Menengai Crater 2: Hot Rocks Earth Cache EarthCache

Menengai Crater 2: Hot Rocks Earth Cache

The cache is located at one of the ‘most spectacular geological features on earth’ where the proximity of hot rocks has great importance . . .

To reach the cache location: see GC5R5MK Menengai Crater 1: Tall Tales Territory for a description of the approach route from Nakuru and related waypoints.

Menengai is a huge extinct shield volcano characteristic of the East African Rift (see figure for its location in the rift system). Liquid magma rose along the deep seated faults associated with the rift. Rifts form where the forces of plate tectonic cause large blocks of the upper mantle and earth crust to move apart, allowing magma to rise from the upper mantle. The lavas of the crater are trachytes, whereas pumice, obsidian and scoria form the rim. The crater is 8-12 km in diameter and 485 m deep.

The young volcano formed about 200,000 years ago with the growth of a 30 cubic km lava shield. The prominent 142 sq km elliptical caldera formed about 8,000 years ago with the last eruption in 6050 BC. The caldera floor is covered with numerous post-caldera lava flows. It is considered one of the best-preserved Krakatoa-style calderas in the world. There is very little sediment in the caldera which is a thick mass of lava boulders and inaccessible ridges. Volcanic activity continues underground and is in the process of being exploited for geothermal power generation. Kenya is developing its vast geothermal reserves faster than any other country and the geothermal power potential is estimated at 7,000 MW compared with its current total power generation of 1,400 MW.

The south and western portions of the crater are complete. In the centre of the caldera there is a small cone rising above surrounding lava, which shows fumarolic activity. Four fumaroles have been measured with temperatures up to 94^oC.

The pre-caldera volcanic rocks are almost continuously exposed in the caldera wall from Lion's Head (at 2,272 m the highest point of the caldera rim on its south side where the caldera wall is 300m high) westwards to the western cliffs.

Two post caldera maars are located south of Menengai and NW of Lake Nakuru. Magma was probably injected from beneath Menengai along a N-S trending fissure to produce the eruption.

A caldera is the large, usually roughly circular, depression with a steep relatively low rim left behind when, after a series of very large eruptions, the top of a large composite volcano collapses or when the surface rock of a shield volcano collapses into the void left after the magma chamber empties, ie. when the magma chamber collapses. Usually a new volcano starts to grow inside, eventually covering it. Caldera-forming eruptions are the largest on Earth and calderas are much larger than single volcanic craters. The central floor of the caldera is later filled by subsequent lava flows. Thus, caldera is both a process as well as a feature that starts with the collapse of the overlying unstable rocks and is completed with lava filling the floor.

In contrast a volcanic crater is a relatively small bowl-like depression at the top of a volcano around an opening (vent) used for eruption of ash, magma and lava. The depression results from weakening of the rocks by hot lava and subsequent sinking in due to high pressure.

See here for an animation explaining caldera formation and here for a simple demonstration of this.

Geothermal Activity at Menengai

Detailed surface exploration was completed and exploration drilling commenced in July 2011 with four drill rigs (see here and here for a detailed descriptions of the drilling process and its numerous challenges). 15 wells with a capacity of 60 MWe (ie. electric power) were completed by July 2013 to depths of around 2-3,000 metres. Phase I targets 400 MWe of a planned total of 1,600 MWe and is expected to be complete around 2017. The field has water systems, a road network and a basecamp. The environment section runs a tree nursery where seedlings are given to the local community for social afforestation. Automatic weather stations at the site monitor daily weather in the area. More indigenous trees are being planted within the caldera to improve tree cover that had disappeared following years of logging. (Source: Geothermal Development Company).

Geothermal energy is thermal energy generated and stored in the Earth which originates from the original formation of the planet (20%) and from radioactive decay of materials (80%). Geothermal power is cost effective, reliable, sustainable, and environmentally friendly, but has historically been limited to areas near tectonic plate boundaries, although with the advance of new technologies that is changing. Geothermal wells release greenhouse gases trapped deep within the earth, but these emissions are much lower per energy unit than those of fossil fuels. As a result, geothermal power has the potential to help mitigate global warming if widely deployed in place of fossil fuels.

The Menengai caldera is a trachytic central volcano underlain by a high-level magma chamber (see figure). It is one of the high-temperature fields in Kenya located within the central Rift Valley and presumed to be at the rift triple junction where the failed Nyanzian Rift joins the main Kenya Rift Valley that extends from Turkana in the north to Lake Natron in Northern Tanzania and is part of the East African Rift System.

The presence of the high level magma chamber below the Menengai caldera means that the geothermal reservoir can be accessed at the relatively shallow depths compatible with economic application of geothermal power generation.

The graphic shows the basic process of power generation using geothermal energy. Water is injected into the geothermal reservoir where it is converted into steam to drive the electricity generating turbines. The steam condenses to water which is re-injected.

A trial direct use project in Menengai currently in the experiment phase uses heat from the geothermal system to warm two greenhouses. In one there is a large tank with 3,000 catfish. The ammonia-rich fish-waste is regularly filtered out of the water and diluted to water bean plants. These in turn are very rich in nitrogen which is extracted and diluted with water to feed strawberry plants which can then produce 2kg of strawberries from 1 plant! There are also plans for a spa as another means of directly using the geothermal energy abundantly available.

Sources:

https://en.wikipedia.org/wiki/Menengai

http://www.volcanolive.com/menengai.html

http://www.travelinggeologist.com/2015/01/menengai-caldera-in-kenya-with-helen-robinson/

Questions:

1. What are two differences between volcanic craters and calderas?
2. Give examples of two other types of crater?
3. How many plumes of venting steam can you see from the viewpoint?
4. What can you see is the major difference between the south side of the caldera where the viewpoint is and the north side some 7km away? Why do you suggest this is?
5. What makes Menengai caldera a promising site for geothermal power generation?
6. Not obligatory: take a photo of yourself highlighting a feature of the caldera and post this with your log.

Notes:

To comply with Earth Cache logging requirements you will need to:

1. Visit GZ
2. Correctly answer the questions above (as well as you can)
3. Submit your answers individually (ie. not as a group)
4. Send the answers to the Cache Owner (via the geocaching.com link on the CO's profile page)
5. State in your log that this has been done (or will be done in a reasonable time)