Heard Island is covered in interesting geology, with windows into the past, the present, and the Earth’s interior. In the coming weeks, I expect this will be a recurring topic, so if there are parts you’d like me to elaborate on, please leave a comment!
There are three main rock types on Heard Island: volcanic rocks, marine sediments, and increasingly, glacial sediments. We will focus primarily on the first two, as the glacial sediments are quite recent and not yet lithified, and less has been written about them.
At the base of the stratigraphic sequence accessible above sea level are marine sediments, specifically limestone. These sediments are composed of the carbonaceous shells of micro-organisms, and were deposited when the water was very shallow, but in an open-ocean setting. From the types of shells found, these limestones were deposited between about 60–30 million years ago (Ma).[1 and references therein] (Refresher on the geologic timescale)
Overlying the limestone is the Drygalski Formation, which is of volcanic origin, and begins around 10 Ma. The Drygalski formation includes pillows, which are small (<1 m diameter) blobs of rock which form when lava is erupted underwater, which rapidly cools the outsides.* Other rocks in this formation include hyaloclastite, which is also characteristic of submarine volcanism. Glacial sediments in the form of tillite (wide range of sizes of sediment with clasts (rock chunks) supported by much finer grain sizes) are also present in this formation, indicating the presence of glaciers in the area.
Volcanism has begun again more recently, no later than about 1 Ma (based on K-Ar dating), and created the modern volcanic structures on the Laurens Peninsula and Big Ben itself. This volcanism continues in the present-day, and eruptions have been observed by satellite in 2013 (see picture above). Not only are there the volcanoes of the Laurens Peninsula and Mawson Peak atop Big Ben, but there are numerous small volcanic cones along the perimeter of the island. The age of these cones is unknown, but their small size and fresh appearance suggest they are quite recent (100-5000 years?).
On the subject of volcanism, Sand Atlas had a good post recently explaining the different types of lava flows. Heard has pahoehoe, a’a, and pillows, and I expect there may be some columns as well.
Dr. Will Powell of Macquarie University has a number of good field photos from a trip to Heard Island in 2000, as well as a bit of commentary to go along with them. One of the pictures is of a basaltic dike intruding rocks on the north end of the Laurens Peninsula. That dike is where magma squeezed up from below, possibly to a surface eruption above which may have been subsequently eroded away.
Finally, Heard Island has glacial sediments (till). This is being deposited in the lagoons and on the land as the glaciers retreat. I expect to find medial and terminal moraines when I am there, and some of the moraines are presently visible in the satellite imagery. There is a terminal moraine (or is it a ground moraine?) at the end of the glacier in the upper right of the picture above, at around the 2:00 position in relation to Big Ben. It is manifested as a brown patch between the glacier and the lagoon.
So, that’s the brief overview of the rocks of Heard Island. All the rocks are from the Cenozoic (<66 Ma), with the oldest being limestones, then some much younger volcanics and glacial sediments on top, with both the volcanics and glacial sediments depositing presently. I can’t wait to get there and see them in person!
 Clarke, I., McDougall, I. & Whitford, D.J., 1983: Volcanic evolution of Heard and McDonald Islands, southern
Indian Ocean. In Oliver, R.L., James, P.R. & Jago, J.B. (Eds): ANTARCTIC EARTH SCIENCE. Australian Academy of Science, Canberra: 631-635.
 Arthur Scholes, Fourteen Men, E. P. Dutton, 1952.
* For more on pillow lavas, with some great pictures, check out this post at Magma Cum Laude.