Isla Clarión, formerly called Santa Rosa, is the second largest, westernmost and most remote of the Revillagigedo Islands, 314 km west of Socorro Island and over 700 km from the Mexican mainland. The island is 8.544 km by 3.686 km in size, with an area 19.80 km². . It has three prominent peaks. The westernmost and tallest peak, Monte Gallegos, is 335 m (1000 ft) high. The central peak is called Monte de la Marina (280 m), and the eastern peak Pico de la Tienda (292 m). The coasts are backed by perpendicular cliffs, 24 to 183 m high, with the exception of the middle part of the southern coast in the vicinity of Bahia Azufre (Sulphur Bay), which is the location of a small military garrison manned by 9 men.
Clarion was sighted again by the English privateer George Shelvocke on the Speedwell, August 21st, 1721. The name of the island goes back to the American brig Clarion, Capt. Henry Gyzelaar, who was engaged in the North Pacific trade around 1820.
A few seabird species breed on Clarión or formerly did so. The island is near the north(east)ern limit of the breeding range of several of these, but their continued presence needs confirmation:
For reasons not fully known, Clarión seems to be more attractive to shorebirds and other vagrant or migrant birds than Socorro; perhaps this is due to the presence of Red-tailed Hawks on the latter island. Great Blue Heron, Snowy Egret, White-faced Ibis, Pacific and possibly American Golden Plover, Spotted Sandpiper, Wandering Tattler, Ruddy Turnstone, Black-necked Stilt, Western Gull and Barn Swallow are examples of the species that can be encountered on Clarión with some regularity; most of the shorebirds congregate in the sheltered shallows of Sulphur Bay. Others, such as Blue-winged Teal, Western Sandpiper, Least Sandpiper, Short-eared Owl, Belted Kingfisher, Northern Mockingbird and Brown-headed Cowbird have only been recorded on very few occasions, sometimes only once.
In addition, the local population of the Western Raven (Corvus corax sinuatus or C. sinuatus) was formerly considered a distinct subspecies clarionensis, but this is not usually accepted at present. It can be expected that this question will soon be resolved as there is currently renewed interest in the phylogeny of the Common/Western/Chihuahuan Ravens. If the Revillagigedos population is indeed distinct, it might an endemic of Clarión as the only other local subpopulation, on San Benedicto, was destroyed by a volcanic eruption on August 1, 1952; on the other hand, mainland birds have sometimes been assigned to clarionensis also.
Brickellia peninsularis var. amphithalassaBulbostylis nesiotica, Cheilanthes peninsularis var. insularis, Cyperus duripes, Euphorbia anthonyi, Nicotiana stocktonii, Perityle socorrosensis, Spermacoce nesiotica and Zapoteca formosa ssp. rosei are Revillagigedo endemics which Clarión shares with either San Benedicto or Socorro. Whether Teucrium townsendii var. townsendii is the same plant as on San Benedicto is not conclusively determined.
There has not been recent dedicated research on the impact the sheep and rabbits had on the local flora. While no plants seem to have gone extinct on the other Revillagigedo Islands, Clarión is one most heavily affected by introduced herbivores.