This biochemical sedimentary rock is formed primarily from tiny SiO2 (silica) secreting planktonic organisms like radiolaria or diatoms and accumulates in the deep ocean. A Coal B Biochemical Chert C Biochemical Limestone D Sandstone
The correct answer and explanation is:
Correct Answer: B. Biochemical Chert
Explanation:
Biochemical chert is a fine-grained sedimentary rock that is formed from the accumulation of silica (SiO₂) in the form of tiny skeletons or shells secreted by microscopic planktonic organisms such as radiolaria and diatoms. These organisms extract dissolved silica from seawater to form their hard parts. When they die, their silica-rich remains settle to the ocean floor, especially in deep marine environments where conditions favor silica preservation over carbonate preservation.
Over time, these silica-rich sediments compact and lithify into a hard, dense rock known as biochemical chert. This rock is distinct from inorganic chert, which forms from the chemical precipitation of silica from solution, without biological assistance. Biochemical chert can form in deep ocean basins where the water is undersaturated with respect to calcium carbonate but rich in dissolved silica. This allows for better preservation of siliceous microfossils.
Biochemical chert often contains visible remains of the original microscopic organisms when examined under a microscope, offering direct evidence of its biological origin. The silica in chert is typically in the form of microcrystalline quartz, making the rock very hard and resistant to weathering.
Now, let’s look briefly at why the other options are incorrect:
- A. Coal forms from plant material, mainly in swampy environments, and is composed mostly of carbon, not silica.
- C. Biochemical Limestone is composed mainly of calcium carbonate from organisms like corals and foraminifera, not silica.
- D. Sandstone is a clastic sedimentary rock composed of sand-sized mineral particles, mainly quartz, but is not formed by the accumulation of planktonic organisms.
Therefore, among the given choices, Biochemical Chert is the only rock that fits the description of forming primarily from tiny SiO₂-secreting planktonic organisms in the deep ocean.