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Gumdrop, Pioneer, and Guide Seamounts

Gumdrop, Pioneer, and Guide Seamounts are located about 120 kilometers off the California margin, north of Monterey Bay.

They and other seamounts along the California and Baja California margin, including Davidson Seamount and Rodriguez Seamount (MBARI Mapping Team, 2000), have a similar structure and origin.

Magnetic anomalies near Guide and Pioneer Seamounts are not symmetrical but correspond to Chron 6B and 6C, respectively, indicating ocean crust of about 22 to 24 million years old.

Formation of these complex, northeast-trending seamounts offshore central California, including Davidson Seamount to the south, is incompatible with a hot spot model or formation as near-mid-ocean ridge seamounts.

All may have been formed by eruption through structurally weak zones at abandoned spreading centers, with volcanism occurring as much as 8 million years after the ridge stopped spreading.

Lavas apparently originated and partly crystallized in the shallow mantle (Davis et al., in press).

Trace element ratios and isotopic compositions suggest origin by small degrees of melting from a mildly but variably-enriched mid-ocean ridge source.

Gumdrop, the northernmost of the three seamounts, has a series of aligned cones separated by sediment-filled troughs, but these structures are poorly defined.

The shallowest cone rises to 1207 meters below sea level.

The volume is difficult to estimate because the base of the seamount is poorly defined. However, we estimate its volume at about 100 km3. Samples recovered from Gumdrop are highly vesicular, alkalic basalt, hawaiite, and mugearite, but their age is not determined