Compared with mouse-based interaction on a desktop interface, touch-based interaction on a mobile device is quite limited: most applications only support tapping and dragging to perform simple gestures. Finger rolling provides an alternative to tapping but uses a recognition process that relies on either per-user calibration, ex- plicit delimiters or extra hardware, making it difficult to integrate into current touch-based mobile devices. This paper introduces ThumbRock, a ready-to-use micro gesture that consists in rolling the thumb back and forth on the touchscreen. Our algorithm rec- ognizes ThumbRocks with more than 96% accuracy without cali- bration nor explicit delimiter by analyzing the data provided by the touch screen with a low computational cost. The full trace of the gesture is analyzed incrementally to ensure compatibility with other events and to support real-time feedback. This also makes it possible to create a continuous control space as we illustrate with our MicroSlider, a 1D slider manipulated with thumb rolling gestures.