In the passage describing the building of the Mizbeach, there are 2 contradictory verses. One verse says “Make for Me an altar of earth (אדמה)”. The very next verse says “And if you make for Me an altar of stones (אבנים).”
The Midrash resolves the contradiction. The word ‘if’ in the second verse does not mean that it is optional. Building the Mizbeach out of stone is obligatory. When the Torah mentions “an altar of earth”, it is not referring to the construction material, but rather to the requirement that the Mizbeach sit directly on the ground and not be built over tunnels or columns.
This obligation to build the Mizbeach out of stone only began upon entering Eretz Yisroel. The Mizbeach in the Mishkan was not made of stone. It was built as a hollow wooden structure and covered with copper-plate. When they would camp, the hollow would be filled with sand and was literally “an altar of earth”.
The difference between earth and stone is that stone has greater strength and durability.
In our Avodah, the Mizbeach represent davening, which is in place of the Korbanos. Davening is called the ‘service of the heart’, an experience of emotion.
Chassidus explains that there are two modes of emotional connection to Hashem. One level is through the emotions that are developed through meditation. Limited by our understanding, this love is more external and weak and can therefore fade after davening is finished. A deeper level of emotional connection is through revealing the Neshama’s powerful desire for Hashem that transcends intellect and reason. This love is strong and lasting.
Being in the desert represents a state of spiritual infancy when we are at the beginning of our service of Hashem and not yet able to experience the deeper connection. And so the Mizbeach of the Mishkan, which was only a temporary structure, was an “altar of earth”.
Arriving in Eretz Yisroel represents a state of spiritual maturity and open G-dly revelation when we are able to experience the deeper and stronger emotional relationship with Hashem that transcends intellect. And so the Mizbeach in the Beis Hamikdash was “an altar of stone”.
~ Based on Sefer Hamaamarim 5658 ד"ה זה יתנו