According to Scerrato (1980) the Iwans plans of Seljuk Mosques were mainly developed for multi-functions including, prayer, teaching, lodging of teachers and students, libraries and charitable activities like stopping stations on the road of pilgrims.
Summarised extracts from a full article:
Muslim Architecture under Seljuk Patronage (1038-1327) by Rabah Saoud
In building types, the Seljuks made considerable changes to the traditional hypostyle mosque. The development of the Iwan plan revolutionised the form and function of the mosque, and later introduced new types of buildings involving the Madrassas, the hospitals and the caravansaries which spread in Anatolia, Syria and Iran respectively.
The Iwan plan, according to Godard (1965), was derived from the plan of houses of Khurasan(1). This was first applied in the Madrassa, then transmitted to the Mosque and later even to secular buildings and palaces. The Iwan was also used by the Sassanians as vestibule leading to the main domed ceremonial hall (Godard, 1965). Scerrato (1980) added a new formula arguing that the Iwans were mainly developed to serve a number of functions including, prayer, teaching, lodging of teachers and students, keeping books and as a reading library as well as a number of charitable activities in stopping stations on the road of pilgrims intended to serve the ill and the needy. Scerrato links the dome it to the ceremonial and royal symbolism derived from the Sassanian tradition. The dome in the Mosque case indicates the princial space.
"It seems highly probable that the concept of domed hall linked to royalty was not lost in Islamic Iran, especially if we consider that, in line with Sassanian usage, the exceptionally heavy royal crown, weighed down with symbols, was not actually worn by the Ghaznavid sovereigns but suspended over their head. The crown of Masud I (1030-1041) was held up by four statues and consequently speaking it was in fact a dome. The throne room of Masud III in his Ghazni palace (completed in 1111) was made up of an Iwan standing in front of a hall that was probably domed" Scerrato
The above view is untenable as the introduction of the Mihrab/dome appeared earlier in the Umayyad Great mosque Mosque of Damascus (745) and later in the Kairawan Mosque (836), long before its introduction to Iran. If it was a Sassanian inspiration it would have first appeared in Persian mosques before anywhere else.
The first mosque to adopt the four Iwan plan was the Friday Mosque of Isfahan (Masjid-I-Jami) which was originally built by the Abbasids in the 9th century. The Seljuk Caliph, Nizam Al-Mulk (1029-1092) made some additions including the construction of a large brick dome, in front of the Mihrab, in the old hall on its Abbassid cylindrical piers imitating Umayyad and Abbasid mosques.
Historic sources indicate that Nizam Al-Mulk copied the Umayad Great Mosque of Damascus which he had visited in 1086 (Hattstein and Delius, 2000). His political rival, Taj Al-Mulk(2) built (between 1088/89) a great domed pavilion, at the opposite end on the axis of the Mihrab and the southern dome of Nizam Al-Mulk. The dome consisted of similar formal elements of its sister in the south in terms of the hemispheric shape, the eight tripartite squinches, as well as the brick construction techniques. However, it ‘attained a perfection seldom equalled and never surpassed' (Hoag, 1987, p.95). This perfection is especially seen in the verticality expressed by the alignment of the decorated blind panels of the walls and the squinches and windows above them. In the words of Hoag (1987) ‘He (the architect of Taj Al-Mulk) achieved a structural consonance and a hierarchy of ordered parts not again approached until the High Gothic of thirteen century Europe' (p.95) The four Iwans were believed to have been erected after the fire of 1121/1122, in which both domes escaped damage(3) (Hoag, 1987) (figure 1).
1 Initially in Parthian Assur Palace (2nd century A.D).
2 Hoag (1987) suggested that this edifice was built for Terkan Khtun, the wife of Malik Shah and daughter of the Qarakhanid Sultan Tamghach Khan.
3 According to Scerrato (1980) the fire took place between 1120-1121.