Islam, The religion of peace, percolated through to west Africa via the Sahara trade routs around the 10th century but made little progress until an Songhai Emperor in 1468 named Ali, captured swiftly a large region of Sahel, in particular surrounding towns of Niger, Djenne, Timbuktu and Gao.
Djenne known as West Africa’s Eternal City, its use of local materials, such as mud and palm wood, its incorporation of conventional architectural styles, and its adaptation to the hot climate of West Africa are expressions of its elegant connection to the local environment. Such earthen architecture, which is found throughout Mali, will last for centuries as it’s commonly maintained.
The current Great Mosque of Djenne was built in 1906-1909 on the ruins of a 13th century building, this mosque covers a site of 40×40 meters. Constructed in traditional clay, it testifies to the continued existence of regional architecture. The relevance of cultural identity is demonstrated in a ‘The Great Mosque’ of Niono, Mali, Which was built in 1973 using the same method and styles as the older one at Djenne.
The Great Mosque incorporates Islamic Architectural elements seen in mosques right through the Muslim world, it reflects the aesthetics and materials used for centuries by the people of Djenne. It is built on a raised plinth platform of rectangular sun dried mud bricks that are held together by mud mortar and plastered over with mud.
Overall, walls vary in thickness among sixteen and twenty-four inches(400/610MM), also varies based on the height. These massive walls are needed in order to bear the weight of the lofty structure and also provide a padding from t he sun’s heat. 
The repair or maintenance of the Great Mosque to return it to its former glory is a great excuse for an annual festival. The unusual wooden poles that extend from the structure are actually inbuilt scaffolding to enable easy access during times of repair.
Non-Muslims used to be welcome to enter the mosque until 1996 when permission was granted to French ‘Vogue Magazine’ for a fashion shoot inside the building & due to inappropriate a action by the magazine, Non-Muslims been banned from entering the structure ever since.
It was declared as UNESCO World Heritage Site since 1998.
from: http://www.islamicarchitecture.org/architecture/thegreatmosquemali.html
Al Masjid 