Movers and shakers

Architecture has entered a sculpted, Plasticine age. The smudge, smear and gloop are replacing right angles and logically ordered forms. And Venice – the Arsenale and Giardini di Castello, to be precise – is where this transformation, involving some of the profession's biggest names, is currently laid bare – though not quite as expected.

Architecture has entered a sculpted, Plasticine age. The smudge, smear and gloop are replacing right angles and logically ordered forms. And Venice – the Arsenale and Giardini di Castello, to be precise – is where this transformation, involving some of the profession's biggest names, is currently laid bare – though not quite as expected.

There, by the chipped-jade waters of the lagoon, one experiences a weird crossover effect. If you visit the Biennale's Eighth International Architecture Exhibition, you will see some damn fine sculpture; pop across the water to the Peggy Guggenheim Collection's sculpture show, and what do you encounter? Riveting architecture.

This new fluidity is nothing new – think of Le Corbusier's fungoid chapel at Ronchamps. But now there's a much wilder collapse of boundaries. Rectilinear ennui has set in, though not in the German pavilion, where dozens of far too closely placed plinths support what can only be described as Things You Can Do With A Cube. This was a low point in the show's Giardini of good and awful: these objects could have been delivered with infinitely more variation by screen-saver software.

The meltdowns of form to be seen in the two vast halls of the nearby Arsenale provide the most telling after-images of an exhibition devised by the design commentator, Deyan Sudjic. He has taken the more-is-more route, setting out models, drawings and multimedia displays featuring the work of more than 100 practices. It's well enough done: one does not feel overwhelmed by minutiae because the narrative is a trail-mix of jagged and smooth architectural evidence.

And Sudjic allows one interesting googly into his tightly controlled show. In the City of Towers section, one finds a display of one-off objects from Alessi's experimental Tea and Coffee Piazza show of two decades ago. In it, architects including Will Alsop, David Chipperfield and Dominique Perrault put avant-garde spins on the pot so beloved of café society. The result is surreal sculpture. And within metres of this dated array, we find a dateless effluvium. Future Systems' tower model is a fluidly ascending splash; Zaha Hadid's is a beautiful skyscraper in the clasp of bulging arteries; and Toyo Ito's may have a rectilinear outer casing, but an irregular vortex-space accelerates up through it like the tail of a Kansas twister. That vortex-effect is even more startlingly pursued by Michele Saee, whose design for Paris's mod-iconic Publicis Drugstore has an interior shot through with surfaces that twist and flail like strips of stir-fried cuttlefish.

Everywhere you look, plasticity. Ito's Relaxation Park project for Torreviega, near Alicante, is made up of three low, horizontal enclosures that have taken croissant design and given it the most elegant of twists. They are buildings, but could easily be large-scale sculptural riffs on the local topography. Herzog and de Meuron, creators of Tate Modern, are digging even deeper in Tenerife where their huge waterfront makeover will use architecture to deliver what they describe as a sudden, volcanic event. One section of this two-level quay system features surfaces that have been vulcanised on a large scale; contorted and pierced, as if solidified after cooling. The architectural metaphor may be unremarkably obvious, but the effect on walkers will be of a strange and grandly naturalistic sculpture.

The same applies to Hadid's hotel project in Guadalajara, Mexico. This is going to be as brilliantly sense-deranging, though on a vaster scale, as her stunning little museum at the Vitra factory in Basle. Imagine a cliff-face made up of horizontal layers of rock – then shatter those layers into irregularly projecting blocks. Is this architecture, or random tectonic sculpture? Guadalajara, incidentally, will be a hot spot for this kind of creativity: the city's JVC Cultural Centre will contain a dirigible-like building by Enrique Norten, and a Children's World complex by Philip Johnson that plays games with angled, truncated cones.

And can it really be true that David Chipperfield has also been infected by this dangerous tendency? His design for the Ansaldo Cultural Complex in Rome may embody his highly developed trademarks – careful massing, interlocking and façade detailing, but right in the middle of it is a huge, flat-topped amoeba that turns out to be a courtyard within a courtyard, a "random assemblage of volumes".

Massimiliano Fuksas is more obviously known for his Play-Doh forms, and his Centro Congressi Italia in Rome will encase a wonderfully swirling, translucent hall within an otherwise unremarkable rectilinear glass and steel outer shell. Just another day at the sculpture, really.

As it always is with Will Alsop. His All Barnsley Might Dream project, under the auspices of Yorkshire Forward's Urban Renaissance panel, is designed to electrify and revive a town centre, whose core dates back to the 13th century, with architecture that erupts out of the streetscapes like randomly proportioned inflatables; the new wrenching itself from the clasp of the old. This is masterplanning right on the edge of possibility, an alchemical alembic that seeks to turn base urban muddle into something like a golden sense of locale and aspiration.

There's no muddle to be found in the British pavilion in the Giardini. Here, sponsored by the British Council and NEC, is a well-realised evocation of Foreign Office Architects' partly completed Yokohama Port Terminal. This, too, is a kind of sculpture – architecture as topographical manipulation. This time, though, nothing is random; the form may flow, but it's symmetrical. Function is the ruthless bottom-line.

But not for the visitor. Two years ago, the British Council's commissioner, Andrea Rose, decided to feature the work of five architects. This time – with curator, Ruth Ur – she decided to deliver a solo show. And the architects have used the four spaces very effectively: one walks across floors glowing ultraviolet with projected and magnified detail-drawings; one watches workmen on site; one gets pleasantly skewed by architectural geometries. The design and construction processes have been arranged to make for an informative but highly sensual experience.

Not unlike spending an hour at Venice's Guggenheim Museum, where Thinking Big delivers a taste of avant-garde British sculpture that often turns out to be chunks of proto-architecture. Shirazeh Houshiary's "Untitled" might well be a maquette for a barley-twist skyscraper; Peter Hide's Plainsong is surely a random section through one of Daniel Libeskind's buildings; Tony Cragg's "calypso-collapso" forms in marble and bronze might just as easily have arisen in Future Systems' collective unconscious; Marcus Vergette's Either Either is pure – no, make that impure – Frank Gehry; William Pye's delightful whirlpool in a glass tube is Ito's and Saee's vortex architecture brought to glittering life. And is Sheila Vollmer aware that her "Well II 2000" would make a fabulous ziggurat-cum-hotel?

The Eighth International Architecture Exhibition, Venice, to 3 Nov, www.labiennale.org; Thinking Big, Peggy Guggenheim Collection, Venice, to 6 Jan, www.guggenheim-venice.it