More properly termed The Mohorovicic Discontinuity - in geology the word "discontinuity" is used for a surface at which seismic waves change velocity - it was discovered in 1909 by Andrija Mohorovicic , a Croatian seismologist. Mohorovicic showed that the velocity of a seismic wave is related to the density of the material that it is moving through.
At the beginning of the 20th century, scientists were almost certain that the Earth contained many layers, but they did not know exactly where these layers started and ended. Andrija Mohorovicic already had an interest in the precise measurement of time of astronomical and seismic events and records of an earthquake in Croatia on 8 October 1909 revealed the existence of 2 different sets of shock waves from the same earthquake. Because the second set of waves exactly mirrored the first set, albeit offset in time and realizing that the velocity of a seismic wave is affected by the material that it is moving through, he suggested that there must be a change in the composition of the rocks at depth, with higher density material being present at depth. By gauging this time difference he was able to calculate the depth of the denser material.
Following the discovery of the Moho, scientists have been able to gauge the depth of the boundary layer, discovering that it varies not only in depth but also its thickness.
At Coverack (pronounced Cover-ack) you will be able to see this boundary layer, which, was once about 5Km beneath the ocean floor, and is now exposed at the surface and laid flat.
Progressing from South to North along the Moho you will be effectively travelling up through the earth’s interior.
Starting at the South end of the beach, near the harbour, you will find serpentine rock from the upper mantle; continuing along the Moho, you will enter into the transition zone, characterised by the intermingling of serpentine and gabbro and the intrusion of basalt. In addition, there is a rare, highly coloured red and white rock known as troctolite present here. Proceeding further north you finally enter the area of gabbro marking the start of the oceanic crust.
It is necessary to go onto the beach in order to complete the logging tasks, please be aware of the state of the tide/sea conditions..
COVERACK BEACH IS A SITE OF SPECIAL SCIENTIFIC INTEREST PLEASE DO NOT REMOVE ANY ROCKS FROM THE AREA.
There are currently (May 2014) several information leaflets, on many subjects, designed to help you to discover more about the local area available from the village post office and local shops (these cost £2 which is refundable if you return the leaflet).
Update January 2018, logging tasks changed, but we will still accept the previous tasks answers.
In order claim this Earthcache please complete the following tasks and email us your answers:
Task 1. Visit the serpentine outcrop at the south end of the site, find some serpentine and describe its appearance/texture.
Task 2. The area between the serpentine and gabbro outcrops is the Mohorovicic Discontinuity (ie the transition from mantle to crust) - measure or estimate the distance between the serpentine and gabbro outcrops.
Task 3. Visit the gabbro outcrop at the north end of the site, find some gabbro and describe its appearance/texture.
Task 4. In the gabbro area at N50 01.671 W005 05.683 you will find an example of a basalt dyke, compare and contrast the appearance of the gabbro and basalt
You may log your visit prior to sending the email, but logs without answers WILL be deleted.
Sources used for development
www.coverack.org.uk
http://geology.com
www.earthquake.usgs.gov
www.enotes.com
The Geology and Landscape of Cornwall and the Isles of Scilly by Simon Camm. ISBN 13 978-0-906720-83-7