction.

Amy Postcard 2

New Zealand is one of the few places on earth that still has glaciers. When glaciers begin to melt, often a river is formed. One example of this is the Waiho River in the South Island of New Zealand. Behind the river is the Franz Josef glacier. The river drains into the Tasman Sea. The water is very cloudy in color, and is very turbid and murky. This could be caused by the high amounts of precipitation in the area and high release amounts from the mountains behind the river. Throughout the river there are bigger boulders and rocks left there by the glacier retreating in the past. Also, glacier flour is left behind; this is from the glacier dragging over the bedrock. Also, due to the type of river that it is, you can often see rocks floating in the river. This is loose terrain from the mountain, where ice melts from the glacier and the debris that has acquired on the top falls down. Around this area you can see small moraines left that formed. As you look downstream, there is the Waiho loop moraine that is flat and was formed during the last ice age when the ice was retreating.

Amy's Postcards

Terraces can be seen all over New Zealand along with other parts of the world. Marine terraces can be seen on the North Island, New Zealand along the shore in this photo due to uplift events as a result of subduction occurring. The most recent event was in 1855, the Wairarapa earthquake which had an 8.2 magnitude. These marine terraces may have occurred during the last 10,000 years (Holocene). Marine terraces are wave-cut platforms that are ancient shore lines. They usually form as a result of waves that hit against the cliff and corrosion occurs. At these sea terraces, earlier sea levels can indicate reoccurrence intervals if dated. Older sea terraces are often more fragmented due to erosion processes. The terraces when standing at them are often clear cut; you are on a flat surface. Then you go up a few feet (slope upwards) and there is another flat surface, this indicates one of the steps up. Often scientists date shells and human artifacts (that may still be preserved where uplift occurred) to date back occurrence.

Champagne Pool

This steamy pool is located in a large area of surface thermal activity in the Taupo Volcanic Zone. It is a 700 year old explosion crater formed by a hydrothermal eruption. The diameter is 65 meters with a depth of 62 meters, making it the largest hot water spring in the district. The water would not be good for a nice swim. The temperature of the pool starts at 230 degrees Celsius and cools to around 74 degrees Celsius. Besides these scorching temperatures, the water is acidic with a pH of 5.4. All along the surface bubbles of carbon dioxide are released. By taking a closer look at the pool, you will notice this orange colored solid along the sinter edge. This is made up of arsenic and antimony. Gold, silver, mercury, sulphur and thallium are also found in the steamy water.

Mt. Ruapehu

One of the highlights of my trip to New Zealand was climbing and sliding back down Mount Ruapehu. Mt. Ruapehu is the highest point in the North Island of New Zealand at a height of 2797 meters. Between its high peaks, there is a crater lake that fills up between eruptions. This volcano is one of the three volcanoes located in the World Heritage Tongariro National Park, the first national park of New Zealand. This land was given to the world by the Maori to protect the land that is home to the volcanoes Mount Ruapehu, Mount Ngauruhoe, and Mount Tongariro Ruapehu is an active composite volcano that is always covered in snow and ice, making it popular for skiers, snowboarders and hikers. Since it is an active volcano, there is a danger of eruptions and lahars. There were eruptions in 1945, 1995, 2006, and 2007. During the most recent eruption, the Crater Lake partially drained into dangerous lahars. A teacher needed to be rescued from the Dome Shelter after crushing his leg during the 2007 lahar.

Turakirae Head

This area (seen in the photo) shows evidence of large scale uplift events through the presence of multiple marine terraces. These uplifts are caused by high magnitude shaking events that raise the land preserving past shorelines. Most of these earthquakes can be attributed to the Hikarangi Trough, a large subduction zone on the North Island, and other major faults that run through the area. The shoreline along the Turakirae Head consists of many marine terraces formed during the Holocene. The most recent marine terrace was caused by the Wyroba earthquake in 1855 with a Richter scale magnitude of 8.2, which raised the land 1 to 6 meters. The abandoned marine terraces form an anticline or convex fold in the ground. Many geologists have worked to date these terraces in order to help map a pattern of earthquake reoccurrence. They use various dating techniques, such as radiocarbon dating. Since radiocarbon dating can only date things up to 50,000 years of age, cosmogenic radionucleotide dating (CNR) is used to date cobbles found on very old high terraces. If a pattern of reoccurrence is developed, it could help to predict future earthquakes potentially saving the lives of many people. The photograph displays the multiple marine terraces leading up to the oldest ones. The older terraces can be identified by their fragmentation, which is caused by the years of erosion.