The main source of surface energy is solar radiation. The amount of solar energy received by the Earth at different times varies, resulting in changes in surface temperature. When the Earth receives very little solar energy, it becomes very cold, and snow in many places never melts, accumulating and compacting over time to form ice. Flatlands gradually develop into thick ice sheets, such as those in Antarctica and Greenland. Mountainous ice, due to the force of gravity, moves down valleys like rivers, hence called glaciers. Periods when the Earth was covered by ice sheets and glaciers are referred to as ice ages, to distinguish them from warmer climatic periods.
There have been five major ice ages in Earth's history. The first was the Huronian Ice Age, which occurred between 2.4 billion and 2.2 billion years ago; the second was the Neoproterozoic Ice Age, between 750 million and 630 million years ago; the third was the Early Paleozoic Ice Age, between 460 million and 430 million years ago, which was relatively short-lived and had limited impact; the fourth was the Carboniferous-Permian Ice Age, also known as the Late Paleozoic Ice Age, between 350 million and 270 million years ago; and the fifth was the Late Cenozoic Ice Age, or the Quaternary Ice Age, from 2.6 million years ago to the present. The Earth is currently in the interglacial period of the Quaternary Ice Age.
Among these five major ice ages, the duration of each varies greatly, and the degree of coldness also differs significantly. The longest ice age was the first one, which lasted for 200 million years. The coldest was the second ice age, during which ice covered large areas of Asia, Europe, America, Africa, and Australia like a lid. Some of the aforementioned regions were located in the low latitudes of the equator at that time. If these regions were all covered by ice, then the Antarctic and Arctic regions would have been even more so. In other words, the Earth's surface was largely covered by ice and snow, forming a giant snowball, which is why scholars refer to this event as the Snowball Earth and call the Earth at that time the Snowball Earth.