Rhyolite

Rhyolite is an igneous, volcanic rock, of felsic (silica-rich) composition (typically > 69% SiO₂—see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, sanidine and plagioclase (in a ratio > 2:1—see the QAPF diagram). Biotite and hornblende are common accessory minerals. It is the extrusive equivalent to granite.

Geology

Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolite melts are highly polymerized and form highly viscous lavas. They also occur as breccias or in volcanic plugs and dikes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal structures. Some rhyolite is highly vesicular pumice. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra/tuff or of ignimbrites.

Aquacade (satellite)

Aquacade, previously designated Rhyolite, was a class of SIGINT spy satellites operated by the National Reconnaissance Office for the United States Central Intelligence Agency. The National Security Agency (NSA) was also reportedly involved. The program, also known by SIGAD AFP-720 and SIGAD AFP-472, respectively, is still classified. During the same period, the Canyon SIGNIT satellites were in use with an apparently somewhat different set of capabilities.

The name of the program, originally "Rhyolite", was changed to "Aquacade" in 1975 following the disclosure of the codeword "Rhyolite" in the trial of Christopher Boyce and Andrew Lee.

The Rhyolite/Aquacade satellites, made by TRW, are rumored to have an umbrella-like reflecting dish 20 meters in diameter. They were succeeded by the Magnum/Orion and Mentor series of satellites.

A major purpose of the Rhyolite satellites was reportedly the interception of Soviet and Chinese microwave relay signals traffic. During the 1960s-70s, much of the long distance telephone and data traffic in both the US and Eastern Europe was carried by terrestrial microwave relay links, each consisting of a dish antenna on a microwave tower that transmitted a narrow beam of microwaves to a receiving dish in a nearby city. A good deal of the microwave beam would miss the receiving dish and, because of the curvature of the Earth, radiate out into space. By placing a satellite in a geosynchronous orbit at a position in the sky where it could intercept the beam, the US government was able to listen in on Soviet telephone calls and telex cables during the Cold War.